Methods of Weed Management and Control PDF

**Methods of Weed** **Management and Control** **259** **FUNDAMENTAL CONCEPTS** Weed prevention, control, eradication, and management are different concepts, and each uses and combines technologies differently. Prevention of invasion is the best strategy to combat weeds. Many important weeds in any country are escaped imports. Mechanical, nonmechanical, and cultural weed control techniques each have distinct advantages and disadvantages. No weed control method has ever been abandoned. Each new method introduced in large-scale crop culture has reduced the need for human and animal power. Cultural weed control is intuitively sensible. **LEARNING OBJECTIVES** To know the defi nition and relative merits of weed prevention, control, eradication, and management. To be familiar with weed seed laws and the federal noxious weed law. To understand the importance of planting clean crop seed. To know the practices that prevent introduction and spread of weeds. To know the advantages and disadvantages of each mechanical, nonmechanical, and cultural weed control technique. To know the present role and to consider future weed management roles of living mulches and companion cropping. To appreciate the role of minimum and no-tillage in weed management. *Fundamentals of Weed Science* Copyright © 2007 by Academic Press, Inc. All rights of reproduction in any form reserved. 260 **Fundamentals of Weed Science** **I. THE DEFINITIONS OF WEED** **PREVENTION, CONTROL, ERADICATION,** **AND MANAGEMENT** When students who are taking a weed science class are asked what the class is about, they often respond "weeds" or "weed control." Those who work on weeds often spend a great deal of time on weed control, but weed science is not only about weed control. Weed scientists try to answer fundamental questions about weeds and weed management. For example, they want to know *why* weeds are problems---that is, what is the nature of weed-crop competition? Why are some weeds problems in many places and others in relatively restricted habitats? Why do different weed management strategies work differently in different cropping systems? Why are some plants so successful as weeds? Answers to these and similar questions lead to hypotheses and theories and greater clarity about what ought to be done to manage weeds and why. **A. WEED PREVENTION** The most diffi cult part of weed management is *prevention*, defi ned as stopping weeds from contaminating an area. It is a practical means of dealing with weeds, but it takes time and careful attention to many details. Experience has shown that it is much easier to make the case and gain support for controlling weeds. After all, if control is successful, as it frequently is, results are easily observed, and something good has happened. Prevention addresses a potential problem, one that does not yet exist, and results of preventive efforts are harder to observe and measure. It is hard to demonstrate that because of weed prevention, a weed did not appear. Science cannot prove a negative. But it is as true for the agricultural ailment, weeds, as it is for human ailments: an ounce of prevention is worth a pound of cure. Effective preventive techniques may reduce short-term economic gain. Here are a few weed prevention measures: Isolating imported animals for several days Not importing weeds or weed seeds in animal feed (buying only clean hay) Using only clean crop seed that is free of weed seed Cleaning equipment between fi elds and especially between farms Preventing weed seed production, especially by new weeds Preventing vegetative spread of perennials Scouting for new weeds **Methods of Weed Management and Control** 261 Small patch treatment to prevent patch expansion and large infestations Education about weeds (e.g., weed identifi cation) **B. WEED CONTROL** Weed control includes using several techniques to limit weed infestations and minimize competition. These techniques attempt to achieve a balance between cost of control and crop yield loss, but weed control is used only *after* the problem exists; it is not prevention. Weed control techniques have been adopted widely because control is the easiest thing to do, and it is usually effective. The problem is known or can be seen, and actions can be tailored to the observed problem. Control works well with short-term economic or cultural planning goals. **C. WEED ERADICATION** Weed eradication is the complete elimination of all live weeds, weed parts, and weed seed. It is 100% or complete control. It sounds easy, but it is very diffi cult to achieve, and eradication efforts have rarely been completely successful. It is usually easy to eliminate live plants because they can be seen. It is diffi cult to eliminate seed and vegetative reproductive parts in soil. Eradication is the best program for small populations of perennial weeds, but present technology does not make it easy. In weed science, as in medical science, prevention is better than control, but control is required because weeds and other pests arrive without notice and are present before they can be prevented. Prevention and eradication require long-term thinking and planning. **D. WEED MANAGEMENT** Weed management is the combination of the techniques of prevention, eradication, and control to manage weeds in a crop, cropping system, or environment. Weed managers recognize that a fi eld's or area's cropping history, the grower's management objectives, the available technology, fi nancial resources, and a host of other factors must be combined to make good management decisions. Complete weed control in a crop may be the best decision in some cases, but it is not automatically assumed to be the goal. Maintenance of a weed population at some level in a cropping system may be the most easily achievable and fi nancially wise goal for a weed management program. 262 **Fundamentals of Weed Science** **II. WEED PREVENTION** People want to be and stay healthy. When we become ill, we are pleased to have competent physicians, hospitals, and medical services. People would rather remain healthy than have to cure an illness. The same logic applies to weed management. Weed control tries to cure but does not prevent weeds. A good weed management program includes vigilance or watchfulness. The good weed manager can identify weed seeds, seedlings, and mature plants, and has a management program for each crop and fi eld and appropriate follow-up programs. The good manager is ever watchful for new weeds that may become problems and whenever possible emphasizes prevention rather than control. Several preventive practices can be included in management programs: 1\. Isolation of introduced livestock to prevent spread of weed seeds from their digestive tract. 2\. Use of clean farm equipment and cleaning of itinerant equipment, including combines, cultivators, and grain trucks. 3\. Cleaning irrigation water before it enters a fi eld. 4\. Mowing and other appropriate weed control practices to prevent seed production on irrigation ditch banks. 5\. Inspection of imported nursery stock for weeds, seeds, and vegetative reproductive organs. 6\. Inspection and cleaning of imported gravel, sand, and soil. 7\. Special attention to fence lines, fi eld edges, rights-of-way, railroads, and so on as sources of new weeds. 8\. Prevention of deterioration of range and pasture to stop easy entry of invaders such as downy brome (Mack, 1981). 9\. Seed dealers and grain handlers should clean crop seed and dispose of cleanings properly. 10\. Cleanings should be heated or ground to prevent seed dispersal. 11\. Fields should be surveyed regularly to identify new weeds. 12\. When identifi ed, small patches of new weeds should be treated to prevent growth and further dispersal. The fi rst rule for weed prevention and the fi rst step of any good weed management program is the purchase and planting of clean seed. The US Federal Seed Act of 1939 regulates transport and sale of seeds in foreign and interstate (but not intrastate) commerce. The law is enforced by the US Department of Agriculture, which has provided supplementary rules aimed primarily at interstate movement of parasitic plants and noxious weed seeds. The Federal Seed Act and state laws mandate labeling of crop seed to show the kind of seed, its variety, and the state and specifi c locale where it was grown. Complete labels also show percent pure seed, percent weed seed, percent other crop seed, **Methods of Weed Management and Control** 263 percent inert matter, percent germination of pure seed, percent hard seeds (those seeds that are viable but not capable of immediate germination), and the date on which the tests were performed. Seed labels also include the name and number per pound of each noxious weed seed. Each US state has a noxious weed seed law that identifi es and regulates sale and movement of crop seed containing what the state law has identifi ed as noxious weed seed. These laws may prohibit importation of crop seed with greater than a certain percentage of specifi c noxious weed seed and require identifi cation of each noxious weed seed. The presence of noxious weed seed in excess of 1 gm in 10 gm of the crop seed results in exclusion from sale in most states. For large-seeded crops, such as beans, the exclusion is often 1 gm in 100 gm of crop seed. These laws may also regulate import and sale of crop seed screenings because they contain viable weed seed. State seed laws are designed to protect seed consumers (farmers and other purchasers). These laws do not mean and should not be viewed as implying equal regulation of weedy plants that may be detrimental to agriculture or the environment. On October 30, 2004, President Bush signed the Noxious Weed Control Act of 2004 (Public Law 108-412). It was passed after several years of effort as an amendment to the Federal Plant Protection Act of 2000. It is a fi rst step, and only a bit (\$15 million) of the funding requested was received. But it demonstrates the benefi t of groups working together to pass federal legislation and an increasing recognition of the importance of weed management. Seed standards are not restricted to the United States. The regulations of the Canada Seeds Act of 1987 allow various levels of weed seed to be present, depending on the crop and the level of classifi cation desired. The standards apply to barley, buckwheat, lentils, rye, and sainfoin and with minor variation for wheat, canola, fl ax, and oats. The seller must supply a certifi cate, on request, that states the number and kinds of weed seed present. A bushel of clover seed weighs 60 pounds and was 88% clover, with 35% germination. Therefore, in 1 bushel, there was 18.5 pounds of live clover seed, 34.3 pounds of dead clover seed, 4.2 pounds of weed seed that represented 11 different species, 2.8 pounds of inert matter, and 0.2 pounds of other crop seed. The purchased seed contained 7,800 Canada thistle seeds per bushel; 5,700 curly dock seeds per bushel; and 114,000 wild mustard seeds per bushel. This bargain seed cost \$5.90 per bushel or \$19.14 per 60 pounds of 100% viable seed. The same variety of certifi ed clover seed could have been purchased for \$8.40 per bushel. That bushel had 99.15% purity and 95% germination, or a cost of \$8.84 per 264 **Fundamentals of Weed Science** About one-third of US states have no limitation on total weed seed in crop seed. Limitations range from 1 to 4%. Most state laws exempt seed sold by a grower without advertising. All state laws designate certain weeds as noxious. About 20 states have no limitation on prohibited or restricted noxious weeds. Prohibited noxious weed seeds are usually seed from perennial, biennial, or annual plants that are highly detrimental to crop yield and diffi cult to control. The presence of these seeds in any amount prohibits sale of crop seed for planting purposes in many states. Restricted noxious weed seeds are seeds of plants that are very objectionable in fi elds, lawns, or gardens but can be controlled by good cultural practices. Over 175 different species are named as noxious weed seeds by the 48 continental United States. An additional 50 species are named in Hawaii. It is important to note that these are legal, not botanical, defi nitions that are informed by agronomic and horticultural practice. Most states have state seed laboratories that determine seed quality. One aspect of quality is the number of weed seed or other crop seed in a sample (Tables 10.1 and 10.2). In these examples, too many weed seeds were sown when the purchased seeds were sown. Planting clean seed is an easy method of preventing weeds. In 1975 weed prevention took a major step forward when the federal noxious weed law empowered the secretary of the US Department of Agriculture to control import, distribution, and interstate commerce of weeds declared to be noxious. Previous laws regulated just seed, not plants. Nearly all US states list some prohibited agricultural weeds in addition to those included in the federal noxious weed law. At present these laws provide some protection but in most states it is inadequate for agriculture and the environment. The federal law includes 93 weedy species, but at least 750 weeds that meet the act's defi nition remained unlisted in 1993 (US Congress, 1993). Many of these are agricultural problems, but some infest other environmental areas such as wetlands and natural areas. Invasive species are discussed in Chapter 7; four important invasive weedy species are purple loosestrife, Brazilian peppertree, Eurasian watermilfoil, and smooth cordgrass (US Congress, 1993). 60 pounds of 100% viable seed. The difference in cash cost (\$8.40 -- \$5.90 per bushel) was \$2.50. The cash cost is the only thing most buyers care about. The bargain seed cost \$19.14 for 100% good seed versus \$8.84 for 100% good seed in the second source---a difference of \$10.30 per bushel in favor of the second source (Barnes and Barnes, 1960). Purchasing bargain seed or cheap seed is rarely a good idea and can create weed problems. **Methods of Weed Management and Control** 265 **TABLE 10.1. Sample Seed Analysis from Colorado State** **Seed Testing Laboratory.** Bromegrass, smooth---61% germination Seeded @ 4--6 lb/A 136,000 seeds/lb Redroot pigweed 27,968 seeds/lb Japanese brome 512 seeds/lb Stinkgrass 256 seeds/lb Barnyardgrass 64 seeds/lb Oldfi eld cinquefoil 64 seeds/lb 28,864 seeds/lb Timothy 448 seeds/lb Barley 64 seeds/lb Sweetclover 64 seeds/lb Sand dropseed 64 seeds/lb Bentgrass 64 seeds/lb 704 seeds/lb **TABLE 10.2. Sample Seed Analysis from Colorado State Seed Testing Laboratory.** Alfalfa, Sample 1 84% germination, 84% live 224,000 seeds/lb seeded at 8--10 lb/A dodder 432/lb mallow 180/lb groundcherry 90/lb At 10 lb/A 4,320 dodder and 2,240,000 alfalfa seeds will be sown per acre. Alfalfa, Sample 2 66% germination, 84% live Russian knapweed 9/lb Chicory 270/lb Netseed lambsquarters 360/lb Kochia 180/lb Buckhorn plantain 117/lb Other weeds 189/lb Other crop Red clover 6,930/lb At 10 lb seed/A, 1,478,400 alfalfa seeds, 11,250 weed seeds, and 69,300 red clover seeds will be sown/A 266 **Fundamentals of Weed Science** A survey of weed and seed laws in fi ve contiguous western states---Idaho, Oregon, Utah, Washington, and Wyoming (US Congress, 1993)---showed the laws provided adequate to inadequate protection based on the likelihood of unlisted weeds causing economic or ecological problems. Many potential threatening weeds were omitted (Table 10.3). Federal and state laws do not include enough weedy plants, and they regulate only agricultural and vegetable seed. The laws do not cover horticultural seeds, including known sources of weed seed such as wildfl ower and native grass mixtures (US Congress, 1993). In spite of existing laws, regulations are not stringent, and it is not surprising that 36 weed species now resident in the United States were imported and escaped to become weeds---in some cases, noxious weeds (Williams, 1980). Of the 36, 2 were imported as herbs, 12 as hay or forage crops, and 16 as ornamentals. Weeds were imported as a windbreak (multifl ora rose), for possible medicinal value (black henbane), for use in aquaria (hydrilla), as a fi ber crop, just for observation, and as a dye (dyer's woad). Bermudagrass, a valuable forage species in the southern United States and many other parts of the world, is also an important weed in many areas and was introduced into the United States as a forage crop. In 1849 the US Cotton Offi ce proposed and introduced a new forage grass crabgrass (Brosten and Simmonds, 1989). More recent introductions of grassy weeds include sorghumalmum promoted as a drought-resistant, emergency hay/forage crop with names such as perennial sudangrass, sorghum grass, and Columbia grass. It is a hybrid between johnsongrass and grain sorghum and was fi rst described and cultivated in Argentina (Brosten and Simmonds, 1989). Wild proso millet was fi rst recognized as a weed in the north central United States in the early 1970s and now infests several million acres in Wisconsin and Minnesota, as far west as Colorado, in the midwestern states, and Canada. It is the same species as cultivated millet and diffi cult to control in corn. A major reason it is such a **TABLE 10.3. A Survey of Weed and Seed Laws in Five Western States** **(US Congress, 1993).** Number of species Adequacy of Number of potential State listed protection threats omitted Idaho 47 adequate 6 Oregon 67 more than adequate few Utah 23 inadequate 11 Washington 75 more than adequate few Wyoming 34 barely adequate 11 **Methods of Weed Management and Control** 267 good weed is that its seed germinates throughout the growing season rather than in a short period, as the crop's seed do, and it thereby escapes control by nonresidual herbicides and single cultivations. The latter two cases are interesting cases of failure to prevent and because of their implications for biotechnology. Hybridization of weeds and crops is uncontrolled and may be uncontrollable. Cross-pollination is inevitable when two phenologically similar, outcrossing plants share a small area (exist in an overlapping range). Research to determine the potential for gene transmittal, in cropped fi elds, from weeds to crops or vice versa is ongoing. There is a possibility that a crop that was genetically engineered for high yield or herbicide resistance will contribute to the generation of new, diffi cult to control weed hybrids (Brosten and Simmonds, 1989). Two species of toadfl ax were introduced as ornamentals and became weeds. Jimsonweed and kochia were brought to the United States for use as ornamentals, and kochia was studied as a forage crop. The artichoke thistle escaped to become a weed in artichokes and is a recurring problem in California (Brosten and Simmonds, 1989). Waterhyacinth was introduced from South America to the United States by Japanese entrepreneurs as part of a horticultural exhibit at the Cotton Centennial Exposition in New Orleans in 1884 (Penfound and Earle, 1948). It originally came from the Orinoco River in Venezuela, and single plants were given away at the Cotton Exposition. It has been introduced around the world primarily because its fl owers are pretty. At the New Orleans exposition, people liked it so much that they took it home and put it in ponds and gardens, after which it escaped because people discarded it or water fl owed out of these places and carried the weed with it. It reproduced profusely in ponds and escaped to the St. John's River in Florida, where it became a major weed problem by clogging the waterway. Waterhyacinth was brought to the Tonkin region of China (now Vietnam) in 1902 as an ornamental. It reached southern China and Hong Kong in the same year. Soon after it was observed in Sri Lanka and then India, where the sluggish rivers of east Bengal were ideal for its growth. In the 1950s it was discovered in Africa (Vietmeyer, 1975), and in 1958 it had infested over 1,000 miles of the Nile River from Juba in the south to the Jebel Aulia dam in northern Sudan (Heinen and Ahmad, 1964). It is a serious weed problem in all of these places and many others but not in Venezuela, where its spread is controlled by natural enemies. Cogongrass or Alang-Alang, a perennial, was introduced at Grand Bay, Alabama, and McNeil, Mississippi (Tabor, 1952). Bare-root orange plants were imported to Grand Bay in 1912, and the cogongrass that lined boxes the plants were shipped in was discarded. In McNeil, scientists were searching for better forage plants, and cogongrass escaped from farmer's fi elds and the experiment station and spread rapidly. 268 **Fundamentals of Weed Science** Kudzu, a nonindigenous species, was introduced to the United States at the Philadelphia Centennial Exposition in 1876 (Shurtleff and Aoyagi, 1977). It was promoted by the US Department of Agriculture for erosion control and forage, but it became a major weed and now grows in many areas throughout southeastern United States and has spread to some midwestern states. Further evidence of distribution of the world's weeds and the necessity for vigilance to prevent introduction of new species is shown in Table 10.4. Most **TABLE 10.4. Origin and Distribution of Some of the World's Most Serious Weeds** **(Holm et al., 1977).** Distribution (number Associated Weed Origin of countries) crops Purple nutsedge India 92 52 Bermudagrass Africa or Indo-Malaysia 80 40 Barnyardgrass Europe and India 61 36 Junglerice India 60 35 Goosegrass China, India, Japan, Malaysia 60 36 Johnsongrass Mediterranean 63 30 Cogongrass Old world 75 35 Spiny amaranth Tropical America 54 28 Sour paspalum Tropical America 30 25 Tropic ageratum Tropical America 46 36 Itchgrass India 28 18 Carpetgrass Tropical America 27 13 Hairy beggarticks Tropical America 40 31 Paragrass Tropical Africa 34 23 Mexico, West India, tropical South America 23 13 Smallfl ower umbrella sedge Old world tropics 46 1 Rice fl atsedge Old world tropics 22 17 Crowfootgrass Old world tropics 45 19 Eclipta Asia 35 17 Globe fringerush Tropical America 21 (rice) Witchweed Europe or South America 35 2 Halogeton Asia unk. rangeland Russian knapweed Asia unk. \>10 Quackgrass Eurasia \>80 many **Methods of Weed Management and Control** 269 of our important weeds have come from somewhere else, and vigilance is necessary to prevent new problems. Among 300 nonindigenous weeds in the western United States, 8 were former crops, and 28 escaped from horticultural areas (US Congress, 1993). All is not lost because weed entry is not prevented. Most imported plants don't become weeds. In the United Kingdom about 10% of the invaders became established but only 1% of those became weeds (Williamsen and Brown, 1986). In Australia only 5% of introduced plants became naturalized and only 1 to 2% of those became weeds (Groves, 1986). Once a plant is naturalized in an area, whether it remains insignifi cant or becomes a weed problem depends on the absence of damaging natural enemies and the presence of suitable soil, crops, land use and weed management practices, and how the plant responds to the local climate (Panetta and Mitchell, 1991). The few that become weeds can be costly problems. Although the chance is small, the consequences can be great. We can identify areas at risk of invasion, but weediness cannot be predicted as easily (Panetta and Mitchell, 1991). **III. MECHANICAL CONTROL** No weed control method has ever been abandoned completely. New techniques have been added in large-scale agriculture, but old ones are still used effectively, especially in small-scale agriculture. Mechanical weed control methods have a long history. They are a primary weed control method in many crops and in many of the world's developing countries. Although they have been used widely, they have not been studied extensively. Improvement of mechanical methods of weed control is required if they are to become acceptable alternatives to chemical control. This is especially true in organic agriculture where chemical control is forbidden and hand-weeding, if hand labor is available, is expensive, arduous, and often not effective because it is delayed. Mechanical control of intrarow weeds is often unsuccessful for the following reasons: 1\. Cultivation is delayed and weeds are only susceptible to uprooting and subsequent drying in early growth stage. 2\. Achieving crop-weed selectivity is diffi cult in early crop growth stages. 3\. The weed response to mechanical damage is highly dependent on weather conditions after cultivation (Kurstjens et al., 2004). Mechanical weed control is also expensive due to the time required, the cost of equipment, and the cost of fuel. Successful mechanical weed control nearly always requires more trips over the fi eld than chemical control, precise timing, and favorable subsequent weather. More knowledge of the weed and crop is 270 **Fundamentals of Weed Science** required by the farmer. In other words, to be successful with mechanical control, farmers must rely more on skill and planning to get the timing right and to select the proper mechanical tool (Kurstjens et al., 2004) than is required with what many refer to as the brute force of chemical control. **A. HAND-PULLING** Hand-pulling weeds on the grounds of the Imperial Palace, Kyoto, Japan. Hand-pulling is practical and effi cient, especially in gardens, but it is hard work. It's very effective for annual weeds but not for perennials capable of vegetative reproduction because shoots separate from roots that then produce a new shoot. A disadvantage is that hand-pulling doesn't get the job done when it's most needed. Most of us are too busy or too lazy to go out and weed before weeds become obvious. By the time they become obvious, easy to grab, and pull, yield reduction due to weed competition has occurred. **B. HAND-HOEING** For the home or small plot grower, unwanted, plants-out-of-place (sigh, *weeds*) are a continuing challenge, especially when situations (my small garden), **Methods of Weed Management and Control** 271 attitudes, or other reasons dictate that herbicides should not be used. The best weeding, then, is an integration of cultural tactics with arduous (i.e., no fun at all), sweat-inducing, manual control that may be complemented by mechanical control with an array of hoes, weed diggers, weed pullers, weed twisters, weed poppers, weed whips, weed hooks, and others. A novel website, http://www.ergonica.com/ergonica\_frame.htm?weeder\_features.htm&1, can aid those who must sweat as they decide whether to consider circle hoes, push-pull weeders, serrated-edge hoes, oscillating hoes, or even traditional hoes. A chart compares physical descriptions, dimensions, and user accounts of operating performance for the likes of the Angle Weeder, Weed Hound, Weed Claw, Weed Eezy, Uproot Weeder, Weed Ninja, Weed-Ho, and the Speedy Weedy. I still use my trusty, usually dull but not rusty, old hoe that is stored in the pump shed. It keeps my garden fairly clean and me fairly well exercised. Hand-hoeing has been used for weed control for many years. It is still the method of choice for most gardens and ornamental plantings and is used regularly in many vegetable crops, although California became the fi rst US state to ban weeding of commercial crops by hand in 2004.1 Hand-hoeing controls the 1Olsen, M. 2004. The end of weeding. E-mail from postmaster\@metrofarm.com; accessed September 30, 2004. Hand-hoeing weeds in rice in the Philippines. 272 **Fundamentals of Weed Science** most persistent perennials if it's done often enough, although it may take years to achieve complete control. Although effi cient and widely used, it takes a lot of time and human energy. Some data on the time required to hand-weed some crops in several different places are shown in Table 10.5. If human labor is abundant, and labor cost is not high, hand-pulling or hoeing is an acceptable but arduous method of weed control. If human labor is not abundant and it is expensive, hand methods are cost-prohibitive and not effi cient. **C. TILLAGE** When most people think of mechanical control, the fi rst thing that comes to mind is tillage with an implement to disturb, cultivate, or mix the soil. On arable land, tillage alone or in combination with cropping or chemical treatment may be the most economical system of weed control. Tillage turns under crop residue, conditions soil, and facilitates drainage. It controls weeds by burying them, separating shoots from roots, stimulating germination of dormant seeds and buds (to be controlled by another tillage), desiccating shoots, and exhausting carbohydrate reserves of perennial weeds. **TABLE 10.5. Time Required for Hand-Weeding.** Crop Location Hours per hectare to hand-weed Soybeans Peru 360 if 6 hour day Transplanted tomatoes Ohio, US 71 after herbicide, 133 after cultivation Corn Zimbabwe 24--48 for 6 hour day Beans Wyoming, US 4.4--15.5 after broadcast herbicide, 32 if no herbicide Sugarbeet Washington, US 2--111 after broadcast herbicide, 141 without herbicide Vegetables California, US 10 after broadcast herbicide Rice Several 16--500 depending on location and rice culture Wheat 101 Sorghum 50 Millet 88--298 Cotton 50--700 Jute 140 Groundnut 102--293 Cassava 115--1069 Source: Newsletter. 1979. *Int. Weed Sci. Soc.* 4(1). **Methods of Weed Management and Control** 273 Other reasons for tillage include breaking up compacted soil, soil aeration, seed bed preparation, trash incorporation, and intrarow cultivation in a crop. All of these are important, but the main accomplishment of most tillage done in the world's developed countries is weed control. The advent of no-till farming and minimum-till farming has shown that tillage is not essential to grow crops and may do no more than control weeds. Too frequent tillage can increase soil compaction---a disadvantage. Other disadvantages include exposure of soil to erosion, moisture loss, and stimulation of weed growth by encouraging germination of dormant seeds and vegetative buds. In some soils, without tillage, soil can crust, and there will be poor water penetration. Decisions about the role of tillage must be made for each soil type and farming system. Cultivating for weed control in beans. Tillage is usually divided into primary and secondary. Primary tillage is initial soil breaking or disturbance. The depth varies from at least 6 (except where primitive tools are used with limited animal power) to as much as 24 inches. Primary tillage implements include moldboard and chisel plows. These cut and invert soil and bury plant and other surface residue. Primary tillage is often the fi rst step of seedbed preparation. It was made possible by Jethro Tull's (1774--1834) invention of a cast iron plow in England in 1819. That was 274 **Fundamentals of Weed Science** followed by a steel-blade plowshare introduced by John Lane in England in 1833\. John Deere (1804--1886) introduced the fi rst steel moldboard plow in the United States in 1837. The moldboard plow may have been the most important invention of the era. It lifted and inverted soil and greatly expanded the ability of a farmer to till more land. Its invention came at a time when the English were never far from starvation and, quite literally, saved humanity (Faulkner, 1943). Farmers had trouble then, as they still do, keeping unwanted plants from growing in their crops. Plowing, because it buried plants and debris, gave the farmer time to get the crop up before the weeds appeared. Agricultural scientists welcomed the plow, without question, for its crop production and weed control benefi ts. They developed what Faulkner (1943, p\. 53) called "an unquestioning reverence for the plow." Only later were the disadvantages of the plow and the intensive tillage it enabled recognized. The advantages of plowing were clear but few realized that each plowing buried weed seeds for future recovery and germination (Faulkner, 1943, p\. 151). Secondary tillage implements may be subsequent to primary tillage, or they may be the fi rst tillage operation. Soil is disturbed, often vigorously, but upper layers are usually not inverted. A wide selection of tools is available (see Kurstjens et al., 2004). Secondary tillage is fast, inexpensive, and its tools are appropriate for large areas. Secondary tillage implements have been used for a long time; the fi rst revolving disk harrow was invented in 1847. Tools available to modern farmers include the double disk, several kinds of harrows, torsion and fi nger weeders, fi eld cultivator, rotary hoes, vertical row brushes, spring tooth harrows, rototillers, rod-weeders, and the cultipacker (combination of harrow and roller). This diverse group of implements tills soil from a few inches to a maximum of 5 or 6 inches. Secondary tillage implements break clods and fi rm soil as they remove weeds. Many regard secondary tillage implements as both weed control and seedbed preparation tools. Primary and secondary tillage is followed, in many row-crops, by selective inter-row cultivation. Tractor-mounted cultivators or animal-drawn implements move soil between crop rows to loosen it and control weeds. In general, inter-row tillage is just that: It works between crop rows. Some implements prepare inter-row areas for furrow irrigation (water runs down furrows between crop rows). Implements used for inter-row cultivation include a wide range of tine (long, fi ngerlike rods) and fl ared or straight steel shovel-like tools at the end of solid or fl exible (fl at, steel) shanks that travel through soil at shallow depths (1--2 inches). They break soil crusts and facilitate irrigation, but their main purpose is weed control. Research (Schweizer et al., 1992; VanGessel et al., 1995) has shown that, in corn, intrarow cultivators require early-season weed control (cultivation or herbicide) for optimum effi cacy. Intrarow cultivators are more effi cient **Methods of Weed Management and Control** 275 Most disking accomplishes weed control *and* seedbed preparation. (Courtesy of Deere and Co., Moline, Illinois.) Plowing is used to prepare land for planting *and* it controls weeds. (Courtesy of Deere and Co., Moline, Illinois.) 276 **Fundamentals of Weed Science** (control more weeds) that inter-row cultivators. Without herbicides, weeds in corn were always controlled better by an in-row cultivator than by the standard inter-row cultivator when each operation was performed at the right time. In-row cultivators have special tools (Figure 10.1 shows some examples) that disturb soil around crop plants and uproot weeds in rows. The tools include spyders (toothed disks that move soil toward or away from crop rows) and torsion and spring hoe weeders that fl ex vertically and horizontally to uproot weeds in crop rows. Spinners displace weeds in crop rows. Standard inter-row crop cultivators are most effective on weeds 15 cm or shorter. Interrow cultivators are most effective on weeds less than 6 cm tall (Schweizer et al., 1992). These cultivators do not work well in row crops when weed density is high. There are situations where plowing and subsequent tillage do not prepare land for planting. These include land that is heavily infested with perennial sod-forming grasses, a situation often encountered in developing country agriculture. Many tillage implements give inadequate results in the crop row after the crop has emerged and begun to grow. Tillage between rows is effi cient and can be done to within a few inches of crop plants. Tillage is not as effi cient in the crop row except when soil is moved and weeds are buried. To maximize tillage benefi ts, uniform spacing of crop rows, straight rows achieved by preci- **FIGURE 10.1.** Types of tlllage implements used for in-row cultivation (Schweizer et al., 1984). Reproduced with permission. **Methods of Weed Management and Control** 277 sion planting, gauge wheels, and instrument depth guides are needed. Uneven stands and driver error often lead to damage from mechanical cultivation and destruction of some crop plants. Successful weed control with tillage is determined by biological factors: 1\. How closely weeds resemble the crop. Weeds that share a crop's growth habit and time of emergence may be the most diffi cult to control with tillage, especially when they grow in crop rows. Weeds that emerge earlier or later than the crop are often easier to control. 2\. If a weed's seeds have a short, specifi c period of germination, it is easier to control them by tillage as opposed to those whose seeds germinate over a long time. 3\. Perennial weeds that reproduce vegetatively are particularly diffi cult to control with tillage alone. Successful mechanical control of weeds is also determined by human factors. Gunsolus (1990) noted that science could explain why certain weed management practices work the way they do. Science develops basic principles to guide action. Human cultural knowledge is different from scientifi c knowledge, although each may work toward the end of good weed management. Cultural knowledge tells one when and how to do something on a given soil and farm. Tillage is a cultural practice, and therefore, by defi nition, it requires cultural knowledge. It requires the mind of a good farmer who knows the land. Successful mechanical control requires managerial skill (cultural knowledge) that cannot be acquired from scientifi c knowledge. Cultural knowledge is acquired by doing and by observing those who have done things well. Cultural knowledge is the art of farming whereby one knows how to select and apply scientifi c knowledge to solve problems. Successful mechanical control of weeds, regardless of the implement used, is always related to the timeliness of the operation. Research can determine when to do something, but knowing when to act on a particular farm is part of the cultural knowledge good farmers have. For example, a three-year study in Pennsylvania showed that corn yields did not differ among no-till, zone-till (surface tillage in narrow rows where corn is to be planted), strip-till (deep tillage in the row where corn is to be grown), and full tillage (chisel plowing followed by disking) (Duiker et al., 2006). The study recommended farmers use no-tillage because it saved fuel, reduced soil erosion, and improved soil and water quality. Cultural knowledge will determine whether farmers will adopt the recommended no-till practices. The scientifi c knowledge of what is possible will be combined with the cultural knowledge of what should be done on a piece of land. The operative principle for use of tillage for control of perennial weeds (number 3 in the preceding list) is carbohydrate depletion. The vegetative 278 **Fundamentals of Weed Science** reproductive system of perennial weeds is a carbohydrate storehouse. When shoots grow and photosynthesize, eventually the storehouse will be replenished. If shoots are cut off, the plant calls on its reserve to create new growth. When tillage is done frequently, the management assumption is that reserves will be depleted and plants will die because of exhaustion of root reserves and increased susceptibility to other stresses (e.g., frost or dryness). Unfortunately, root reserves are vast and outlast human patience and time. Tillage may have to be so frequent that crops cannot be grown. If tillage and destruction of foliage are delayed from a few days to up to a week after emergence, the greatest depletion of root reserves occurs. With most perennial weeds, the great majority of roots and vegetative buds are in the top 6 to 12 inches of soil. Tillage done when a crop is growing cannot go this deep without disturbing crop roots---a disadvantage for control of perennial weeds. Early research showed that if fi eld bindweed was tilled 12 days after it fi rst emerged, 16 successive tillage operations at approximately 12-day intervals were required to approach eradication. If it was tilled immediately after emergence, about twice as many tillage operations were needed. The effi cacy and impracticality of tillage are also illustrated by a 1938 study that showed that purple nutsedge could be controlled in Alabama by disking at weekly or biweekly intervals for 5 months (Smith and Mayton, 1938). Obviously no crop can be grown during the 5 months. Buhler et al. (1994) demonstrated over 14 years that greater and more diverse populations of perennial weeds developed in reduced-tillage systems than on areas that were moldboard plowed. Practices used to control annual weeds and environmental factors interacted with tillage to regulate (but not eliminate) perennial weeds. It is often thought, incorrectly, that as long as one tills, it doesn't depend on how or when it is done as long as the weed is there to be controlled (Schweizer and Zimdahl, 1984). Studies were established in a fi eld where corn had been grown continuously for 6 years. Half of the plots received regular chemical weed control each year, while the other half had herbicides for the fi rst 3 years, then no herbicide, and only cultivation for the last 3 years. Plots that received herbicide for 3 years also received optimum supplemental weed management including cultivation in each of the 6 years. In the plots with herbicide for the fi rst 3 years but only cultivation thereafter, redroot pigweed dominated. At the end of the 6-year experiment, the fi eld was divided in half; one-half was plowed in January and disked in April prior to normal spring planting, and the other half was disked in January and again in April prior to normal spring planting. More redroot pigweed emerged when the fi eld was disked in the fall than when it was plowed. Where herbicide and optimum weed management had occurred for 6 years, almost no redroot pigweed survived to produce seeds for the last 3 years of the study, and tillage did not make any difference in the redroot pigweed population in the 7th year (see Figure 10.2). Smith (2006) working in Michigan demonstrated the importance **Methods of Weed Management and Control** 279 of tillage timing. Spring tillage led to weed communities dominated by spring annual forbs and C4 grasses, whereas fall tillage created communities dominated by later-emerging forbs and C3 grasses. The traits that determined species' susceptibility to tillage included the seed germination process and the plant's life cycle, which infl uence how a species responds to changes in soil resources and light availability that are related to the seasonal disturbance regime (the tillage). **FIGURE 10.2.** Population of redroot pigweed seedlings following several conventional tillage practices and atrazine use in continuous corn. In weed management system Ia, 2.2 kg/ha of atrazine was applied preemergence for 6 consecutive years. In weed management system Ib, the same rate of atrazine was applied for the fi rst 3 years and discontinued thereafter. In the fall one-half of each system Ia and Ib plot was plowed (hatched line) and the other half disked (solid line) (Schweizer and Zimdahl, 1984). 280 **Fundamentals of Weed Science** Disking soil (secondary tillage) in plots that had only cultivation for 3 years enhanced germination of seeds on the soil surface by bringing them nearer the surface. Plowing (primary tillage) buried seeds. Therefore, in the experiment, if weed control has not been good, disking instead of plowing made the weed problem worse. If weed control had been good, the kind and time of tillage didn't matter (Schweizer and Zimdahl, 1984). Another example of the importance of tillage timing is from land to be planted to wheat in North Dakota (Donald, 1990). Moldboard plowing 18--20 cm or chisel plowing 9--15 cm deep in the fall (primary tillage) followed by a combined fi eld cultivator-harrow in spring (secondary tillage) controlled established foxtail barley on previously untilled sites. Foxtail barley is a problem only in no-till spring wheat and other spring sown no-till crops in the northern Great Plains. Often it could be managed by changing tillage practices (e.g., rotating from no-till to primary tillage). If land was chisel-plowed in spring and then harrowed, the weed was not controlled (Donald, 1990). Research to determine the infl uence of the type of tillage implement and the timing of tillage leads to understanding how land management and weed control may actually create weed problems. Roberts and Stokes (1965) showed that plowing distributes weed seeds throughout the plow layer. Rotary cultivation leaves 50% of weed seeds in the top 3 inches and 80% in the top 6 inches, where they germinate best. Regardless of the type of cultivation, between 3 and 6% of the viable weed seeds in the top 10 cm of soil can be expected to produce seedlings after cultivation (Roberts, 1963; Roberts and Ricketts, 1979). Thus, one concludes that tillage can create more weeds to control. Spring soil disturbance reduced seedling emergence of large crabgrass, giant foxtail, smooth pigweed, and common ragweed by 1.4 to 2.6 times, but emergence of eastern black nightshade and velvetleaf was unaffected by spring soil cultivation (Myers et al., 2005). The same study showed that the infl uence of soil disturbance on yellow foxtail and common lambsquarters varied between seasons and location. One must conclude that the type of tillage implement and tillage timing can determine the weed problem. But the effect of tillage is also determined by the weeds present and the time of year tillage is done. One longs for precise generalizations, but weed management is too complex for simple rules. In a rare study of tillage over time, Wicks (1971) grew winter wheat annually for 12 years and studied the effect of a sweep plow, one-way disk, and moldboard plowing (all primary tillage implements) after harvest on downy brome. The moldboard plow eliminated the downy brome population after 12 years compared to 94 plants per square meter for sweep plowing and 24 for the one-way disk. Sweep plows do not bury seed as deeply as moldboard plows. The moldboard buries seed that germinate but cannot emerge. Spread of **Methods of Weed Management and Control** 281 downy brome is hastened by changing from spring to winter wheat because land is then plowed and prepared for seeding at exactly the right time for the winter annual life cycle of downy brome (McCarty, 1982). The same kind of evidence about the effects of timing and type of tillage is found in several farming systems. Evidence from rice culture shows that the method and timing of land preparation infl uenced the subsequent weed population. In fi elds where tractor plowing during the dry season was followed by two harrowings in the wet season, junglerice was over 85% of the weed population in rice, and purple nutsedge was negligible. In the same region, where two plowings and two harrowings occurred in the wet season, junglerice was virtually nonexistent, and purple nutsedge was the dominant weed (Pablico and Moody, 1984). Annual grass weeds are likely to remain a problem with use of minimum cultivation in cereal production, particularly when early planting is practiced (Froud-Williams et al., 1981). Other, previously unimportant, weeds became more prevalent, especially weedy species of brome in winter cereals in the United Kingdom. Buhler and Oplinger (1990) working with spring-sown crops in the United States showed that common lambsquarters' density was not infl uenced by tillage method, but redroot pigweed density was usually higher in chisel plow systems prior to planting soybeans. Moldboard plowing (primary tillage) followed by cultipacking (secondary tillage) always had greater densities of velvetleaf than no-till, and no-till always had more foxtail than plowing. Giant foxtail and redroot pigweed became more diffi cult to control when tillage was reduced, whereas velvetleaf was less of a problem. Growers need to be aware of the effect of tillage type and timing on weed populations and, whenever possible, choose a system that contributes to weed control. That is good management, and the integration of techniques will follow. Reduced cultivation encourages establishment of wind-disseminated species, and annual broadleaved species decline. In corn, green foxtail density was greater in chisel plow and no-till systems than with moldboard plowing, and ridge tillage had lower green foxtail density than all other systems (Buhler, 1992). Common lambsquarters' density was nearly 500 plants per square meter after chisel plowing, whereas it was only 75 in other tillage systems. Redroot pigweed responded differently to tillage with average densities of 307 and 245 plants per square meter after no-tillage and chisel plowing versus only 25 plants per square meter after moldboard plowing or ridge tillage. Weed populations were affected by tillage, but corn yield was not. Many weed seeds require light to stimulate germination (see Chapter 5). Weed scientists have asked if germination could be reduced if soil tillage or cultivation was done at night. In Oregon's Willamette Valley, cultivating agricultural land during the day increased germination 70 to 400% above levels found after nighttime tillage (Scopel et al., 1994). The effect was attributed to 282 **Fundamentals of Weed Science** the light seeds are exposed to during tillage. Buhler and Kohler (1994) showed that tilling soil in absolute darkness can reduce germination of some weed species up to 70%. Night tillage is most effective against small broadleaved species such as pigweed, smartweed, ragweed, nightshade, wild mustard, and common lambsquarters. It is not effective to reduce germination of foxtail or barnyardgrass, and it has no effect on large-seeded broadleaved weeds such as velvetleaf, giant ragweed, and cocklebur. Hartmann and Nezadal (1990) were the fi rst to report, after 7 years of study, that tillage between 1 hour after sunset and 1 hour before sunrise reduced weed emergence as much as 80% compared to day tillage. They saw night tillage as a way to manipulate and control weed populations on a purely cultural basis. They also advocated daytime tillage to photostimulate germination of dormant weeds seeds with the goal of diminishing the soil seed bank. They recommended that early primary tillage (plowing) should be carried out in full sunlight to encourage seed germination. Secondary tillage to prepare the seed bed should be done after dark to destroy emerged seedlings and not encourage germination of seeds. However, do not become too enamored of this idea. While it is true that exposure to light favors germination of many weed seeds, some are light insensitive. Light is only one of many environmental factors that affect weed seed germination. Regulating light exposure will favor management of some weeds and enhance chances for success of others. In weed management, absolute rules are hard to fi nd. When undisturbed in soil, most light-sensitive seeds are not photoinduced to germinate by light penetration below 1 cm. Germination stimulation comes from brief (a few seconds or less) exposure to light during soil disturbance in daylight. This observation is consistent with early work by Wesson and Wareing (1969), who showed weed seed germination was dependent on exposure of seeds to light during soil disturbance. Most weed seeds germinated within 2 weeks after exposure to light. They also demonstrated that stirring soil for 90 seconds in bright light increased weed seed germination up to 60%. Minimum or no-tillage agriculture is practiced for many reasons, including economic ones, and a desire to reduce soil erosion. As just emphasized, tillage, including minimum or no-tillage, affects the weed population. Any method of weed control that minimizes tillage is potentially of benefi t to soil structure. The data in Table 10.6 on ecofarming encourage minimum tillage for production of crops grown under low rainfall conditions. The point is that minimum tillage wheat and minimum tillage grain sorghum yield as well and frequently have lower production costs than more intensive tillage systems. Minimum tillage, nonirrigated corn does not yield what irrigated corn does, but production costs are lower. In vegetable fi elds in California, reduced tillage compared to conventional (more vigorous) tillage increased the density of shepherd's-purse in the top **Methods of Weed Management and Control** 283 15 cm of soil (Fennimore and Jackson, 2003). Shepherd's-purse emergence and soil seed bank densities were always lower in plots that had been organically amended (cover crops and compost). The authors suggested that organic matter additions may lead to reduced weed emergence. The extent of use and weed control implications of no- or minimum-tillage have been reviewed for developing countries (Akobundu, 1982; Buckley, 1980). It has been shown that these systems rely on herbicides and may complicate soil management due to presence of crop residues. With an abundance of weed seed in soil, the best approach may be to use minimum or no-tillage and let natural factors deplete the population of buried seed. If weed control fails one year and the soil weed seed bank has been depleted, the best strategy will be to plow deeply and then use minimal tillage thereafter (Mohler, 1993). In the fi rst year after minimum tillage begins, no tillage will have more seedlings than tillage, but in subsequent years, fewer weed seedlings will emerge unless dormancy is high or there is good survival of seed near the soil surface (Mohler, 1993). There are important advantages to minimum and no-tillage (Phillips, 1979): 1\. Soil erosion is reduced. (A primary disadvantage of tillage is the possibility of increased erosion.) 2\. Because of reduced erosion, land subject to erosion can be used more intensively. 3\. Reducing tillage saves energy. 4\. There is less compaction with decreased travel over soil. 5\. Because land is continually covered, soil moisture is not as limiting as it can be on bare soil. **TABLE 10.6. Yield and Production Costs for Different Cropping Systems in Southwest** **Nebraska (Klein, 1988).** Average yield Production cost Crop Tillage (bu/A) (\$/bu) Wheat clean fallow 37 3.88 Wheat stubble mulch 43 3.44 Wheat ecofallow-reduced tillage 45 3.30 Sorghum conventional 40 3.09 Sorghum ecofallow-reduced tillage 65 2.42 Corn conventional tillage with center-pivot irrigation 140 2.59 Corn ecofallow-reduced tillage 65 2.52 284 **Fundamentals of Weed Science** 6\. Irrigation requirements are lower because post-tillage evaporation of soil moisture is reduced. 7\. Less horsepower is required for land preparation and machinery costs can be reduced. It is generally agreed that reduction or absence of tillage increases problems with perennial weeds. Tillage may increase or decrease weed seedling density (Mohler, 1993). Some studies have found more seedlings in tilled plots, and others have found more without tillage. The effects of tillage vary between species, season, and locations. Froud-Williams et al. (1981) reviewed changes in weed fl ora associated with reduced tillage systems. They found several studies where perennial monocot and dicot species increased in the absence of tillage. They suggested that perennial monocot weeds with rhizomes or stolons would be the greatest threat to successful adoption of reduced tillage systems. Murphy et al. (2006) found over 6 years that tillage systems had a major effect on weed diversity and density. No-tillage promoted the highest (20 species), and moldboard plowing the lowest weed diversity. Chisel plowing was intermediate. The soil seed bank declined from 41,000 seeds per cubic meter of soil to 8,000 over 6 years under no-tillage. Crop yield was not affected by the tillage system. There are equally important disadvantages to reducing or eliminating tillage (Akobundu, 1982): 1\. Average soil temperature is lower, and this may delay spring planting and subsequent crop emergence. 2\. Insect and disease problems may increase because plant residues on the soil surface provide a good environment for insects and disease pathogens (Musick and Beasley, 1978; Suryatna, 1976). 3\. A greater degree of farm managerial skill may be required because: a\. Fertilizer requirements and application techniques must be changed. b\. Crop establishment may be more diffi cult because of surface residue. c\. Irrigation systems may have to be modifi ed. d\. Weed control is essential but as species change methods must change. e\. The variety of available herbicides is not great. Disadvantages have not deterred growers from learning required skills and shifting to no- or minimum-tillage. In the United States, no-till acreage increased from 10.6 to 32.9 million acres from 1972 to 1980 (Triplett, 1982) and continued to grow. Triplett (1982) suggested that 80% of US crop acreage would be planted using some form of reduced tillage and 50% of the acreage will be no-tillage. Seed burial studies (see Chapter 5) support the contention that the shift to minimum- or no-tillage systems of crop production will not eliminate the need **Methods of Weed Management and Control** 285 for weed management. The need will continue, but the weeds to be managed will change as tillage systems change. Data from seed burial studies show that as tillage is reduced, biennial weeds invade cropland, partially because their seeds survive longer when buried (Burnside et al., 1996). Other annuals, adapted to no-till, will appear in cropping systems. Federal farm programs promote conservation tillage and require maintenance of plant and residue cover on the soil surface to reduce wind and water erosion. **D. MOWING** Mowing to remove shoot growth prevents seed production and may deplete root reserves on some upright perennials. If repeated often enough, it can be used to control upright perennials in turf. Prostrate perennials such as fi eld bindweed and dandelion survive mowing. Mowing followed by application of 3.3 kg/ha of glyphosate to resprouting perennial pepperweed can enhance the weed's control (Renz and DiTomaso, 1998). A similar technique has been successful for control of other perennial weeds. Renz and DiTomasso (2004) proposed that the technique was successful because mowing changed the canopy structure of perennial pepperweed and there was greater deposition of the herbicide on basal leaves with subsequent increased translocation to roots. "The delay between mowing and re sprouting synchronized maximal belowground translocation rates with herbicide application timing." Brecke et al. (2005) showed similar results for a similar reason for control of purple nutsedge with herbicides. To maximize mowing's benefi ts, it must be done before viable seeds have been produced. Weeds should be cut in the bud stage or earlier. Table 10.7 shows the percentage of germinable seeds produced at various stages of maturity. Mowing is a useful technique but rarely accomplishes much weed control because it is done late. It removes unsightly growth and, if done at the right time, can prevent seed production, which is important in control of annuals and biennials. Its effectiveness for control of the biennial musk thistle is shown in Table 10.8. The foregoing deals with mowing performed to control weeds or clean up an area. Mowing is a normal cultural operation for some crops (e.g., turfgrass and hay) and is properly regarded as a potential weed management technique rather than solely a necessary part of producing the crop. Norris and Ayres \(1991) showed that cutting interval (but not irrigation timing after cutting) affected yellow foxtail biomass in alfalfa and alfalfa yield. Percent yellow foxtail ground cover was greatest after a 25 day cutting interval and least after a 37 day interval (Figure 10.3). Yellow foxtail biomass was also greatest for the 286 **Fundamentals of Weed Science** **TABLE 10.7. Germination of Weed Seeds from Plants at** **Three Stages of Maturity (Gill, 1938).** Cut Weed in bud Flowering Medium Ripe Ripe Annual sowthistle 0 100 100 Canada thistle 0 0 38 Cat's ear, spotted 0 0 90 Common chickweed 0 56 60 Common groundsel 0 100 100 Curly dock 0 88 84 Dandelion 0 0 91 Meadow barley 0 90 94 Shepherd's-purse 0 82 88 Soft brome 0 18 96 Corn speedwell 0 69 70 **TABLE 10.8. Seed Production by Musk Thistle** **(McCarty, 1982).** Time of harvest Seeds/plant Full bloom 26 +2 days 72 +4 days 774 Mature plant 3,580 short cutting interval and least for the longest interval. In the 3 years of the study, the 37-day cutting interval always had a higher yield than the 31- or 25-day interval (Table 10.9), thus demonstrating the utility of mowing for weed management. **E. FLOODING, SALT WATER,** **DRAINING, AND CHAINING** These techniques cause ecological change. If a normally dry area is fl ooded or a normally wet area is drained, ecological relationships are changed, and weed species will change. The techniques are effective only when an area is immersed or drained for 3 to 8 weeks. Immersion, an anaerobic treatment, is **Methods of Weed Management and Control** 287 not equally effective on all weeds; lowland or paddy rice fi elds have weeds such as barnyardgrass and junglerice that survive fl ooded conditions of the rice paddy as well as rice does. Flooding does not eliminate all weed problems, just some of them, and it creates an environment where other weeds succeed. Weeds found in lowland rice are generally different from those found in upland rice. Purple nutsedge occurs in both systems. Flooding will control established perennials such as silverleaf nightshade, camelthorn, and the knapweeds in arid areas, but the expense of creating dikes and obtaining water make the practice economically unfeasible (Slife, 1981). Reestablishment of natural fl ooding in the southwestern United States may be useful as a way to reestablish native cottonwoods. Flooding can be risky because some invasive species such as tamarisk (tamarix) can also be encouraged. Research by Sher et al. (2000) demonstrated that because native **FIGURE 10.3.** Percent yellow foxtail cover in relation to cutting frequency and duration of irrigation delay following cutting. Columns with different letters are different at P = 0.05 according to the LSD (Norris and Ayres, 1991). **TABLE 10.9. Alfalfa Dry Matter Yield in Relation to Cutting** **Interval (Norris and Ayres, 1991).** Alfalfa yield tons / acre with different cutting intervals (days) Year 25 31 37 1 10.0 12.8 14.9 2 15.0 21.7 24.0 3 11.0 16.2 20.0 288 **Fundamentals of Weed Science** cottonwoods were larger and had superior competitive ability, they dominated when historical fl ooding regimes were restored, even in the presence of an invader like tamarisk that responds well to disturbance. Ocean water with its salt content has been shown to be an effective method to control mimosa-vine and large crabgrass in seashore paspalum and bermudagrass turf on the island of Guam (Wiecko, 2003) but was less effective on yellow nutsedge. The turf was not fl ooded. Ocean water was applied as an herbicide at concentrations up to 55 dS/m (decisiemen per meter, a measure of electrical conductivity). Draining is an excellent control for cattails, bulrushes, and reed canarygrass that grow best in wet areas. Draining and fl ooding are not applicable to most agronomic or horticultural environments, but they should not be forgotten when considering weed management for appropriate sites. Chaining has been employed on rangelands to destroy emerged vegetation. A large chain similar to a ship's anchor chain is dragged between two bulldozers and uproots sagebrush, rabbitbrush, and other range weeds. Chaining removes emerged growth and completely controls annuals but not perennials that reproduce vegetatively. The technique is not suited for most cropland. Chains are also used to stop passage of weeds in irrigation channels in many countries. Removing collected weeds from the impoundment created by the chain is a labor-intensive, smelly, unpleasant operation. **IV. NONMECHANICAL METHODS** **A. HEAT** **Flaming** Many plant processes are susceptible to high-temperature disruption attributed to coagulation and denaturation of protein, increasing membrane permeability, and enzyme inactivation. Photosynthesis is decreased or stopped. Initial thermal disruption of cellular membranes is followed by dehydration. Heat, short of setting fi re to an area, usually does not kill by combustion. The thermal death point for most plant tissue is between 45° and 55°C (113° to 131°F) after prolonged exposure. Temperatures of the fl ame in a fl amer used for weed control approaches 2,000°F but fl amers may be used selectively when distance from the crop and speed are controlled. A fl amer directs a petroleum-based fuel emitted under pressure and ignited. Plant size at treatment infl uences effi cacy much more than plant density. To achieve 90% control of white mustard with one to two leaves required at least 40 kg/ha (36 lb/A) of propane, whereas plants with 2 to 4 **Methods of Weed Management and Control** 289 leaves required 70 kg/ha (62 lb/A) (Ascard, 1994). Required dose increased with growth stage, and some species of annual weeds are more tolerant than others. The most tolerant species cannot be controlled with one fl aming regardless of dose (Ascard, 1994). Weeds with unprotected meristematic areas and thin leaves such as common lambsquarters, common chickweed, and nettle were completely killed by 20 to 50 kg/ha of propane when they had less that fi ve true leaves (Ascard, 1995). Shepherd's-purse and pineapple-weed have protected growing points and were killed by fl aming only at very early growth states. Annual bluegrass could not be killed with a single fl aming regardless of its size or the propane rate. Plants with up to four true leaves were killed by 10 to 40 kg propane ha−1, whereas those with 4 to 12 leaves required 40 to 50 kg/ha (Ascard, 1995). Corn between 2 and 12 inches tall cannot withstand fl aming. Before corn is 2 inches tall its meristematic region is underground and will regenerate the plant. After 12 inches, the fl ame can be directed at the plant's base and used selectively if the weeds are shorter than the crop. Intensity and duration of exposure are important. If one held a fl ame on a corn plant for several minutes, the plant would die, so fl amers must be kept moving and speed affects selectivity. Flame has been used selectively in cotton and onions. When cotton stems are 3/16 inch in diameter or greater, fl aming can be used. Flaming kills green shoots where tillage is impractical, such as along railroad tracks. Buried weed seeds or perennial plant parts are not affected. Dry seeds withstand high temperatures and rather long exposures because soil protects and insulates. Burning can destroy weed seeds but only if they are on the soil surface. Even a small layer of soil will protect most seeds. Therefore, fl aming is effective only for controlling emerged weeds. Burning mature weeds destroys debris but doesn't prevent crop losses from competition. Flaming has no residue, a problem with chemical methods of control. Other than high rainfall conditions, fl aming is not affected by prevailing environmental conditions. It may induce erosion by eliminating vegetation that holds soil. Heat could induce germination of dormant seeds or create conditions favorable for their germination by eliminating emerged, competing plants. This is especially true when brush is burned. Controlling a fl amer's direction eliminates drift, and one can achieve some degree of insect and disease control. An additional advantage is immediate observation of results. Flaming is often used to eliminate vegetation along irrigation ditches. In spite of its advantages and proven success, fl aming is not used much in crops due to its cost and the success of other methods. The presently high cost of propane and other combustible fuels indicates fl aming is probably not economically sensible. Burning is nevertheless a valid, useful weed control method. Regular fi re has played a signifi cant role in development and stability of many ecosystems 290 **Fundamentals of Weed Science** (Hatch et al., 1991). Native plants often depend on regular fi res to reduce competition, remove thatch, scarify seeds (break dormancy), and cycle nutrients (Kyser and DiTomaso, 2002). In many grassland and forest communities, fi re is not a hazard but a necessary part of community stability. In the absence of periodic natural or planned fi res, it may be much more diffi cult and perhaps impossible to maintain grasslands in a natural state and prevent invasion of weedy species such as yellow starthistle (Kyser and DiTomaso, 2002). Burning has been combined successfully with an herbicide (clopyralid) for management of yellow starthistle in California (DiTomaso et al., 2006). The combination was most effective when burning in the fi rst year was followed by clopyralid in the second year. **Solarization and Heat** It is feasible to use the heat of the sun to control weeds in a process called solarization. Weed seed germination is suppressed by high soil temperatures and seedlings are killed. Transparent and opaque polyethylene sheets raise soil temperature above the thermal death point for most seedlings and many seeds. Solarization uses plastic sheets placed on soil moistened to fi eld capacity and thus heats soil by trapping solar radiation just as a greenhouse does (Horowitz et al., 1983). Its effectiveness for weed control is dependent on a warm, moist climate and intense radiation with long days to raise soil temperature enough to kill weed seeds and seedlings. Moisture increases soil's ability to conduct heat and sensitizes seeds to high temperatures (Horowitz, 1980). Solarization also can control soil-borne diseases and increase crop growth due to soil warming. When different types of plastic were used for four weeks in Israel, the temperatures under clear plastic exceeded 45°C. Temperatures under black plastic exceeded 40°C about half the time, but did not reach 45°C. UV-absorbing transparent plastic raised temperatures above 50°C. At 5 cm, temperatures increased 9° for black and 19° for clear plastic. The effects of solarization on weed emergence were apparent for a short time after plastic was removed. During the fi rst two months after removal, the number of emerging annuals was less than 15% of an untreated check, and clear plastic was more effi cient. Only clear plastic reduced weed populations for one year after solarization (Horowitz, 1980). Table 10.10 shows some data on the sensitivity of annual weeds to solarization. In other work, a month after solarization, fi eld bindweed, annual sowthistle, and prostrate pigweed covered 85% of the soil surface in plots not solarized compared to only 18% in solarized plots (Silveira and Borges, 1984). A oneweek period of solarization reduced the percentage of buried seeds of prickly **Methods of Weed Management and Control** 291 sida, common cocklebur, velvetleaf, and spurred anoda in soil in Mississippi (Egley, 1983). Solarization reduced emergence of all weeds except purple nutsedge. Total weed emergence was reduced 97% one week after removal of plastic and up to 77% for the season (Egley, 1983). Work in Hawaii (Miles et al., 2002) showed a different effect on purple nutsedge tubers. Five weeks of solarization with clear polyethelene fi lm raised mean soil temperature, 15 cm deep by 5.8°C in spring and 7.2°C in summer, and both increased the fi nal sprouting percentage of purple nutsedge tubers from 74 to 97% in the spring and from 97 to 100% in summer. These increases, especially only 3% in summer, may seem small, but because purple nutsedge is such an aggressive weed, complete or increased tuber germination should lead to more complete control. Solarization has been combined with a green manure crop in a study of annual bluegrass survival (Peachey et al., 2001). Clear polyethylene fi lm (0.6 mil) applied for 53 or 59 days reduced annual bluegrass 89 to 100% in the upper 5 cm of soil but did not affect survival below 5 cm and may have even enhanced it. Green manure, cover crops of barley, rapeseed, and sudangrass generally increased survival of annual bluegrass seed buried 2.5 to 15 cm deep. Combining green manure crops and solarization did not improve annual bluegrass control over solarization alone, although solarization signifi cantly improved the effi cacy of metham (a soil fumigant) for control of annual bluegrass seed. Solarization with transparent polyethylene were combined with a chicken manure mulch to study the effect on scarifi ed and nonscarifi ed fi eld dodder seed (Haidar et al., 1999). Only seeds on the soil surface were affected consistently. For scarifi ed seed, 95% germination reduction occurred after 10 days under the plastic. Chicken manure reduced the required period of solarization for nonscarifi ed seed from 6 to 12 weeks, but the effect of manure on total seed germination disappeared after 6 weeks. Solarization for 2 to 6 weeks with or without chicken manure reduced weed growth in cabbage, but manure **TABLE 10.10. The Sensitivity of Annual Weeds to** **Solarization (Horowitz et al., 1983).** Weeks of solarization to reduce seedling Weed numbers to less than 10% of control Blue pimpernel 2--4 Bull mallow \>8 Fumitory 6 Heliotrope 4 Horseweed \>8 Pigweeds 2 292 **Fundamentals of Weed Science** increased yield (Haidar and Sidahmed, 2000). Solarization with clear plastic for 60 days during tomato growth killed 95% of branched broomrape seed and induced secondary dormancy in the remaining seed (Mauromicale et al., 2005). In solarized soil, no broomrape shoots emerged and no parasitic attachment to tomato roots was detected. The authors recommended solarization as a good technique for organic farming. The major effect of high soil temperature (up to 150°F) is killing weed seedlings that germinate under the plastic. Solarization has not been employed on a large scale in fi eld crops but is used effectively in high-value vegetable crops in California's Imperial Valley. Because there is no cold winter season, solarization is used for 6 weeks before crops are planted. The plastic is removed prior to planting and must be disposed of---a problem all by itself---but solarization nearly eliminates use of herbicides. Solarization has potential to improve weed management, but costs, compared to other methods, preclude widespread adoption in other than high value crops. Research by Campbell's Soup Company in California has used solarization in a different way (Hoekstra, 1992). The previous comments related to use of plastic mulch to heat soil and kill weeds. D. Larsen of Campbell's Soup has experimented with a solar-powered lens that heats soil and kills weeds. The curved lens is an acrylic sheet made of an array of small lenses. It is cheaper and lighter than glass and not as easily damaged. Lens concentration of solar energy has two primary disadvantages: 1\. It does not work on cloudy days. 2\. The lens must be pulled slowly over the fi eld to focus energy suffi ciently to kill seedling weeds. Stronger lenses capable of concentrating more energy may enable faster movement. Steam (heated water vapor) has been used to sterilize greenhouse and nursery soil for many years. Its use has been limited in the fi eld, especially for weed control. Kolberg and Wiles (2002) studied steam as an alternative weed control method that does not have the disadvantages of herbicides and lacks environmental persistence. Emergence of a few common annual weeds was not affected, and control was similar to glyphosate. The amount of steam applied, the speed of application, the weed species, and their growth stage at application determined steam's effectiveness. **B. MULCHING** Mulching excludes light and prevents shoot growth. Wide mulches are required to control perennials that can creep to the edge of a mulch and emerge. Mulches increase soil temperature and may promote better plant growth. **Methods of Weed Management and Control** 293 Several different materials have been used for mulch, including straw, hay, manure, paper (fi rst used on sugarcane in Hawaii), and black plastic. It is common to see mulches used in greenhouses where plants grow in soil. Mulches are used most in high-value crops grown on small areas and in crops (e.g., sugarcane) where laying the mulch can be mechanized. Hartwig and Ammon (2002) reviewed the status and promise of cover crops and living mulches for vineyards, orchards, and some agronomic crops in terms of their benefi cial effects on soil erosion, nitrogen budgets, weed control, management of other pests, and the environment. Shredded paper was one of the fi rst mulches used in a crop. It has been replaced by plastic mulch but use of either is rare. Pellett and Haleba (1995) evaluated use of chopped paper in perennial nursery crops over two seasons. Their work showed that paper was an effective mulch that provided weed control over two seasons, especially when the paper was wetted and rolled after application. They applied 2.3 or 3.6 kg/m2. The higher rate was 15 cm thick. The equivalent rate per hectare was almost 38 tons, and the cost of hand application of baled paper, in Wisconsin, was over \$2,500 per hectare. The mulch provided good weed control for two years, and it was possible to rototill paper into soil with power equipment. A tackifi er (a substance to make the paper sticky) was important to prevent paper from blowing away or piling due to wind. Cost of the paper and its application prohibit consideration of use of paper mulch in any but high-value crops. As the amount of wheat straw mulch increased in a wheat-corn-fallow dryland production system, weed growth decreased (Crutchfi eld and Wicks, 1983). Others have shown that planting no-till corn into a desiccated green wheat cover crop reduced morningglory biomass 79% compared to a nonmulched, tilled treatment (Liebl et al., 1984). Rye mulch was also successful in reducing biomass of three annual broadleaved species in three crops (Liebl et al., 1984). Rye has been used successfully as a crop mulch in the fall and winter before corn (Almeida et al., 1984), a practice known as green manuring. The rye contributed to weed control in corn because of its allelopathic activity. Its foliage was dense enough so a contact herbicide had to be applied before corn planting. Penny and Neal (2003) showed that mulching helps to control mulberry weed, a new invasive weed of container nurseries and landscapes in the southeastern United States. Light stimulates mulberry weed seed germination, and mulches that prevent light penetration effectively prevented seed germination. Yellow sweetclover residues left after growth ceased provided excellent weed suppression of annual and two perennial weeds (dandelion and perennial sowthistle) in Canada (Blackshaw et al., 2001). Weed suppression was similar whether yellow sweetclover was harvested as hay after growth as a green 294 **Fundamentals of Weed Science** manure fallow replacement crop or its residues were incorporated in soil or left on the surface as a mulch. Allelopathy was possible. A mulch compost made from swine bedding material and swine manure was tested for its effects on corn (Liebman et al., 2004) and soybean (Menalled et al., 2004) yield and growth of weeds associated with each crop. The compost consistently increased corn height but had no effect on yield compared to corn grown without swine manure compost but with nitrogen fertilizer. Similarly, the compost did not increase soybean yield, but it did increase the competitiveness of common waterhemp. The authors concluded that if composted swine manure is to be used in corn or soybeans, effective weed management practices must be considered. In these cases the compost/mulch provided nitrogen fertility, which was equally benefi cial to the crop and weeds. Black polyethylene mulch was about 1.5 times more effective (72% reduction in shoots) than clear polyethylene mulch (46% reduction) for control of yellow nutsedge in Georgia. Neither mulch was effective for control of purple nutsedge (Webster, 2005), indicating a possible shift to purple nutsedge in mulched vegetable production systems. A synthetic black cloth available for mulching is sold commercially in rolls about 6 feet wide and can be applied by machine when trees are planted. It is easy to spread and prevents emergence of most annual weed seedlings. **C. SOUND AND ELECTRICITY** Use of high-frequency energy and electricity has been considered since the late 19th century. Ultra-high-frequency (UHF) fi elds are selectively toxic to plants and seeds and the fi rst use of sound for weed control was patented in 1895. UHF fi elds produce thermal and nonthermal effects, but thermal effects are the chief source of toxicity. There is a linear and positive correlation between seed water content and susceptibility to electromagnetic energy. Lower frequencies have broken seed dormancy. Commercial weed control devices using UHF fi elds have been developed, patented, and commercialized but without lasting commercial success. These have been used for selective vegetation control in cotton and for aquatic weeds but have not achieved great commercial success. They require a great deal of power but can be used preemergence or postemergence. Postemergence use forces plants to conduct current and in effect "boils" plant solutions and ruptures cell walls. Vigneault et al. (1990) reviewed what they called "electrocution" for weed control. They concluded that use of electricity may have a place in high-value, specialty crops such as fi ne herbs. It may be especially appropriate when the treated area is small, no herbicides are available, and cultivation is undesirable because of the potential for root damage and the risk of soil erosion. Advantages include lack of any chemical residue and no soil disturbance. **Methods of Weed Management and Control** 295 **D. LIGHT** Agriculture students are well aware of the role of light in seed germination and photosynthesis. It may not be as common to think of the difference in plant's light refl ectivity as an aid in weed management. Research has demonstrated that different plant surfaces refl ect light differently and that the difference can be used to differentiate weed from crop plants and to determine if weeds are present on a particular patch of ground. Optical sensing and optical refl ectance (e.g., the ratio of red to near-infrared light---650 nanometers vs. 750 nm) can be used in weed management (Shropshire et al., 1990). Machines have been developed that use optical refl ectance to determine if a weed is present and then turn on an herbicide spray. This reduces the amount of herbicide applied, saves money, and is environmentally benefi cial. **V. CULTURAL WEED CONTROL** Cultural weed management is an important part of nearly all weed management systems, even when it is not recognized. Cultural weed management techniques are especially important in crops where other weed management options are limited or not available. They should be included in weed management programs although they should not be regarded as solutions to all weed problems. Similarly, despite the outstanding success of herbicides, absolute reliance on them to solve all weed problems is economically and environmentally unfeasible (Gill et al., 1997). Gill et al. provide a complete review of nonmechanical and cultural methods of weed management. **A. CROP COMPETITION** The techniques of cultural weed control are well known to farmers and weed scientists. In fact, they are employed regularly but often are not conscious attempts to manage weeds. Planting a crop is a sure way to reduce growth because the crop interferes with the weeds. It is a fundamental method of weed management, but most often cultural weed control just happens rather than occurring as a planned addition to weed management programs. Methods of cultural weed management include conscious use of crop interference, use of cropping pattern, intercropping, soil amendments, and no or minimum tillage. Weed scientists have investigated the relative competitiveness of crop cultivars. As reported by Mohler (2001) and reviewed in Zimdahl (2004), "The role of crop genotype in weed management has received growing attention over the past 30 years." The reports indicate there has been attention but the 296 **Fundamentals of Weed Science** role of genotype has not been a major area of weed science research. As cited in Mohler (2001), Callaway (1992) reviewed the literature on crop varietal tolerance to weeds, and Callaway and Forcella (1993) examined the prospects for breeding crops for improved weed tolerance. There are differences in crop varietal tolerance (often defi ned as competitive ability) to weeds. Mohler's \(2001) Table 6.3 identifi es 25 crops in which such differences have been found. For many crops only a few reports are included, but for the major crops (barley, beans, corn, rice, soybean, and wheat) there are many reports (e.g., 14 for soybean). However, despite many years of research and several reports, few crops have been bred to be more competitive (Caton et al., 2001). The essence of the problem is that neither weed scientists nor plant breeders know what makes a plant more competitive. Several crops exhibit genotype differences in competitiveness (Burnside, 1972; Monks and Oliver, 1988). Weed biomass differences up to 45% have been reported among soybean genotypes (Rose et al., 1984). Wild oat competition with wheat was greater than intraspecifi c competition in wheat. The competitiveness of six wheat cultivars with wild oat was similar for all factors measured (Gonzalez-Ponce, 1988). The most weed-suppressive of 20 winter wheat cultivars reduced weed biomass 82% compared to the least-suppressive cultivar (Wicks et al., 1986). With weed interference, the lowest yielding varieties produced 66 and 54% of the highest yielding varieties of wheat (Ramsel and Wicks, 1988) and rice (Smith, 1974), respectively. Other work showed that short-stemmed cultivars were more affected by taller wild oats because of light competition (Wimschneider and Bacthaler, 1979). The quest to develop integrated weed management systems has encouraged research on the competitiveness of crop cultivars. That cultivars differ in competitive ability was amply demonstrated several years ago in soybeans (McWhorter and Hartwig, 1972; Table 10.11). Research in Denmark showed that spring barley varieties vary in weed-suppression ability (Christensen, 1995). Weed dry matter in the most suppressive variety was 48% lower than the mean dry matter of all varieties, whereas it was 31% higher in the least suppressive variety. More vigorous, taller, faster growing cultivars are likely to be better competitors, but too little is known about what makes a cultivar competitive and whether it is a trait that plant breeders can select for and develop. Christensen's \(1995) work demonstrated no correlation between varietal grain yields in pure stands and competitiveness, suggesting that breeding to optimize yield and competitive ability may be possible. Research is being done to develop crop cultivars that can be bred or managed for high levels of crop interference via high rates of resource uptake or possible allelopathic (see Chapter 8) interference with weeds (Jordan, 1993). Alfalfa and other hay crops are smother or cleaning crops. Land is not plowed when they are grown, making it hard for annuals to succeed, but **Methods of Weed Management and Control** 297 perennial weeds do well in perennial crops such as alfalfa. Sudangrass, planted in dense stands, can compete effectively against many, but not all, weeds. Crops can be favored by knowing and using the effect of row width and crop seeding rate. Khan et al. (1996) showed that spring wheat yields were as great or greater when early seeding or a double seeding rate was used as a substitute for a postemergence herbicide to control foxtail species. Early and middle seeding dates favored the increase of green foxtail over yellow foxtail, whereas late seeding favored yellow over green. Spring wheat competing with foxtail had a higher yield when the seeding rate was 270 kg/ha (twice the normal rate) than when it was 130 or 70 (1/2 normal rate) kg/ha unless the seeding was late. Yenish and Young (2004) demonstrated that seeding rate of winter wheat in Washington had a consistent effect on wheat yield. Yield was about 10% higher when the seeding rate was 60 as opposed to 40 seeds per meter of row when jointed goatgrass was the competing weed. Tall wheat varieties competed best. Early, high-seeding rates increase crop density and biomass early in the season and this suppresses weed growth. Seeding wheat at higher than normal rates in Alberta, Canada, improved performance of herbicides used to control wild oats (O'Donovan et al., 2006). Increasing wheat seeding rate from 75 to 150 kg/ha reduced wild oat biomass up to 18% and the soil seed bank up to 46% even when herbicides were not used. On average, wheat yield improved 19% and net economic return 16% with the higher seeding rate. Decreases in weed growth have been observed in narrow (about 8 inch) versus wide (about 30 inch) row spacing in several crops. For example weed growth was reduced 55% in peanuts (Buchanan and Hauser, 1980) and 37% in sorghum (Wiese et al., 1964). Varying row width uses the principles of plant **TABLE 10.11. Yield Reduction in Selected Soybean** **Varieties Due to Johnsongrass or Cocklebur Competition** **(McWhorter and Hartwig, 1972).** Yield reduction Soybean with weed competition from % variety Johnsongrass Cocklebur Davis 34 56 Lee 41 67 Semmes 23 53 Bragg 24 57 Jackson 30 67 Hardee 23 26 298 **Fundamentals of Weed Science** population biology to achieve competitive interactions that favor the crop. Research is proceeding in the midwestern United States to devise narrow row production techniques for soybeans. When these are combined with minimal tillage and the right herbicides, yield is maintained or increased, soil erosion is reduced, and excellent weed management is obtained. Row spacing is not always an effective weed management technique. Esbenshade et al. showed that row spacing had little effect on burcumber emergence or control in corn (2001a) and soybean (2001b). Tharp and Kells (2001) showed that corn yield was not affected by row spacing and corn population, and row spacing did not infl uence weed emergence following glufosinate application. Common lambsquarters' biomass was reduced as corn row width was reduced from 76 to 38 cm spacing. In Minnesota, narrow rows (51 vs. 76 cm) did not affect lateseason weed density, but corn grain yield increased in two of three years (Johnson and Hoverstad, 2002). Other work showed a signifi cant reduction in weed density by careful selection of early-maturing corn hybrids planted in narrow (38) versus wide (76 cm) rows (Begna et al., 2001). Combining narrow rows and high population density increased corn canopy light interception 3 to 5%, decreased light available to weeds, which produced 5 to 8 times less biomass. In contrast, Norsworthy and Oliveira (2004) suggested that increasing corn population in the row might be a more effective strategy to reduce weed competition than decreasing row width. They found light interception and the critical period for weed control were similar in narrow-row (48 cm) and widerow (97 cm) corn, and the end of season weed biomass was similar. An interesting study of the effect of soil amended with residue of the weed wild radish showed that the competitiveness of tomato and bell pepper with yellow nutsedge was enhanced by the weed residue compared to soil with no residue (Norsworthy and Meehan, 2005). This work illustrates the previously suspected but undemonstrated potential of weed residue in weed management and crop competitiveness. Intercropping is a common, small-scale farming system among farmers of the developing world. The main reasons for mixing crops or planting in close sequence are to maximize land use and reduce risk of crop failure. Intercropping maintains soil fertility, reduces erosion, and may reduce insect problems (Altieri et al., 1983). Intercropping also gives greater stability to yield over seasons and provides yield advantages over single crop agriculture (Altieri, 1984). The National Agricultural Library published a useful bibliography of citations on green manure and cover crops (MacLean, 1989). The positive and negative effects of Brassica cover cropping systems have been reviewed by Haramoto and Gallandt (2004). It is claimed (Altieri et al., 1983; Moody and Shetty, 1981) that one reason for intercropping is weed suppression, but other than work in Nigeria (Chikoye et al., 2001), there has been little experimental evidence to support this con**Methods** **of Weed Management and Control** 299 clusion (Shaw, 1982). Similarly, there is little evidence that intercropping requires less weed control. It is assumed that intercropping saves labor because weeding is less critical, and some operations such as planting a second crop and weeding the fi rst can be combined (Norman, 1973). Intercropping's effectiveness for weed control depends on the species combined, their relative proportions, and plant geometry in the fi eld. All reports recommend additional weeding with intercropping, and weeds can often be worse than in sole crops (Moody and Shetty, 1981). Successful use of interseeded cover crops in vegetables has been limited by their tendency to inadequately suppress weeds or to suppress weeds and the crop. For example, winter rye sown in broccoli was successful only when sown at high density, in locations or seasons with low soil temperatures (e.g., spring), and when combined with other weed management methods (Brainard and Bellinder, 2004). When these conditions were not met, rye was often detrimental to weed management and reduced broccoli yield. Rye sown as a cover crop in soybean reduced total weed density and biomass compared to no cover crop. However, costs were higher and the rye cover crop system was less profi table than soybean grown without a cover crop where weeds were controlled with conventional technology (Reddy, 2003). Several cover crops were compared in the moist savanna regions of Nigeria (Ekeleme et al., 2003). Weed density was negatively correlated with percent ground cover of fi ve legume cover crops. Only one, lablab (hyacinth bean), produced adequate ground cover and good weed suppression in all locations independent of varying duration, distribution, and amount of rainfall. Others were successful in high-rainfall regions. Readers must note the variation between rainfall regions. The same variation will be observed across the regions of the United States or Europe. No system will be developed that will work equally effi ciently in all regions. Other work with cover corps in Nigeria has been quite successful. For example, 12 months after planting corn, cassava, or a corn/cassava intercrop plots with cover crops had 52 to 71% less cogongrass (a hardy, diffi cult to control perennial weed) and 27 to 52% more corn grain yield at three locations in Nigeria (Chikoye et al., 2001). The cover crops were centro, copwea, hyacinth bean, egusi melon, tropical kudzu, or velvetbean all known as tropical food crops (cowpea and egusi melon) or green manure crops. Higher crop yield was a result of one or a combination of three things: reduced weed competition from the cover crop, a mulching effect that conserved soil moisture and prevented weed growth, and a contribution of nitrogen from the leguminous cover crops. It has been demonstrated that cover crops such as hairy vetch can improve corn and soybean productivity, and, when they are combined with reduced rates of environmentally benign herbicides, will minimize the requirements for herbicides (Gallagher et al., 2003). Annual intercrops can enhance weed suppression and crop production compared to sole crops. Studies in Canada with wheat-canola and 300 **Fundamentals of Weed Science** wheat-canola-pea intercropping demonstrated that intercropping tended to provide greater weed suppression compared to sole cropping; there was a synergism of weed suppression among the intercrops compared to any sole crop (Szumigalski and Van Acker, 2005). Studies of intercropping do not confi rm that any plant grown with a crop will always provide adequate weed control. Intercropping is a common practice in many agricultural systems, and these systems should be studied to develop complementary plants, control soil erosion, and prevent or reduce weed growth. It is undoubtedly true that plants that are not crops are classifi ed by most farmers in the developed world as weeds. Other farmers classify noncrop plants in a way that judges their potential use or their effects on soil and crops. Western farmers see noncrop plants as weeds, but subsistence farmers have a different understanding of the use and value of plants that are neither crop nor weed. A variation on intercropping is the intentional growth of spring-seeded smother plants for weed management. The intent is to eliminate the plants after the crop has grown and is a better competitor and before the smother plants become competitive, as intercrops often do. Berseem clover, four species of medic, and yellow mustard were planted immediately after corn and soybean planting in a 25 cm band over the crop row. All species achieved 45% or greater ground cover within 10 weeks of seeding. Yellow mustard grew most rapidly, and it and sava medic gave greater weed suppression than other species. When the medic was killed 30 days after planting, it reduced weed suppression but did not increase corn yield compared to season long presence (Buhler et al., 2001). Research on these alternative, generally nonchemical systems of weed management is continuing as environmental concerns, sustainability questions, and debate over long-term effi cacy

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