Herbicides and Plants PDF

**Herbicides and Plants** **437** **FUNDAMENTAL CONCEPTS** There are several environmental, chemical, and physiological factors that affect an herbicide's activity and selectivity. The most important determinants of herbicide selectivity are the rate of absorption and the amount absorbed,...

**Herbicides and Plants** **437** **FUNDAMENTAL CONCEPTS** There are several environmental, chemical, and physiological factors that affect an herbicide's activity and selectivity. The most important determinants of herbicide selectivity are the rate of absorption and the amount absorbed, translocated, and metabolized by two species. Several plant and environmental factors interact to determine selectivity. Sprayer calibration is one of the most important and neglected aspects of herbicide application. Forward speed, pressure, and nozzle tip orifi ce size are the primary things that can be adjusted to change a sprayer's calibration. **LEARNING OBJECTIVES** To understand the difference between herbicide drift and volatility and the importance of each. To know techniques to control drift and volatility. To understand the fundamental importance of sprayer calibration. To know the external factors that infl uence spray retention and herbicide absorption. To know the effect of moisture, temperature, and light on herbicide action. To know the relative advantages and disadvantages of foliar and soil-applied herbicides. To understand the difference between shoot and root absorption of herbicides. To understand the role of absorption, translocation, and metabolism as determinants of selectivity. *Fundamentals of Weed Science* Copyright © 2007 by Academic Press, Inc. All rights of reproduction in any form reserved. 438 **Fundamentals of Weed Science** **I. FACTORS AFFECTING** **HERBICIDE PERFORMANCE** This discussion of factors affecting herbicide performance in plants assumes that users have an applicator appropriate to the task and that it has been calibrated to apply the correct volume and the proper amount of active ingredient per acre. The discussion also assumes that the correct herbicide has been selected and that it will be applied at the right time. If these things are not ensured, they will nearly always negatively affect herbicide performance and environmental quality, but because human errors and their results are not precisely predictable (we can't plan our accidents), the discussion herein assumes human error has been avoided. This chapter discusses factors that affect performance from the time an herbicide molecule leaves the applicator (usually this means the nozzle tip) until it hits a plant target and acts. **II. GENERAL** **A. SPRAYER CALIBRATION** It is important to understand the equipment required to apply herbicides properly. Although size and reliability of equipment have changed, it remains basically the same (McWhorter and Gebhardt, 1987). More than 90% of all herbicides are still applied with hydraulic sprayers that have the same four basic components: a tank, pressure regulator, pump, and spray nozzles. The conventional hydraulic sprayer continues to be the most acceptable and most widely used method of herbicide application whether herbicides are applied to plants or soil. Great advances in herbicides and formulations have been made, but while application technology has improved, it has not advanced at the same pace. Most herbicides are still broadcast as an aqueous mixture from a hydraulic sprayer that uses simple nozzles to break the pressurized liquid stream into droplets. As long as the fuel, the herbicide, and the farmer's time were inexpensive and environmental contamination was a minor concern, a cheap method of herbicide application was appropriate. These conditions have changed, and more effort is now being expended to improve herbicide application. In most cases herbicides are applied as broadcast sprays to an entire area, whether the area is an entire fi eld or a band over the crop row. Not all of the area sprayed may have weeds, but it is all sprayed. This means that herbicide is commonly applied where there are no weeds. This, while **Herbicides and Plants** 439 appearing ineffi cient, has been effi cient because it has been easier and less expensive to spray an entire area and the technology to spray just the weeds has not been available. There has been no way to detect each weed. Weed scientists know that weeds usually exist in patches in a fi eld, not as uniform stands, and spraying the entire fi eld is not necessary. Recent research (Felton, 1990) makes it possible to apply one herbicide to one species and another herbicide to a second species in one pass across a fi eld. Weed species are detected because the leaf tissue of each species differs in refl ectance. Microprocessors turn the sprayer on only when weeds are sensed. The system reduces total spray, herbicide use and cost, doesn't waste herbicide, reduces environmental presence, and reduces the likelihood of off-target movement and nontarget effects. When morphological and foliar refl ectance characteristics of different species are incorporated, specifi c weed control will be possible. These advances combined with global positioning system (GPS) technology will allow an applicator to know and the machine to remember where species are and be very precise with herbicide application. Several years ago, there was great interest in controlled droplet applicators (CDA technology), but that has waned. In principle CDA technology produces droplets over a narrow size range. The principle holds for low-volume applications, and CDAs are quite effective for drift reduction (see the following section on drift). Herbicides can be, but usually are not, applied as granules with applicators capable of being calibrated. Granule application can often be combined easily with crop planting. Because of its exclusive foliar activity, glyphosate led to the development of wiper application. Wipers could be nylon ropes that act as wicks but do not drip the herbicide on nontarget species. Weeds that emerge above a crop canopy receive a lethal dose of glyphosate when wiped by the rope. Shag carpet--covered rollers have been used, but they were replaced by rope wicks. Both technologies are now rarely used and are primarily of historical interest. Each kind of herbicide applicator can be calibrated with the same basic technique. The applicator is driven over a known area, and output is measured, or output is measured for a certain time with the applicator stationary. Special devices are available to assist with calibration by direct reading during spraying or while stationary. No technique is diffi cult or complex but each takes time before herbicide application. Even with sophisticated, specialized knowledge of herbicide chemistry, mechanism of action, application timing, rate of application, selectivity, and activity, herbicides may fail to control the weeds they should control, achieve desired crop selectivity, and may leave undesirable environmental residues. A major reason for failure is not a lack of knowledge about how the herbicide 440 **Fundamentals of Weed Science** acts, but rather that herbicides are frequently not applied properly. A Nebraska study (Reichenberger, 1980) found that two of every three pesticide applicators made application errors due to inaccurate calibration, incorrect mixing, worn equipment, or failure to read and understand the product label. These mistakes caused over- and underapplication and cost farmers between \$2 and \$12 per acre in added chemical expense, potential crop damage, and lost weed control. When results were extrapolated to the entire United States, a billiondollar application blunder was made each year. Other studies of farmer's sprayers have shown similar problems (Ozkan, 1987). It is not totally inaccurate to say that a major problem with agricultural chemicals is the people who apply them. In spite of all the specialized research and technology required to develop and market an herbicide, the end result is often dependent on decisions made by a user, just prior to use. These quick decisions are frequently wrong. The reason more accidents haven't occurred is that herbicides are developed to be reasonably foolproof, but they are not completely so; all mistakes are not tolerable. Because of application blunders and concern for human and environmental safety, government regulation of herbicides has increased. No legislative body can enforce a law against stupidity, but all can pass laws that make penalties for stupidity greater and encourage use of reasonable intelligence. Such laws become more likely when reasonable intelligence is not the norm. The metallic salts, the fi rst selective herbicides, were applied at 100+ lbs/A in at least 100 gallons of water per acre. Some may also have been applied in relatively low volume by brushing or wiping (Gebhardt and McWhorter, 1987). Invention of the compressed air sprayer in the early 1900s improved application (Gebhardt and McWhorter, 1987) but didn't reduce the amount of herbicide required for weed control. Early weed sprayers were high-volume sprayers with wooden tanks. Later sprayers, capable of applying lower volumes, had steel tanks. As mentioned, the fi rst sprayers and modern sprayers have basically the same parts: a tank, pressure regulator, pump, and nozzles. Today, 90% of all herbicides are applied with low-pressure ground sprayers drawn by a tractor (Felton, 1990). Herbicides are also sprayed by airplane and with large, self-propelled ground implements. Spraying may be followed by soil incorporation to reduce or control volatility, put the herbicide in position to maximize plant uptake, and promote control of emerging seedlings, or root uptake. Failure to incorporate well is a frequent reason for poor herbicide performance. Power rototillers are the best incorporation implements but are not used on most farms. Disking is probably the most common incorporation technique and works best if done twice with the second pass at right angles to the fi rst. A single disking produces zones of high herbicide concentration and other areas with virtually no herbicide because of the tendency of the disk to ridge soil. **Herbicides and Plants** 441 Herbicides can be applied by injection into water fl owing in furrows or ditches and through sprinklers. This technique, called herbigation, is effective for herbicides taken up by plant roots from soil, but is not effective for all herbicides. **B. REACHING THE TARGET PLANT** **Drift** Spray drift is movement of airborne liquid spray particles. It is often unseen and may be unavoidable. It can be minimized. Drift increases with wind speed and the height above the ground at which drops are released and decreases as spray droplet size increases. Ideally, uniform drops between 500 microns (moderate rain) and 1 mm (1,000 microns = heavy rain) in diameter are desired. Drops of this size minimize, but do not eliminate, drift, especially if spraying is done when wind speed is less than 5 mph. It is not uncommon, especially in arid environments, for water to evaporate within 200 to 300 feet of the point of delivery, so only the herbicide and associated organic solvents remain to drift. Table 14.1 shows spray droplet size, droplet lifetime, and the potential effect on drift (Brooks, 1947; Hartley and Graham-Bryce, 1980). For comparison, a number 2 pencil lead is about 2,000 microns, a paper clip is 850, a toothbrush bristle 300, and a human hair is about 100 microns in diameter. Nozzle tips give pattern to sprays and break up the liquid stream into small particles. Hydraulic nozzles produce a range of droplet sizes rather than just one. Droplet size is a function of orifi ce size, operating pressure, and surface tension of the spray solution. Smaller nozzle orifi ces, higher pressures, and lower surface tensions produce more small drops. All hydraulic nozzles produce a normal (Figure 14.1) distribution of spray drop sizes. As size decreases and pressure increases a greater percentage of small droplets is produced. The infl uence of wind on droplets is illustrated in Table 14.1. Small droplets will drift a long way in a light breeze. Large drops decrease drift problems. Spraying in strong wind should be avoided, but it is diffi cult when large areas must be sprayed with herbicides that require application at particular growth stages or before crop emergence. Farmers and other applicators must apply herbicides at the proper time. However, if other considerations take precedence over drift avoidance, problems may ensue when the applicator's or a neighbor's crops are injured or the environment is contaminated by improper application. Sprayer boom height is normally fi xed. However, as illustrated (Table 14.1), release height infl uences drift potential simply by allowing drops to remain suspended longer. 442 **Fundamentals of Weed Science** **TABLE 14.1. The Effect of Spray Droplet Size on Evaporation and Drift.** Evaporating water Droplet Lifetime fall Time to Distance traveled diameter while falling 10 feet Type of droplet Precipitation Drops Drop life distance fail 10 feet (microns) (in./hr) (No./in. in a 3 mph wind 2) (sec) (in.) (sec) 5 Dry fog 0.04 9,220,000 0.04 \