Crop 221: Weed Control Lecture Notes PDF

1/23/2025 OBJECTIVES At the end of this lecture, you should be able to: state the basic principles of weed control...

1/23/2025 OBJECTIVES At the end of this lecture, you should be able to: state the basic principles of weed control suggest the most suitable method of weed control in a particular crop and locality identify suitable biological methods of controlling certain weeds. CROP 221: WEED CONTROL Develop an integrated system of managing weeds Daniel Brain Akakpo, PhD DEFINITION OF WEED AND WEED Classification of Weeds INTERFERENCE Weeds are classified in several ways that include life cycle, habitat, growth habit, Weeds are plants that grow spontaneously without having been planted. degree of undesirability or noxiousness, morphology and by taxonomy or scientific A plant is considered a weed when it interferes with human activity. classification. This interference occurs in three ways: Based on Life cycle i. Production of allelochemicals (noxious Annual weeds species) Biennial weeds ii. Competition (water, nutrient, light, space) Perennial weeds iii. Parasitism (total/partial) 1  1/23/2025 Losses caused by weeds Classification of Weeds Other classifications include: Weeds may cause losses in several ways either i. Common weeds: these are annuals, direct or indirect losses: biennials, or simple perennials that are readily controlled by ordinary good farming Direct losses: practices. i. Decrease in crop yield ii. Impairment of crop quality ii. Noxious weeds: These are weeds which are iii. Harboring of plant pest and diseases difficult to control because of an extensive iv. Weeds interfere with harvest operation perennial root system or because of other characteristics that make them persistent. Beneficial Effects of Weeds Losses caused by weeds i. Weeds provide a vegetative cover that protects the soil Indirect losses surface against soil erosion by rain or wind. ii. Weeds play an important role in nutrient recycling. i. Weeds serve as alternative host for many plant diseases and pests iii. Weeds add organic matter to the soil. iv. Many weeds are used directly as food for humans ii. The presence of weeds imposes a limit on farm v. Weeds play an important role as a source of drugs size. used in public health. iii. Weeds can reduce the economic value of lakes vi. Weeds provide food and cover for animals. by limiting fishing activities vii. Weeds serve as important source of genetic materials iv. Weeds become fire hazards in the dry season. for crop improvement. viii. Weeds serve as host to beneficial insects ix. Many weed species help to beautify the landscape 2  1/23/2025 ROGUING ROGUING CONT’D This refers to the act of identifying and removing plants with undesirable For example, to ensure that the crop retains characteristics from agricultural fields. its integrity as regards certain physical Rogues are removed from the fields to preserve attributes, such as colour and shape, the quality of the crop being grown. That is, the individual plants that exhibit differing traits plants being removed may be diseased, be of are removed. an unwanted variety, or be undesirable for Roguing is particularly important when other reasons (e.g. volunteer crop). growing seed crops to prevent plants with undesirable characteristics from propagating into subsequent generations Importance of Weed Control Weed Control, Weeds are plants that thrive under extreme Eradication & conditions but excel under favourable conditions and can easily suppress plant growth when not Management controlled It is therefore important that weeds are Weed control controlled in crop production systems, especially Refers to those actions nurseries that seek to restrict the spread of weeds and Weeds also serve as alternative hosts and hence destroy or reduce their control even when no crop production is on going population in a given is important (fallow management). location. 3  1/23/2025 Weed Control, Eradication & Weed Control, Management Eradication & Weed Eradication Management This refers to the complete removal of all weeds and their Propagules from a habitat. Weed management Weed eradication is difficult to achieve and is This is the ability to uneconomical in most situations. manipulate weeds so that they do not seriously interfere with the growth, development and economic yield of crops and animals. Principles of Weed Control Principles of Weed Control Cont’d The weed should be killed before it has a The cost of controlling weeds on a crop should chance to produce seeds. For this reason, land be weighed against the expected loss in yield that is left fallow should be ploughed at and quality if the weeds are not controlled. intervals in order to kill weeds before they produce seeds. Crops should be properly spaced so that when Weeding should occur before the crop begins the crops are fully grown the ground is to suffer from the competition of the weeds. completely covered, thus depriving the weeds Perennial weeds should be attacked at the of sunlight, therefore can only grow slowly. beginning of the rainy season when the amount of reserved food present in them is minimal. 4  1/23/2025 Methods of Weed Control/Mgt PREVENTIVE METHOD It is a method which minimizes the introduction i. Preventive method of new weeds into an area and prevention of the increase/build-up of already existing weeds. ii. Cultural method It includes all sanitation measures that should iii. Chemical method be routinely used on the farm, including the vigilance that keeps a farmer alert to the presence of a strange plant on his farmland. iv. Biological method PREVENTIVE METHODS Biological Method This refers to the control or suppression of weeds Fallowing by the action of one or more organisms, through natural means or by manipulation of the weed, Preventing weeds from setting seeds organism, or the environment. Examples are: Cleaning major tillage implements after each Vertebrate animals activity Microorganisms such as plant pathogens Using clean planting material from a reputable (microbial control) source Live mulch Allelopathy Plant canopy 5  1/23/2025 Live mulch crop production aims at Live Mulch the following Live mulch is defined as a crop production system in which a - Suppresses weeds food crop is planted directly in - Reduces weed seeds population in the soil the living cover of an established cover crop without destruction - Reduces loss of soil organic matter of the fallow (cover crop) vegetation. - Provide favourable conditions for earthworm Perennial legume cover crops activity have been evaluated and found - Protect the soil from erosion to be suitable for use as live mulch. - Reduces soil compaction - As additional fodder for livestock Advantages of using live mulch Cultural Methods Cultural weed control methods include all aspects of good crop husbandry used to minimise weed interference - Reduces the need to control weeds after with the crop. These consist of the following: harvest. - It prevents the establishment of those weeds Hand weeding (hoeing, slashing, picking/pulling) that colonise fallow land. Mechanical weeding (Tractors, animals) Tillage (zero, primary and secondary) Mulching (plant debris, plastic) Burning Flooding Crop rotation. 6  1/23/2025 Flame Weeding Chemical Methods Chemicals that are used for killing or adversely affecting plant growth are known as herbicides. The practice by which weeds are killed with herbicides is called chemical weed control Mulching Criteria used to classify herbicides - The time when they are normally applied. - Whether they are selective or not. - Whether they normally act through the shoot or the root. Classification of herbicides based Classification of herbicide based on on time of application type of plants killed There are generally three distinct times when All herbicides kill plants either selectively or herbicide may be applied. non-selectively: i. Pre-planting application i. Nonselective herbicides are those herbicides ii. A pre-emergence herbicide application that exert toxic effects on all plants that may iii. A post emergence herbicide application come in contact with them. Examples include diquat, glyphosate, paraquat and sodium chlorate. 7  1/23/2025 Classification of herbicide based on Classification of herbicides based on type of plants killed movement in plants ii. Selective herbicides are those i. Contact herbicides: These are herbicides that that will preferentially kill certain kill the tissues they touch. Contact herbicides plants species at recommended commonly in used in the tropics include the rates but will not harm other following: paraquat, propanil, oryzalin and plants that they come in contact diquat. with. Example: 2,4-D, diclofopmethyl, fluometuron and metolachor iii. Soil-acting herbicides: Classification of herbicides based Are those which act primarily in the soil. They on movement in plants are usually applied to the soil where they retard or ii. Systemic herbicides: These are those herbicides inhibit the germination of that are transported in the xylem or phloem weed seeds. vessels of the treated plants. Such herbicides usually Examples of systemic herbicides are atrazine, have long residual action so that they can prevent dalapon and glyphosate. the growth of weeds for a Systemic herbicides are particularly useful in substantial part of the cropping season. controlling perennial weeds because underground perennating organs and roots are killed in addition to the shoot. 8  1/23/2025 CHEMICAL SPRAYING EQUIPMENT INTEGRATED WEED MANAGEMENT A combination of any two or more of the major methods of controlling weeds is referred to as integrated weed management (IWM) Advantages of IWM Inability of any one method of weed control to completely solve the weed problem of a given crop at all times and without adverse effects. The ability of weeds to develop resistance to herbicides that is frequently used. The End Broad spectrum weed control Long lasting effect of control Prevention of environmental pollution Overall cost-effectiveness 9