Chemical Weed Management

Transcription

1 Chemical Weed Management 1

2 Chemical Weed Management By the end of this lesson, you should know the definition of herbicide be familiar with the history of chemical weed management know several properties of an ideal herbicide explain the advantages and disadvantages to chemical weed management be familiar with herbicide nomenclature understand several herbicide classification systems understand the limits of herbicide use in organic production. 2

3 Chemical Weed Management A herbicide is a chemical used to: control, suppress, or kill plants, or interrupt their normal growth processes Almost 60% of all pesticides applied are herbicides Over 90% of US cropland has been sprayed with herbicides annually for the last 30 years 3

4 History of Chemical Weed Management The use of chemicals to control weeds was documented by philosophers in ancient Greece. Early chemicals Hemlock juice and lupine seeds 1890s copper salts 1902 sodium arsenate 1910 petroleum oils 1920 borate and sodium chlorate 1930 sulfuric acid 4

5 History of Chemical Weed Management The first synthetic chemical for selective weed management was dinoseb, introduced in The era of modern chemical weed management begins essentially in the 1940s with the discovery of 2,4-D

6 The Ideal Properties of a Herbicide Applied in small amounts Applied in many different ways (tank mixes, with fertilizer, etc.) Easy to apply Effective in many environments High selectivity Low volatility Produces minimal site disturbance and impact beyond that intended Rapidly biodegraded (short half-life) Profitable Low soil mobility 6

7 The Ideal Properties of a Herbicide Low mammalian toxicity Low toxicity of adjuvants and inert ingredients Low toxicity of byproducts Do not cause cancer 7

8 Advantages of Herbicides Herbicides reduce the need for human labor. Herbicides control weeds when other measures cannot. Preemergence applications control weeds before they emerge, and residual herbicides control weeds late into the season. Herbicides control perennial and parasitic weeds when other measures are not effective. Herbicides kill weeds without permitting their dissemination. 9

9 Advantages of Herbicides Herbicides can be used in situations where tillage is neither possible nor desired. Cultivation requires: more fuel may injure crop plants increases the potential for erosion compared to the application of herbicides Herbicides can be used when the physical condition of the soil does not permit manual weeding Herbicides are often effective in only one application 10

10 Disadvantages of Herbicides Herbicide applicators must be current on all licensing requirements, training, and fees. Herbicides can pose health risks to humans and the environment. Volatile herbicides may injure off target plants. Crop injury may occur. Glyphosate drift injury on corn. Photo courtesy of Bill Johnson, University of Missouri. 11

11 Disadvantages of Herbicides Some herbicides may persist in the soil longer than anticipated. Some herbicides have an extremely long half-life even under ideal weather conditions. Both scenarios limit crop rotation options. Herbicide resistance may evolve with the repeated use of the same herbicide or herbicides with the same site of action. Russian thistle (Salsola iberica) is one the most widespread resistant weeds in Oregon. 12

12 Understanding Herbicide Nomenclature A herbicide will have only one chemical and one common name, but it may have more than one trade name. The chemical name describes the chemical structure of the compound. The common name is usually a simplified version of the chemical name. The trade name is assigned by the manufacturer; you will often see a TM or symbol next to the trade name. 13

13 Herbicide Nomenclature An example Chemical name N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine Common name pendimethalin Trade names Pentagon DG, Prowl, Pursuit Plus, Squadron, Pendulum, Corral and numerous others pendimethalin chemical structure 14

14 Herbicide Classification Herbicides can be classified or categorized in many different ways. Classifications can be based on: when the herbicide is applied what the herbicide controls the chemical description of the herbicide molecule. There is no single, universal method for classifying herbicides. 15

15 Application Timing An example of herbicide classification Preplant the herbicide is applied to the soil surface before seeding or transplanting. Preemergence the herbicide is applied prior to emergence of either the crop or the weed, and not necessarily both. Usually refers to after planting and before crop emergence. In established crops prior to weed emergence. Postemergence the herbicide is applied after the crop or a specific weed has emerged. Can be further divided into early, late, and directed treatments. 16

16 Application Type An example of herbicide classification Applied broadcast over the entire area Applied in bands with the crop rows Applied as spot treatments 17

17 Selectivity An example of herbicide classification Herbicide selectivity is the ability of a herbicide to kill certain plants without injury to others. Non-selective herbicides kill all plants when applied at the labeled rate. Selectivity is relative and not absolute. 18

18 Selectivity An example of herbicide classification Selectivity can be accomplished also by physical methods. Carbon seeding is most often done during the fall seeding of perennial ryegrass. a slurry of activated carbon and water is applied over the seed during drilling in a one-inch wide band. a non-selective herbicide, such as diuron, is applied over the seedbed. The germinating crop seed is protected from the herbicide by the activated carbon in the charcoal band Weeds that emerge outside this band are unprotected and die. 19

19 An example of carbon seeding

20 A close-up of the carbon-seeding band

21 Selectivity An example of herbicide classification Selectivity can be accomplished by physical methods. The seed of both plants is at the same depth. The coleoptilar node is the primary site of entry for some soilapplied herbicides. Wheat is protected from the herbicide, because its coleoptilar node is below the treated layer. Wild oat is killed by the herbicide. 22

22 (Fryer and Evans 1968)

23 Contact vs. Systemic An example of herbicide classification Contact herbicides do not move within the plant. must have good coverage for these herbicides to work. contact herbicides are generally most effective on annual plants. An example of a contact herbicide is Gramoxone, or paraquat. Paraquat is effective only where the droplets land. This photo shows damage on soybean leaves. Photo courtesy of the University of Missouri. 24

24 Contact vs. Systemic An example of herbicide classification Systemic herbicides move inside the plant in the symplast, the apoplast, or both. Systemic activity is synonymous with translocation. Herbicides that enter the symplast move acropetally and basipetally. 25

25 Contact vs. Systemic An example of herbicide classification Herbicides that enter the apoplast only move acropetally. Systemic herbicides are required for control of perennial weeds. An example of a systemic herbicide is or glyphosate. An example of herbicide movement via the apoplast. The dots represent herbicide application to the soil and foliage. The arrows indicate acropetal translocation. 26

26 Persistence An example of herbicide classification The half-life (T 1/2 ) of a herbicide is used to predict soil persistence. The half-life estimates the time it takes in days for 50% of the herbicide to degrade. Some persistence is necessary for residual weed management. Too much persistence limits crop rotations or results in crop injury. 27

27 Persistence An example of herbicide classification Some commonly used herbicides and their half-lives: No to Low Persistence Herbicide Application Half-life Carfentrazone (Aim) post less than 1 day Eptc (Eptam) preplant 6 days 2,4-D (several) post 10 days Fluroxypyr (Vista) post days Dicamba (Banvel) pre or post less than 14 days Imazamox (Beyond) post days Moderate to High Persistence Herbicide Application Half-life Atrazine (several) pre or post 60 days Oxydiazon (Surflan) pre 60 days Metolachlor (Dual) pre 90 days Picloram (Tordon) post 90 days Prodiamine (Barricade) pre 120 days Diquat (Diquat) post 1000 days 28

28 Chemical Family An example of herbicide classification There are often several herbicides that are structurally related, i.e., they have the same core molecule. These herbicides make up a herbicide family. These herbicides may also have the same mechanism of action. However, there are exceptions. Herbicides within a chemical family may vary greatly also in selectivity or persistence. 29

29 Example: The Imidazolinone Herbicide Family imazapic imazapyr imazaquin imazamox imazethapyr imazamethabenz The molecule core is highlighted in green. 30

30 Mode of Action An example of herbicide classification The entire sequence of events, from first contact with the plant to plant death, is referred to as the herbicide s mode of action. The names describe the major effects that herbicides have on plants simplified into two or three words. 31

31 Mode of Action An example of herbicide classification The eight modes of action are: Amino Acid Synthesis Inhibitors Cell Membrane Disruption Growth Regulators Lipid Synthesis Inhibition Photosynthesis Inhibition Pigment Inhibition Seedling Growth Inhibition Unknown 32

32 Site of Action or Group Designation An example of herbicide classification The site of action is the specific biochemical pathway a particular herbicide acts upon in a plant. Currently, there are over 20 sites of action. The classification system: easily and readily known, recommendations for resistance management herbicide rotation would be simplified 33

33 Site of Action or Group Designation An example of herbicide classification Herbicides with the same site of action were assigned a group number. These numbers are listed on some herbicide labels. Examples The triazinone family (Group 5) The oxyacetamide family (Group 15) 34

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35 Herbicides in Organic Production Weed management on organic farms consists of cultural and mechanical techniques such as crop rotation, mulching, tillage, cultivation, water management, and manual weeding. There are a few natural substances that are used also to manage weeds, but the efficacy of these substances is still questioned. Swezey, S. L., B. Baker, D. Chaney, S. Guldan and D. Granatstein Organic Farming Compliance Handbook: A Resource Guide for Western Region Agricultural Professionals. Davis, CA: UC Agriculture and Natural Resources, Sustainable Agriculture Research and Education Program 36

36 Herbicides Approved for Organic Production The Organic Materials Review Institute (OMRI) is a national nonprofit organization that helps determine which products are allowed for use in organic production. OMRI approved products may be used on operations that are certified organic under the USDA National Organic Program. Weed ZapTM by JH Biotech 30% cinnamon oil (a nonselective herbicide) 37

37 Chemical Weed Management This lesson provided information on the definition of herbicide history of chemical weed management, several properties of an ideal herbicide, the advantages and disadvantages to chemical weed management, herbicide nomenclature several herbicide classification systems the limits of herbicide use in organic production. 38