Principles of Weed Science: Soil-Applied Herbicides - Prof. Robert G. Hartzler, Study notes of Agricultural engineering

Download Principles of Weed Science: Soil-Applied Herbicides - Prof. Robert G. Hartzler and more Study notes Agricultural engineering in PDF only on Docsity! Agronomy 317 Principles of Weed Science Bob Hartzler Soil-Applied Herbicides I. Purpose A. Control emerging seedlings B. Do not affect dormant seeds C. Many have little effect on established plants II. Application timing A. Preplant – applied prior to planting crop 1. Early preplant – applied several weeks ahead of planting. a. Advantages i. Spreads workload ii. Increases likelihood that rain will move herbicide into soil profile before weed emergence. b. Disadvantages i. The earlier the herbicide is applied, the sooner it breaks down. May allow weeds to escape control before crop canopy develops. 2. Preplant incorporated a. Herbicide is mechanically incorporated with tillage b. Reduces rain requirement c. Protects from losses due to volatilization, photodegradation B. Preemergence 1. Applied prior to emergence, may refer to either crop or weed 2. Some preemergence herbicides can be applied after crop emerges, others can cause significant crop injury if applied to crop foliage III. Adsorption to soil colloids A. Factors influencing 1. Soil properties a. Clay content b. Organic matter (4-20 times more adsorptive than clay) c. Soil pH d. Soil moisture 2. Herbicide properties a. Ionic properties 1. Most herbicides are weak acids, weak bases or non-ionic 2. Herbicide adsorbs to surface of soil colloids (clay, OM) 3. Adsorption is reversible, an equilibrium between free and bound herbicide is established b. Water solubility 1. Generally less important than ionic properties 2. Because of low herbicide rates used, there generally is sufficient water in soil to dissolve herbicide 3. Partitioning coefficient (Kd) a. Ratio of herbicide bound to soil and that in water surrounding soil particles (Kd = grams herbicide bound to soil colloids / grams herbicide in soil water solution b. As Kd increases herbicide availability decreases c. Koc similar to Kd but only considers organic matter in soil, not clay B. Importance of adsorption to soil colloids 1. Limits movement of herbicide through soil profile a. Keeps herbicide in zone of weed seed germination b. Limits exposure of deep rooted crops to herbicide c. Minimizes movement into groundwater 2. Most soil-applied herbicides have recommended rates based on soil type, soils with more adsorptive sites (high clay or organic matter content) use higher rates IV. Leaching A. Downward movement through soil profile 1. Desirable for herbicide to move 1-2” into profile to depth of weed seeds 2. Movement below this detrimental due to dilution of herbicide from active zone and potential to reach groundwater B. Factors influencing leaching 1. Kd 2. Herbicide persistence – the longer the persistence of a chemical, the greater chance it has to move through the soil profile. Exposed to more water percolating through the profile. 3. Soil characteristics – leaching indirectly related to CEC 4. Rainfall C. Concerns 1. Excessive leaching can result in herbicides reaching groundwater, tile lines, etc. 2. Adsorption and degradation limit excessive movement of most herbicides 3. Factors contributing to potential problems a. Persistent herbicide b. Weakly adsorbed herbicide c. Soil with little adsorptive capacity d. Shallow groundwater e. Point sources of high herbicide concentration (spills, etc.) f. Routes to bypass soil (sinkholes, ag drainage wells, wormholes, etc.) V. Herbicide runoff A. Movement of herbicide off of target site with water 1. Ranges from <1% to 10% or more B. Closely related to erosion potential of field C. Herbicide may leave field bound to soil colloids or in solution D. First rainfall event greatest risk for significant losses 1. Herbicide on the soil surface most likely to leave field in runoff 2. If first rainfall following application is gentle and moves herbicide from the surface into the profile, runoff losses for remainder of year generally low 3. If first rainfall event creates runoff, significant losses can occur E. Conservation tillage, grass waterways, and other practices that reduce erosion and water runoff limit off-target movement of herbicides VI. Volatilization A. Movement of herbicide in gaseous form B. Directly related to herbicide vapor pressure 1) Most herbicide have v.p. sufficiently low to minimize losses C. Losses increase with temperature D. Losses vary with surface characteristics and moisture E. Herbicide formulation can influence potential for volatilization