Management systems that include reduced tillage and cover crops are gaining popularity. These practices typically increase plant residues at the soil surface and organic matter in the surface soil. In turn, microbial activity is increased, and the soil develops a greater capacity to adsorb and retain many types of farm chemicals, including herbicides. Accordingly, tillage and cover crops variously affect the degradation of herbicides and their movement with surface water runoff and internal drainage.
Herbicide degradation in soil is controlled by the interaction of several factors. These include non-biological properties (such as texture, organic matter and pH) and conditions (moisture and temperature) and biological conditions (numbers, types and activities of microorganisms). The main non-biological effect of tillage on herbicide degradation is probably increased herbicide adsorption onto soil solids because of increased soil organic matter with reduced tillage. This has been shown for several different cotton herbicides including Bladex (active ingredient, cyanazine), Cotoran (fluometuron), Prowl (pendimethalin) and Zorial (norflurazon), among others.
In some cases, increased adsorption of the herbicide in reduced tillage soils has led to its slower degradation and longer persistence, despite typically higher microbial populations and activities. But when the effect of adsorption on reducing the concentration of herbicide susceptible to microbial degradation is factored out, it appears that degradation would actually be faster in the reduced tillage soils. These calculations suggest that reduced tillage usually increases the rate of herbicide degradation. But from the practical standpoint of how much herbicide actually remains over time, there is no clear, general conclusion on how tillage affects herbicide degradation rates.
As with reduced tillage, planted cover crops increase soil organic matter and microbial numbers and activities. Comparison of herbicide adsorption in conventionally tilled soil with and without cover crops has shown that buildup of soil organic matter because of the cover crop increases adsorption. But the differences in herbicide adsorption in reduced tillage soil because of the cover crop tend to be smaller since levels of organic matter are higher.
Given the ambiguous results for effect of tillage on herbicide degradation, it is not surprising that there is no clear trend for effect of cover crop. This is particularly true for reduced tillage soil with cover crops. In some cases, increased herbicide adsorption caused by increased soil organic matter has led to slower degradation in soil planted with cover crops than soil without a cover crop. Whether this occurs may depend on the type of cover crop and the biological and chemical properties of its residue. For example, while there was no difference in the degradation rate of Prowl in no-till Gigger silt loam soil with volunteer native annuals or a wheat cover crop, degradation of Prowl was noticeably slower with a hairy vetch cover crop.
Besides effect of cover crop on herbicide degradation in soil, interception of spray-applied herbicide by crop and cover crop residue in reduced tillage systems may affect herbicide degradation and weed control. For example, Cotoran is relatively easily washed off cover crop residue compared to Prowl. Consequently, interception of Cotoran by a dense layer of vetch residue may have a negligible effect on herbicidal efficacy. On the other hand, since Prowl tends to be retained by residue, little Prowl that is intercepted by plant residue may ever be transferred to the soil by rainfall. Instead, it is degraded by biological and chemical processes within the residue.