Location: Crop Production Systems Research2012 Annual Report
1a. Objectives (from AD-416):
Objective 1: Develop management practices to minimize pesticide fate and transport in Mid-South cropping systems. Objective 2: Develop soil management practices within-field and at edge-of-field to minimize the transport, survival and reinfestation of the field by herbicide resistant weed seeds.
1b. Approach (from AD-416):
Pesticide persistence in soil and soil sorption estimates are integral components of fate, transport and risk assessment models, which are utilized by Environmental Protection Agency (EPA), industry, National Resource Conservation Service (NRCS), and others to make pesticide use decisions. Sensitivity analysis, that is, investigations into how model performance varies along with changes in the key assumptions on which the model projections are based, indicates that inaccurate persistence and sorption estimates adversely affect model predictions; thus, pesticides may act differently than expected depending on many factors. Accurately predicting pesticide behavior increases the probability for successful pest control while simultaneously protecting the environment. Currently, modelers use default input persistence and sorption estimates to predict pesticide transport, and these default input estimates are based on scant data from a few locations scattered across the United States. For example, data for glyphosate, the most widely used herbicide in the United States, is negligible, with dissipation kinetics available for only six soils within our borders. Dissipation and sorption data for herbicides that will augment and/or supplant the Roundup Ready™ system, e.g., glufosinate, dicamba, 2,4-D and mesotrione, are equally scarce, particularly for the Mid-South. Thus, the objective of this project is to determine glyphosate, glufosinate and perhaps dicamba, 2,4-D and mesotrione fate, i.e., sorption, mineralization, dissipation, and bound residue formation, in Mid-South soils under various crop production systems that vary widely in physiochemical properties. Regression analysis will be used to construct regional sorption, mineralization, dissipation and bound residue formation models that more accurately predict the behavior of these pesticides in the environment. In addition, weed managers will utilize our fate and transport expertise to begin addressing resistant weed seed transport in Mid-South soils.
3. Progress Report:
There are two vacancies in this project: Soil Microbiologist since November 1, 2010, and Soil scientist since May 1, 2012. Due to vacancies milestones could not be met. In the last 60-months considerable work focused on enhanced atrazine degradation. ARS scientists group developed two diagnostic assays to confirm enhanced atrazine degradation. Models were developed that accurately predicted symetrical-triazine persistence under field conditions. Scientists demonstrated that genes coding for enzymes able to rapidly degrade s-triazine herbicides are widespread in United States agricultural soils, and that enhanced s-triazine degradation can occur in any soil with a pH greater than 5.1. ARS scientists demonstrated that parameters used by regulatory agencies to model atrazine fate and transport are significantly different than those observed in adapted soils and that regulatory agencies were over predicting transport by 13-fold. Scientists confirmed reduced residual weed control with s-triazine herbicides in adapted soils and cross-adaptation among s-triazines. Scientists also confirmed that crop rotation and s-triazine exclusion from the weed control program slightly reduces atrazine degrader numbers and activity, but levels do not return to baseline during the time frame evaluated. Scientists demonstrated that glyphosate off-site transport potential is minimal, regardless of tillage, and that non-target glyphosate effects on soil chemical properties, microbial community structure and function are minimal and transient. Our work on in-field Best Management Practices indicate that narrow-row cotton and soybean can reduce water loss, erosion, and off-site pesticide transport relative to conventional row spacings, and that if tillage is required to control glyphosate resistant weed biotypes in cotton, then rye cover crops can reduce runoff, erosion, herbicide and nutrient losses in the spring to levels less than or equivalent to that of conventional and no-tillage systems.
1. Water, sediment and metolachlor transport differences between wide- and narrow-row cotton production systems. Changing the way we plant cotton in the Mid-South could improve the regions surface water quality. ARS scientists at Stoneville, MS, evaluated sediment and herbicide loss from 4- to 6-leaf cotton planted in narrow and wide rows. Planting cotton on flat beds with rows spaced 15-inches apart reduced sediment loss by at least 38% relative to cotton planted on raised beds spaced 38-inches apart. Moreover, planting cotton on narrow nows reduced herbicide loss relative to wide-row systems if runoff and factors affecting pesticide movement were similar between row spacings. Converting from wide-to narrow-row cotton, therefore, could positively impact Mid-South water quality.
2. Enhanced atrazine degradation problem and mitigation. Enhanced atrazine degradation is a phonomenon whereby soil borne bacteria have developed the ability to rapidly degrade this economically important herbicide to the point that residual weed control is reduced. ARS scientists at USDA, ARS, Stoneville, MS, postulated that atrazine persistence in affected soils could be restored to baseline levels by eliminating atrazine applications and switching from corn to soybean production. Results indicate that even after four years of not applying atrazine that soil bacteria retained the ability to rapidly degrade the herbicide. Modeling efforts indicate that atrazine would need to be excluded for perhaps 40 yrs before the compounds persistence was similar to that observed prior to bacteria developing the ability to rapidly degrade this pesticide. This means that alternative chemistries should be used in place of or in conjunction with atrazine in affected soils to maintain acceptable weed control.
Dillon, K.A., Walker, T.W., Harrell, D.L., Krutz, L.J., Varco, J.J., Koger, C.H., Cox, M.S. 2012. Nitrogen sources and timing effects on nitrogen loss and uptake in delayed flood rice. Agronomy Journal. 104:466-470.