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United States Department of Agriculture

Agricultural Research Service

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Research Project: Agrochemical and Weed Seed Fate and Transport in Mid-South Crop Production Systems

Location: Crop Production Systems Research Unit

2013 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 Readyâ„¢ 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:
ARS scientists assessed long-term effects of cotton production with rye and balansa cover crops on soil parameters under no-till or minimum till. Cover crops accumulated more soil carbon than no cover crop and nitrogen was highest under clover cover crop. Soil microbial community structure analysis indicated a significant cover crop effect but no tillage effect. Collectively, cotton production with a cover crop and reduced tillage resulted in soil conditions indicative of soil quality. In the past considerable work focused on enhanced atrazine degradation. ARS scientists developed two diagnostic assays to confirm enhanced atrazine degradation. Models were developed that accurately predicted symmetrical-triazine (s-triazine) persistence under field conditions. Scientists demonstrated that genes coding for enzymes able to rapidly degrade s-triazine herbicides are widespread in the 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 reduced 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.


4.Accomplishments
1. Enhanced atrazine degradation problem and mitigation. Enhanced atrazine degradation results in loss of residual weed control. Scientists at USDA-ARS, Crop Production Systems Research Unit in Stoneville, MS, have conducted a 4-yr field study on atrazine adapted soil to determine the effects of triazine exclusion interval (1, 2, 3, or 4 years), crop production system (continuous corn or continuous soybean), and rhizosphere proximity (bulk or rhizosphere soil) on atrazine degrader populations and activity. Results indicate that omitting triazines could reduce degrader populations and activity in adapted soils, but more than 4 years is required to return to nonadapted soil levels, regardless of crop or rhizosphere proximity.

2. Conservation management in cotton production: long-term soil biological, chemical, and physical changes. Conservation practices are critical component of sustainable crop management system and information on their effect on soil characteristics is needed. Scientists at USDA-ARS, Water Quality and Ecology Research Unit, Oxford, MS, and Crop Production Systems Research Unit, Stoneville, MS, assessed soil parameters in no-till or minimum till cotton production with rye and balansa cover crops. Results indicated that cover crops accumulated more soil carbon than no cover crop and nitrogen was highest under clover cover crop. Soil microbial community structure analysis indicated a significant cover crop effect but no tillage effect. Collectively, cotton production with a cover crop and reduced tillage resulted in soil conditions indicative of soil quality.


Review Publications
Krutz, L.J., Zablotowicz, R.M., Reddy, K.N. 2012. Selection pressure, cropping system and rhizosphere proximity affect atrazine degrader populations and activity in s-triazine adapted soil. Weed Science. 60:516-524.

Locke, M.A., Zablotowicz, R.M., Steinriede Jr., R.W., Testa III, S., Reddy, K.N. 2013. Conservation management in cotton production: long-term soil biological, chemical, and physical changes. Soil Science Society of America Journal. 77:974-984.

Last Modified: 9/10/2014
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