ATRAZINE RUNOFF FROM CONSERVATION TILLAGE SYSTEMS - A SIMULATED RAINFALL STUDY

Atrazine losses in surface runoff from a silty clay loam under no-till (NT) chisel (CH) and disk (DY) tillage were measured using simulated rainfall. Atrazine (0.56 kg ha(-1); 0.50 lb acre(-1)) and rainfall (64 nm h(-1), 2.5 in h(-1), for 1.25 hr) were applied to small plots and atrazine in sediment and dissolved in water was determined for all run off samples. Total atrazine losses were CH (58.6 g ha(-1)) > NT (44.1 g ha(-1)) = DK (41.9 g ha(-1)). However, sediment losses were CH (2.84 MT ha(-1)) > DK (2.09 MT ha(-1)) congruent to NT (0.37 MT ha(-1)). Di ssolved atrazine in runoff water accounted for more than 99.8% of tota l atrazine loss, and atrazine lost through sediment transport was insi gnificant Although atrazine partitioning between solution and solid ph ases in soil ir often directly related to soil organic matter content atrazine adsorption by soil organic matter did not control the amount of atrazine last in runoff. Poor relationships between apparent Kd val ues and soil organic matter content across simulated rainfall studies suggest experimental conditions affect runoff losses of atrazine more than atrazine adsorption by soil organic matter No-till systems reduce water and herbicide runoff under natural rainfall conditions, but res ults under simulated rainfall conditions are mixed. Presaturation of p lots with water, high-intensity simulated rainfalls shortly after atra zine application and other experimental renditions may outweigh tillag e effects and mask differences in atrazine losses from land under diff erent tillage systems. Future best management practices (BMPs) used to reduce herbicide runoff from agricultural land will be developed from simulated rainfall studies Identification of successful BMPs that min imize herbicide runoff under environmental conditions requires careful selection of experimental conditions used in simulated rainfall studi es.