Cotton Defoliant Runoff as a Function of Active Ingredient and Tillage

Cotton ( Gossypium hirsutum L.) defoliant runoff was recently identified as an ecological risk. However, assessments are not supported by field studies. Runoff potential of three defoliant active ingredients, dimethipin (2,3‐dihydro‐5,6‐dimethyl‐1,4‐dithiin 1,1,4,4‐tetraoxide), thidiazuron ( N ‐phenyl‐ N ‐1,2,3‐thidiazol‐5‐yl‐urea), and tribufos ( S,S,S ‐tributyl phosphorotrithioate) was investigated by rainfall simulation on strip (ST) and conventionally tilled (CT) cotton in south central Georgia. Simulated rainfall timing relative to defoliant application (1 h after) represented an extreme worst‐case scenario; however, weather records indicate that it was not unrealistic for the region. Thidiazuron and tribufos losses were 12 to 15% of applied. Only 2 to 5% of the more water soluble dimethipin was lost. Although ST erosion rates were less, loss of tribufos, a strongly sorbing compound, was not affected. Higher sediment–water partition coefficients ( k d ) were measured in ST samples. This likely explains why no tillage related differences in loss rates were observed, but it is unknown whether this result can be generalized. The study was conducted in the first year following establishment of tillage treatments at the study site. As soil conditions stabilize, ST impacts may change. Data provide an estimate of the maximum amount of the defoliants that will run off during a single postapplication storm event. Use of these values in place of the default value in runoff simulation models used in pesticide risk assessments will likely improve risk estimate accuracy and enhance evaluation of comparative risk among these active ingredients.