Dissipation kinetics and residues of pendimethalin in soil, straw and grain of rainy season greengram

Pendimethalin [N-(1-ethylpropyl)-3, 4 dimethyl 2, 6 dinitrobenzenamine], is being used for control of majority of grasses and broad-leaf weeds in crops such as peas (Pisum sativum L.), rice (Oryza sativa L.), wheat (Triticum aestivum L.), soybean (Glycine max L. merr.), and vegetables. Dissipation pattern and the residues dynamics of the herbicide in the soil, greengram straw and grain was determined by conducting field and pot experiments during 2017-2019 utilizing the doses of 1000 g/ha for field study and 1 mg/20 g soil for pots. The residue level of pendimethalin in soil were 0.088, 0.080, 0.075, 0.065, 0.056 and 0.048 mg/g at 2, 5, 15, 25, 35 and 45 days, respectively after herbicide application (DAA) in field that had gone down to below detectable level at the time of harvest (65-70 days). However, greengram plants and seeds, at time of harvest, were found free from the pendimethalin residues. The dissipation of pendimethalin in field and in pots was found to operate as per first order kinetic equation [dC₀/dt=K(C-C₀)], therefore, based on dissipation rate constant (K) values, viz. 1.36 × 10^-2 (field) and 1.11×10^-2 (pot), the half-lives (T_1/2) of pendimethalin were calculated as 52 and 62 days in field conditions and pot culture, respectively. The effective period (T_eff.) with respect to weed control was worked out as 21 and 26 days for field and pot experiments, respectively by assuming the concentration of herbicide in field between 1000-750 g/ha. Therefore, this study suggests that the herbicide can only provide effective protection to crop against weed up to a maximum period of 20-25 days in sandy clay loam soil of taxonomical class Typic Usrtochrept.