Residues of Pendimethalin in Field Crops.

Herbicides for the control of weeds has become imminent  in the modern day agriculture for a wide variety of reasons  like non availability of labour, high labour cost, unfavorable climatic conditions for weeding etc. After application of these herbicides to soil, undergoes decomposition and a part may taken by plants accumulating in the edible parts, which are found to be toxic in nature.            Application of recommended dose of herbicides may not pose serious problem for environmental pollution (Adachi et al.,2007).However quantitative determination of herbicide residues helps in understanding the degradation pattern in the soil. Pendimethalin as pre-emergence herbicide found effective for the control of annual weeds in leguminous and other vegetable crops. In recent years, the compound is subjected to increase toxicological and environment concerns, e.g., to cause various physiological changes and endocrine effects in the animal studies including liver, kidney damage, and number of mutagenic effects. (Dimitro et al., 2006). Carrot, groundnut and radish are the important  crops and consumed in daily food product as raw or cooked form. Residue studies of pendimethalin in crops is lacking hence current studies were taken up to determine the terminal residues of pendimethalin.

            Field experiments were conducted to study the residues of pendimethalin in edible parts of radish, carrot and groundnut and to find out the half life of pendimethalin in soil. Pendimethalin was applied @ 1 kg a.i/ha for carrot, groundnut crops and @0.75 kg a.i/ha for radish as pre emergence for control of weeds. The edible parts were collected at the time of harvest for analysis of residues by gas chromatograph. Shimadzu GC 2010 with Electron Capture Detector (ECD) with Ni 63. A ZB-5 column with 30 m length ID 0.53 mm, film thickness 1.50 um was used to detect pendimethalin residues in soil and vegetables. The carrier gas used was Nitrogen with flow rate 53.6 ml/min. Injector temperature was 240⁰C and detector temperature was 260⁰C  with injector split ratio 1:10. The retention time and peak area of the samples and standard were recorded and pendimethalin in the samples was quantified. The recovery studies for pendimethalin was carried out at 2 ppm, 1 ppm and 0.5 ppm.  Percent recovery of pendimethalin varied from 85 to 94 per cent. Standard solutions of pendimethalin was prepared by dissolving 105.8 mg of respective technical grade herbicides in 100 ml of hexane. From this 100 μg mL^-1 solution , 1 μg mL^-1  solution was prepared and  injected  to GC. The peaks by their retention time were identified and the peak area was measured. The amount of residues of herbicide was calculated. Pendimethalin eluted a peak at 10.77 minutes. Pendimethalin was applied @ 1 kg a.i ha^-1 for carrot and groundnut and the observed residue in the edible parts of carrot and groundnut was 0.0061 mg kg^-1 and 0.0015 mg kg^-1 respectively. In radish pendimethalin was applied @0.75 kg a.i ha^-1, the observed residue in the tuber was 0.0063 mg kg^-1. Soil samples were collected at 15 days interval of growing period of crops and the residues of pendimethalin in soil was estimated by using Gas Chromatograph. Based on the estimated residues and time intervals, regression equations were made and half life was calculated by using the formulae t_1/2=0.693/λ. In soils the observed residues were decreased with increase in days after application in all the crops. The observed half life was 7.16 days in carrot grown soil and 25.08 days in groundnut grown soil when pendimethalin was applied @ 1 kg kg a.i ha^-1. In radish pendimethalin was applied @0.75 kg a.i ha^-1, the observed half life was 9.40 days. It was observed that vegetable crops and tuber(under soil) is the edible part for consumption, the observed herbicide residue was more compared to the groundnut crop, where the observed residue at the time of harvest in the groundnut pods was less. This was may be due to the short duration of vegetables and the tubers may occupy large volume in soil compared to groundnut pods. The observed half life was also more in groundnut crop compared to the vegetable tuber crops.