Effect of long-term field application of pendimethalin: enhanced degradation in soil Gita Kulshrestha, 1 * Shashi B Singh, 1 Shashi P Lal 2 and Nanjapur T Yaduraju 3 1 Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi 110012, India 2 Division of Mycology and Plant Pathology, Indian Agricultural Research Institute, New Delhi 110012, India 3 Division of Agronomy, Indian Agricultural Research Institute, New Delhi 110012, India Abstract: The effect of long-term application of pendimethalin in a maize±wheat rotation on herbicide persistence was investigated. Pendimethalin was applied at 1.5 kg AI ha 1 separately as one or two annual applications for ®ve consecutive years in the same plots. Residues of pendimethalin were determined by gas chromatography. Harvest-time residues of the herbicide decreased gradually over the years and at the end of ®ve years less than 3% of applied pendimethalin was recovered from soil as against 18% in the ®rst year. Residues were found distributed in the soil pro®le up to 90 cm depth at the end of the experiment with peak distribution of 0.03 mgg 1 in the surface layer of soil treated with 10 herbicide applications. The minimum distribution was, however, in the deepest soil (75±90 cm) pro®le. Some of the metabolites of pendimethalin ie dealkylated pendimethalin derivative, partially reduced derivative and cyclized product were also traced in surface and sub-surface soils up to 90 cm. A study of the rate of degradation of pendimethalin in ®eld-treated soils under laboratory conditions revealed faster degradation compared to control soils. Only the surface soil (0±15 cm) showed this enhanced degradation of the herbicide, which could be due to the adaptability of the aerobic micro-organisms to degrade pendimethalin. Microbes capable of degrading herbicide were isolated, identi®ed and pendimethalin degradation was con®rmed in nutrient broth. # 2000 Society of Chemical Industry Keywords: long-term effect; pendimethalin; herbicide residues; soil; enhanced degradation 1 INTRODUCTION Pendimethalin [N-(1-ethylpropyl)-2,6-dinitro-3,4- xylidine] is a dinitroaniline herbicide used selectively pre-emergence for the control of grasses and dicoty- ledenous weeds in crops such as cotton (Gossypium hirsutum L), wheat (Triticum aestivum L), soybean (Glycine max L), maize (Zea mays L), peas (Pisum sativum L) and several vegetable crops. 1 It is effectively used in kharif maize and winter wheat cropping systems. Pendimethalin has a relatively low volatility (vapour pressure, 3.0  10 5 mmHg at 25 °C) unlike other dinitroaniline herbicides 2 and, is therefore lost less rapidly from the soil surface 3 by volatilisation. This dinitroaniline herbicide is also less photodegradable than other dinitroanilines. 4 Degradation of pendi- methalin proceeds more rapidly under ¯ooded anae- robic conditions than under aerobic conditions, 5 which is similar to other dinitroanilines. 6±8 Early research indicated that pendimethalin was relatively stable to degradation in soil, 9 with a half-life of about 60 days under tropical ®eld conditions. There was a slower rate of loss from soil rich in clay or organic matter. Residues bound to soil organic matter (15% after six months) have been reported. 10 Although much is known about the persistence of a single application of pendimethalin in a crop, 8 there is no information on its long-term effect following repeated applications, especially in a maize±wheat rotation. The objective of this investigation was to determine the degradative fate of pendimethalin in the soil pro®le of a maize±wheat cropping system when applied continuously for ®ve years separately both as annual and twice per annum applications. 2 EXPERIMENTAL METHODS 2.1 Chemicals Analytical standard pendimethalin (99.9% purity) was obtained from Cyanamide India Ltd. The predicted degradation products of pendimethalin, viz dealky- lated pendimethalin [DA; 3,4-dimethyl-2,6-dinitroa- niline], partially reduced pendimethalin [PR; N-(1- ethylpropyl)-3,4-dimethyl-2-nitrobenzene-1,6-dia- mine] and cyclised product [CY; N-(1-ethylpropyl)- 5,6-dimethyl-7-nitrobenzimidazole] were prepared in the laboratory. 5,11 2.2 Field experiment The ®eld experiment was established in 1989 on a ®ve- (Received 15 June 1999; revised version received 2 September 1999; accepted 5 October 1999) * Correspondence to: Gita Kulshrestha, Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi 110012, India # 2000 Society of Chemical Industry. Pest Manag Sci 1526±498X/2000/$17.50 202 Pest Management Science Pest Manag Sci 56:202±206 (2000)