Persistence and Degradation of Herbicides in Rice, Maize and Soybean Grown in Vertisols of Tamil Nadu, Southern India P. JANAKI Herbicide Residue Chemist DWSR Centre, Department of Agronomy, Tamil Nadu Agricultural University, Coimbatore-641 003, Tamil Nadu, India e-mail: janakibalamurugan@rediffmail.com

Introduction

Production Scenario in India Crop Productivity (kg ha-1) Rice 2203 Maize 1912 Soybean 1063

Herbicides Use in India

Herbicide - Indian market

Purpose Controlling weeds Increasing the yield crops

Herbicide residues concern Prone to leaching and contaminate soil and water Bioaccumulation in crop produce and food chain Health hazard to non target organisms Environmental safety

Residues status in major crops No of locations <MRL >MRL BDL Rice 16 5 6 Soybean 1 4 Maize 3

Objectives Study the persistence and degradation behavior of commonly used herbicides in vertisols applied to rice, maize and soybean crops.

Methodology

Experimental Details Soil Properties Rice soil Maize soil Soybean soil Soil type Clay loam Sandy Clay loam Organic Carbon (g kg-1) 3.9 2.30 3.00 Available N (kg ha-1) 314 137 190 Available P (kg ha-1) 15.24 9.0 11.20 Available K (kg ha-1) 796 904 642 Infiltration rate (cm hr-1) 1.68 1.59 1.52 Bulk Density (g cm-3) 1.23 1.32 1.33 Porosity (%) 42 46 48

Treatments and methodology Crops Herbicides Doses Determination Recovery (%) Rice Butachlor X (1 kg ha-1) 2X (2 kg ha-1) GC-ECD 86-88 Pretilachlor 82-84 Maize Atrazine GC-FID 78-83 Alachlor X (0.75 kg ha-1) 2X (1.50 kg ha-1) > 80 Soybean Pendimethalin 81-82 Metolachlor

Results and Discussion

Initial deposition of applied herbicides

Field dissipation of applied herbicides in vertisols

Half-life Herbicides persistence in the soil is expressed as half life or time required for degradation of fifty percent of the original molecule. The persistence varies with the nature of a chemical, soil and climatic conditions.

Persistence in Rice grown soil Sampling intervals after herbicide application (Days) Butachlor Residue (mg kg-1) x (1 kg/ha) 2 x (2 kg/ha) 0.328 0.658 10 0.194 0.300 20 0.007 0.020 30 BDL 0.005 45 90 Harvest Sampling intervals after herbicide application (Days) Pretilachlor Residue (mg kg-1) x (1 kg/ha) 2 x (2 kg/ha) 0.214 0.486 15 0.096 0.246 30 0.058 0.189 45 0.020 60 BDL 0.014 90 Harvest

Persistence in Soybean grown soil

Persistence in Maize grown soil Sampling intervals after herbicide application (days) Atrazine residue (ppm) 1.0 kg/ha 2.0 kg/ha 0.212 0.426 5 0.156 0.314 10 0.082 0.218 20 0.068 0.152 30 0.050 0.121 45 0.042 0.094 60 BDL 0.052 Harvest Sampling intervals after herbicide application Alachlor Residue (ppm) X (0.75 kg/ha) 2X (1.50 kg/ha) 0.530 1.003 5 0.316 0.586 10 0.112 0.249 20 0.036 0.102 30 BDL 0.018 45 60 Harvest

Degradation rate in rice soil

Degradation rate in maize soil Half Life - 31.45 days for Atrazine

Degradation rate in soybean soil Half Life - 22.42 days for metolachlor

Optimized first order field dissipation curves Herbicides DT50 Goodness of fit Rice Butachlor – x Butachlor – 2x 5.44 6.28 R² = 0.869 R² = 0.790 Pretilachlor – X Pretilachlor – 2 x 13.16 11.72 R² = 0.886 R² = 0.923 Soybean Pendimethalin - x Pendimethalin – 2x 13.24 16.02 R² = 0.828 R² = 0.837 Metolachlor – x Metolachlor - 2x 21.13 24.71 R² = 0.944 R² = 0.957 Maize Atrazine - x Atrazine – 2 x 30.81 32.09 R² = 0.714 R² = 0.784 Alachlor – x Alachlor – 2x 5.15 4.41 R² = 0.771 R² = 0.797

Summary Initial deposits vary with the concentration applied Gradual and continuous dissipation was observed as a function of time Dissipation followed the first order kinetics Half life increased with increase in concentration Mean half life of herbicides : 5.88, 12.44, 31.45, 4.78, 14.63 and 22.42 days for butachlor, pretilachlor, atrazine, alachlor, pendimethalin and metolachlor

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