1. 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 ,
Tamil Nadu, India

2. Introduction

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

4. Herbicides Use in India

5. Herbicide - Indian market

6. Purpose
Controlling weeds
Increasing the yield crops

7. 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

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

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

10. Methodology

11. 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

12. 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

13. Results and Discussion

14. Initial deposition of applied herbicides

15. Field dissipation of applied herbicides in vertisols

16. 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.

17. 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

18. Persistence in Soybean grown soil

19. 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 ( 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

20. Degradation rate in rice soil

21. Degradation rate in maize soil
Half Life days for Atrazine

22. Degradation rate in soybean soil
Half Life days for metolachlor

23. 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

24. 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, and days for butachlor, pretilachlor, atrazine, alachlor, pendimethalin and metolachlor

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