Bioaccumulation and risk assessment of butachlor in the soil ecosystem

Thesis Abstract

Bioaccumulation of butachlor in plants following its application in the farm against weeds was evaluated using Phaseolus vulgaris (bean plant). Also, the risk of the consumption of such plants with some amount of bioaccumulated butachlor by non-target humans was studied using rabbits as animal model. The field experiments were carried out by crop cultivation with the application of 4.0 liters per hectare (2.6 kg a.i/ha), 4.4 liters per hectare (2.9 kga.i/ha) and 5.0 liters per hectare (3.2 kg a.i/ha) concentrations of butachlor at pre-emergence of the bean plant and the leaves of the plant were analyzed for the presence of butachlor residues using GC-MS , the result gave 0.10, 0.13 and 0.20 ppm bioaccumulated butachlor respectively for the concentrations of the butachlor applied to the plots of land. For 28 days three replicate groups of rabbits (4 per group) were fed the leaves containing these different concentrations (0.10, 0.13 and 0.20 ppm) of butachlor while the control groups which were composed of three replicates, were fed the plants cultivated in plots not treated with the herbicide. The rabbits were allowed access to water ad libitum. At the end of this exposure period, significant increases (p < 0.05) were observed in Cytochrome P450 (CYP) protein and increases in Glutathione S-transferase (GST) activity of the post-mitochondria liver fractions which were observed for the groups of rabbits fed the leaves having butachlor at the concentrations of 0.13 and 0.20 ppm in a time- and concentration-dependent manner; The liver marker enzymes, aspartate aminotransferase and alkaline phosphatase (AST, ALP) activities increased significantly (p < 0.05) for the rabbits fed the leaves with butachlor concentrations of 0.13 and 0.20 ppm in a time and concentration dependent manner thus suggesting a possible body defiance mechanism for herbicide detoxification. There was fluctuations in the ALT activity, with a decrease observed in group 4 rabbits that consumed the highest concentrations. Histological sections of the liver tissues of the exposed rabbits thus revealed no pathological alterations on day 28. The pesticide biomarker enzyme results is an indicative of induction and animal exposure to xenobiotics. The two enzymes (CYP and GST) could however be overwhelmed when the concentration of the herbicide increase, or upon chronic exposure, resulting to toxicity. The study shows that the manufacturers’ recommended application rate for butachlor (2.6 kg a.i/ha) poses no health risk; however, the application rates above the recommended rate could pose some risk when butachlor bioaccumulates in edible plants that are consumed. Thus, this work underlines the importance of strict adherence to the manufacturers’ and regulatory bodies directives, in the application of this herbicide butachlor in the soil ecosystem.