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ARS Home » Southeast Area » Houma, Louisiana » Sugarcane Research » Research » Publications at this Location » Publication #252600
Title: Metolachlor and chlorothalonil dissipation in gypsum-amended soil

Author
item White, Paul
item Potter, Thomas

Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2010
Publication Date: 6/18/2010
Citation: White Jr, P.M., Potter, T.L. 2010. Metolachlor and chlorothalonil dissipation in gypsum-amended soil. Journal of Environmental Science and Health, B: Pesticides, Food Contaminants, and Agricultural Wastes. 45(7):728-737.

Interpretive Summary: Many cropping systems require the application of herbicides and fungicides individually or as tank mixtures before and/or at planting followed by sequential application of the same or other pesticides during the growing season. Previous studies demonstrate significant interactive affects of pesticides, including altered efficacy, phytotoxicity, and soil persistence. A laboratory study focusing on potential interactions between the herbicide metolachlor (e.g., Dual Magnum ®) and the fungicide chlorothalonil (e.g., Bravo®) in a peanut (Arachis hypogaea) cropping system was conducted. Gypsum application was included due to its widespread use on peanut and other crops. Results suggest that applying chlorothalonil with metolachlor increases metolachlor soil persistence by 50%. Gypsum alone without chlorothalonil increased metolachlor persistence by 28% and when chlorothalonil and gypsum were used together, the increase was 100%. In all studies chlorothalonil dissipation was rapid (half life <4d). Chlorothalonil and metolachlor are both widely used for peanut in the southeastern USA. Adjusting application rate and/or timing may be possible to get the most economical impact from each of the pesticides, such as reduced metolachlor rates due to increased persistence when co-applied with chlorothalonil.

Technical Abstract: Many cropping systems incorporate pesticides individually or as tank mixtures pre- and or at planting followed by sequential application of other pesticides during the growing season. Additionally fertilizers and amendments may be applied. Previous studies demonstrate significant interactive affects of pesticides, including altered efficacy, phytotoxicity, and soil persistence. This work focused on the interactive effects of the fungicide chlorothalonil (2,3,4,6-tetrachloro-1,3-benzendicarbonitrile) and gypsum on the persistence of the soil-residual herbicide metolachlor (2-chloro-N-(6-ethyl-o-tolyl)-N-[(1RS)-2-methoxy-1-methylethyl]acetamide). Gypsum application was included due to widespread use on peanut (Arachis hypogaea). Both agricultural grade gypsum and reagent CaSO4-2H2O were tested. A laboratory soil incubation was conducted to evaluate interactive effects. Results indicated 1.5X greater metolachlor half-life (DT50) in soil amended with chlorothalonil (37 d) as compared to control soil (25 d). The two gypsum sources alone increased metolachlor DT50 to about 32 d and with the combination of chlorothalonil and gypsum, DT50 was 50 d, 2-fold greater than the control. Chlorothalonil dissipation was rapid (DT50 <4d). A possible explanation for metolachlor dissipation kinetics is a build-up of the chlorothalonil intermediate (4-hydroxychlorothalonil) which limited soil microbial activity and depleted glutathione S-transferase (GST) from chlorothalonil detoxification. Further information related to gypsum impacts is needed. Results confirm previous reports of chlorothalonil impeding metolachlor dissipation and showed the gypsum application was extended persistence even longer. Farming practices, such as reducing metolachlor application rates, may need to be adjusted for peanut cropping systems where chlorothalonil and gypsum are used.