Metolachlor and chlorothalonil dissipation in gypsum-amended soil

Abstract

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 its 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 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.

Keywords:

• Soil dissipation
• persistence
• metolachlor
• chlorothalonil
• pesticide degradates

Acknowledgments

We thank Dr. Thomas DeSutter, Sally Belflower, Margie Whittle, James Jennings, Lorine Lewis, and Josh Camp for laboratory and field assistance. Funding was provided by the USDA-Agricultural Research Service Southeast Watershed Research Unit in Tifton, GA. Mention of trade names or commercial products is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.

Notes

*CLT = chlorothalonil.

**MESA = metolachlor ethane sulfonic acid.

^†MOA = metolachor oxanilic acid.

*CLT = chlorothalonil.

**MESA = metolachlor ethane sulfonic acid.