SOIL RESPONSE TO CONSERVATION TILLAGE IN A COTTON-PEANUT ROTATION
Location: Southeast Watershed Research
Title: Reduction in metolachlor and degradates concentrations in shallow groundwater through cover crop use
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 15, 2009
Publication Date: October 22, 2009
Citation: White Jr, P.M., Potter, T.L., Bosch, D.D., Joo, H., Schafer, B., Munoz-Carpena, R. 2009. Reduction in metolachlor and degradates concentrations in shallow groundwater through cover crop use. Journal of Agricultural and Food Chemistry. 57(20):9658-9667.
Interpretive Summary: Agriculture practices in southern Florida have the potential to impact groundwater quality. The climate and hydrologic conditions include a winter “dry” growing season and a summer rainy season where leaching rates can be high. Growers are frequently requested to apply best management practices (BMPs) to reduce the impacts of agrichemicals on groundwater quality. However, reliable data to base BMPs is limited. In the current study our goal was to measure the effect of a cover crop on the leaching characteristics of a commonly used herbicide, metolachlor. The study was conducted in a sweet corn (Zea mays) field where one half of the plots (n=3) were cover cropped to sunn hemp (Crotalaria juncea L.) during the rainy season. The cover crop was incorporated into the soil prior to planting corn each fall. Metolachlor applications occurred in November 2001 and 2002 prior to corn planting, and groundwater samples were taken bi-weekly from up and downgradient wells and wells located in cover crop and non cover crop plots. Results show that metolachlor degraded rapidly in soil, with a half life of about 40 days. Data show increasing amounts of metolachlor over time in groundwater below non cover crop plots, and lower amounts in upgradient, downgradient, and cover crop test wells. Concentrations of the most commonly detected metolachlor degradate, metolachlor ethane sulfonic acid (MESA), were high during the wet 2002 season in all treated plots, however lower levels were observed in groundwater beneath cover crop plots during the growing season of 2002-2003. Overall data indicate that the practice of cover cropping in South Florida during the fallow period reduces pesticide impacts on shallow groundwater.
Pesticide use during crop production has the potential to adversely impact groundwater quality. In southern Florida climatic and hydrogeologic conditions and agronomic practices indicate that contamination risks are high. In the current study soil dissipation of the widely used herbicide, metolachlor, and levels of the compound and selected degradates in shallow groundwater beneath six 0.15-ha plots in sweet corn production were evaluated over a two-year period at a research station in the region. During fallow periods (May to October), plots were either left bare or cover cropped to sunn hemp (Crotalaria juncea L.). Metolachlor was broadcast applied prior to planting each corn crop at label recommended rates. Groundwater monitoring wells hydraulically upgradient and downgradient, and within each plot were sampled bi-weekly. Results showed that metolachlor dissipation was rapid, with evidence being detection of relatively high levels of the metolachlor ethane sulfonic degradate (MESA) in groundwater beneath plots, and metolachlor soil DT50 = 38 to 43d. Other degradates detected included hydroxymetolachlor in soil and in groundwater metolachlor oxanilic acid (MOA) and a product tentatively identified as 2-chloro-N-(2-acetyl-6-methylphenyl-N-(2-methoxy-1-methylethyl) acetamide, a photo-oxidation product. Significantly higher concentrations of metolachlor and MESA were found in groundwater beneath the non cover cropped plots when compared to the cover cropped plots indicating that cover cropping reduced leaching of these compounds. The study demonstrated that integration of cover crops into agronomic systems in the region may yield water quality benefits by reducing herbicide inputs to groundwater.