ARS | Publication request: Comparison of field-scale herbicide and runoff losses: An eight year field investigation

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 2010
Publication Date: September 14, 2011
Citation: Gish, T.J., Prueger, J.H., Daughtry, C.S., Kustas, W.P., McKee, L.G., Russ, A.L. 2011. Comparison of field-scale herbicide and runoff losses: An eight year field investigation. Journal of Environmental Quality. 40:1432-1442.

Interpretive Summary: Herbicides play an important role in maintaining worldwide food and fiber production by controlling weeds that compete for water and nutrients. Unfortunately, herbicides can also be toxic to humans and are difficult to monitor since they can exit an agricultural field simultaneously through three loss pathways: surface runoff, leaching to groundwater, or volatilization into the atmosphere. The distribution of herbicides between these three loss pathways is a function of herbicide physical and chemical properties as well as soil and climatic variables. To maintain productive and sustainable agricultural systems there is a need to understand field-scale processes governing herbicide use and off-site movement. An eight year investigation using state of the art instrumentation was used to determine how much atrazine and metolachlor (two widely used herbicides) were lost through runoff and volatilization over an eight year period. Results show that volatilization for these two herbicides were much greater than runoff losses even though both have low vapor pressures and runoff was thought to be the major loss pathway. For 6 of the 8 years annual herbicides runoff losses were much less than 1% of that applied. The largest annual runoff loss never exceeded 4.6% of that applied for metolachlor while atrazine runoff never exceeded 1.9% of that applied. On the other hand, metolachlor volatilization losses after 5 days ranged from 5 to 63% of that applied and 2 to 11% of that applied for atrazine. This research demonstrates that volatilization is perhaps least understood yet most critical loss pathway governing herbicide behavior in nearly level, sandy loam soils and will need to be understood if sustainable practices are to be developed.

Technical Abstract: An 8-year study was conducted to compare field-scale herbicide volatilization and surface runoff losses, and to better understand field-scale factors influencing year to year variability in these loss pathways. The 21 ha research site used for this study is located at the USDA-ARS Beltsville Agricultural Research Center, in Beltsville Maryland. Site location, soil properties, herbicide formulations, and agricultural management remained unchanged throughout the duration of the study. Metolachlor and atrazine were co-applied as a surface broadcast spray, while runoff and concentration profiles in the atmosphere were monitored. Herbicide runoff monitoring was initiated a month before application with a 46-cm H-flume and continued through harvesting. A flux gradient technique was used to compute volatilization fluxes for the first 5-days after application using herbicide concentration profiles and turbulent fluxes of heat and water vapor as determined from eddy covariance measurements. Additionally, twenty locations were randomly selected each year and subsequently sampled predawn at 4:30 am (EST) each morning to determine surface soil moisture and herbicide concentrations within the top 5 cm of soil. Results demonstrate that herbicide volatilization for these two herbicides were much greater than runoff losses even though both have low vapor pressures and runoff was thought to be the major loss pathway. Annual herbicides runoff losses were typically << 1%. The largest annual runoff loss for metolachlor never exceeded 4.6% while atrazine runoff never exceeded 1.9% of that applied. On the other hand, herbicide volatilization losses after 5 days ranged from 5 to 63% of that applied for metolachlor and 2 to 11% of that applied for atrazine. This research demonstrates that volatilization is perhaps least understood yet most critical loss pathway governing herbicide behavior in nearly level, sandy loam soils.