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United States Department of Agriculture

Agricultural Research Service

Research Project: DEVELOPMENT OF MODELS AND CONSERVATION PRACTICES FOR WATER QUALITY MANAGEMENT AND RESOURCE ASSESSMENTS

Location: Grassland, Soil and Water Research Laboratory

Title: Assessment of cattle grazing effects on E. coli runoff

Authors
item Wagner, Kevin -
item Redmon, Larry -
item Gentry, Terry -
item Harmel, Daren

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 15, 2012
Publication Date: December 1, 2012
Citation: Wagner, K.L., Redmon, L.A., Gentry, T.J., Harmel, R.D. 2012. Assessment of cattle grazing effects on E. coli runoff. Transactions of the ASABE. 55(6):2111-2122.

Interpretive Summary: Runoff of bacteria from grazing lands has been identified as a significant source of water quality contamination. Development of best management practices to address these bacterial issues is critical to the success of watershed restoration efforts where grazing is a substantial contributor. In the present study, grazing management was evaluated to assess its effectiveness as a best management practice. Specifically, bacteria levels in runoff from grazed and ungrazed pastures were monitored for over 2 yr. Findings indicated rotational stocking, if timed appropriately, reduced bacteria runoff concentrations. The impact of grazing timing in relation to runoff events was more significant than stocking rate on rotationally stocked pastures. When runoff occurred more than two weeks following grazing, bacteria levels were reduced by more than 88% compared to runoff during or soon after grazing. As a result, it is recommended creek pastures and other hydrologically connected pastures be grazed during periods when runoff is less likely and upland pastures be grazed during rainy seasons when runoff is more likely to occur. Background bacteria levels were relatively consistent among sites and should be considered in water quality assessment and implementation efforts. Finally, although water quality standards are not currently applicable to edge-of-field runoff, it is notable that more than 90% of samples exceeded Texas Water Quality Standards for E. coli bacteria, even on ungrazed sites, emphasizing that current water quality standards are not appropriate for and should not be implemented at the field-scale.

Technical Abstract: Runoff of Escherichia coli and other fecal indicator bacteria from grazing lands has been identified as a significant source of bacterial contamination. Development of best management practices to address these bacterial issues is critical to the success of watershed restoration efforts where grazing is a substantial contributor. In the present study, grazing management was evaluated to assess its effectiveness as a best management practice. Specifically, E. coli levels in runoff from grazed and ungrazed pastures were monitored for over 2 yr. Findings indicated rotational stocking, if timed appropriately, reduced E. coli runoff concentrations. The impact of grazing timing in relation to runoff events was more significant than stocking rate on rotationally stocked pastures. When runoff occurred more than two weeks following grazing, E. coli levels were reduced by more than 88% compared to runoff during or soon after grazing. As a result, it is recommended creek pastures and other hydrologically connected pastures be grazed during periods when runoff is less likely and upland pastures be grazed during rainy seasons when runoff is more likely to occur. Background E. coli levels were relatively consistent among sites, with median levels ranging from 3,500 to 5,500 colony forming units per 100 mL. These substantial background levels should be considered in water quality assessment and implementation efforts. Finally, although water quality standards are not currently applicable to edge-of-field runoff, it is notable that more than 90% of samples exceeded Texas Water Quality Standards for E. coli, even on ungrazed sites, emphasizing that current water quality standards are not appropriate for and should not be implemented at the field-scale.

Last Modified: 4/19/2014
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