ASSESSMENT OF CATTLE GRAZING EFFECTS ON E. COLI RUNOFF
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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, runoff from rotationally grazed and ungrazed approximately 1 ha pastures was monitored for more than two years using automated samplers. E. coli levels in runoff were enumerated by membrane filtration using modified membrane-thermotolerant Escherichia coli agar (modified mTEC). The study found that background E. coli concentrations (i.e., those from wildlife and soilborne naturalized strains) were relatively consistent among sites. Median background E. coli concentrations ranged from 3,500 to 5,500 colony forming units per 100 mL, which are not inconsequential levels related to water quality assessment and implementation efforts. Findings also indicated that rotational stocking, if timed appropriately, reduced E. coli concentrations in runoff. The impact of grazing timing in relation to runoff events was a more significant factor than stocking rate on the 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 that creek pastures and other hydrologically connected pastures be grazed during periods when runoff is less likely and that upland pastures be grazed during rainy seasons when runoff is more likely to occur. 2012 American Society of Agricultural and Biological Engineers.
