Evaluation of Methods to Assess and Reduce Bacterial Contamination of Surface Water from Grazing Lands



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Excessive bacterial levels are a major water quality concern. Better methods are needed to quantify the proportion of bacterial loading contributed by various sources, and best management practices are needed to restore water quality. This study assessed the ability of alternative water supplies and grazing management to reduce E. coli loading from cattle and evaluated the ability of quantitative polymerase chain reaction analysis of total and bovine-associated Bacteroides markers (AllBac and BoBac, respectively) to determine the percentage of bovine-associated fecal contamination. Runoff from seven small watersheds, representing ungrazed, properly stocked, and overstocked conditions, was analyzed for E. coli, AllBac, and BoBac to assess grazing management impacts on E. coli runoff and the effectiveness of Bacteroides markers. To determine the effectiveness of alternative water, instream E. coli levels and cattle movement were evaluated before and after alternative water was provided.

The study found that when alternative off-stream water was provided, the amount of time cattle spent in the creek was reduced 43 percent and the direct deposition of E. coli into Clear Fork of Plum Creek was estimated to be reduced from 1.11E 07 to 6.34E 06 colony forming units per animal unit per day. Observed pre- and post-treatment E. coli loads suggested similar reductions; however, this study could not conclusively attribute observed E. coli loading reductions to providing alternative water because of the lack of statistical significance of these observations, possibly due to decreased streamflow during Year 2 (due to drought) and a corresponding increase in E. coli levels.

The study found that rotational stocking, if timed appropriately, was very effective at reducing E. coli runoff. The impact of grazing timing in relation to runoff events was more significant than the impact of grazing management (i.e. ungrazed properly stocked or overstocked) or stocking rate. When runoff occurred more than two weeks following grazing, E. coli levels in runoff were decreased more than 88 percent.

Finally, data suggest that AllBac and BoBac markers are good indicators of recent fecal contamination from cattle. However, although elevated BoBac/AllBac ratios generally aligned well with cattle presence, this ratio appeared to underestimate the percentage of bovine-associated fecal contamination.