Transactions of the American Society of Agricultural Engineers , 49 , 2006

Bekele, A., McFarland, A.M.S., Whisenant, A.J.

In February 2001, the Texas Commission on Environmental Quality (TCEQ) adopted a Total Maximum Daily Load (TMDL) for soluble reactive phosphorus (SRP) along the North Bosque River. Within this TMDL, dairy waste application fields were identified as the major nonpoint-source contribution of nutrients. In September 2000, a manure composting program was initiated that resulted in about 500,000 metric tons of dairy manure being hauled to composting facilities and exported from the watershed through December 2004. To evaluate the impact of the manure composting program on stream water quality, storm event mean concentrations of nutrients and total suspended solids were compared before and after the start of the program at seven stream sites representing a range of land uses and levels of participation in the program. Data were analyzed as a “before/after” monitoring design using analysis of covariance (ANCOVA) with flow as the covariate and Wilcoxon rank sum (WRS) procedures with flow-adjusted data because flow was positively correlated to concentration. Although the manure composting program has only been in place about four years, water quality appeared to be improving at sites with the highest levels of manure removed per cow and watershed area. At these sites, SRP concentrations decreased from 19% to 23%. Significant decreases in SRP were not seen at stream sites with lower levels of manure hauled off, normalized on a per area and cow basis, indicating that the level of participation in the manure composting program might be a major determinant of the level of impact.