Impact of Grazing Dairy Cattle on Phosphorus Cycling and Water Quality in a High Water Table Pasture

During 1990 and 1991 Callie bermudagrass (Cynodon dactylon (L.) Pers.) pastures were used to determine the effects of 3 grazing management systems (short-duration rotational, long-duration rotational and continuous stocking) on P cycling and runoff water quality from intensively managed pastures used to raise replacement heifers (Bos taurus). A hay system also was included to compare soil responses under grazing and clipping. Soil P redistribution was not different among grazing systems, but accumulated in the third of the pastures closest to shade and water. Amounts of soil P were lower in the hay system than in the grazed pastures. Surface runoff samples from the study site never exceeded recommended standards for NO3- N, but soluble reactive PO4-P concentrations were often 2-6 times levels theoretically attainable with the management practices used during the study. It is suggested that fencing be used to exclude cattle from water courses. In addition, forage and soil analyses indicate that P inputs could be greatly reduced at the study site.