Contamination of freshwater systems by nutrients has been studied extensively in the eastern U.S. Little research on the fate of nutrients in grazing conditions has been done in the western U.S. Western agriculture is often irrigated and research addressing the potential for leaching under these conditions is warranted. This study used Management Intensive Grazing (MIG) and eight grass-legume mixtures to evaluate the management and forage choice in relation to nitrogen and phosphorus leaching to ground water. Soil water samples were taken weekly through the growing season from 2001 through 2003. Water samples were analyzed for nitrate, ammonia, and phosphate. Soils were described and a textural discontinuity was discovered in two of the six soil profiles described. Data were analyzed by treatment, grass, legume, and soils by treatment.

Three-year nitrate-N means were found to be in a range of 4.50 to 48.10 and 2.11 to 49.5 mg NO₃^- -N L^-1, at 60 cm and 90 cm, respectively. The amount of leaching depended on the grass-legume mixture as well as the soil characteristics. Significant differences were found between grasses (P=0.0001) with tall fescue leaching the least and orchardgrass leaching the most. Differences in leaching under legumes were also found to be significant (P=0.001), with white clover always leaching more than birdsfoot trefoil when significant differences were observed. Ammonia leaching was nonsignificant at a studywide analysis.

Phosphorus treatment concentrations ranged from 0.025 to 0.41 mg PO ₄^- -P L^-1 and were significantly different (P=0.001). A comparison by grass type indicated that perennial ryegrass leached significantly higher concentrations of P (P=0.01) at 60 cm than the other grasses, but found no difference at 90 cm. No significant differences were found between legumes at either depth. Leaching of P was less affected by the soil textural discontinuity than nitrogen leaching.

Soil differences affect leaching in varying degrees. Nitrate was affected most strongly by soil differences. These differences were more strongly evident at 60 cm than 90 cm. Soils affected phosphorus leaching differently than nitrate, and were more strongly affected at 90 cm than at 60 cm. Ammonia was not affected by soil characteristics.