Phosphorus Release From Fecal Pats in Grazed Pasture As Influenced by Nitrogen Fertilization Regime.

Soil nutrients in grazed pasture are continually replenished via animal excretions; however, rate of movement of nutrients, notably P, from fecal pats to the soil profile is not well understood.  In late October 2010, 6 0.28-ha plots were overseeded with triticale (Triticum secale) and crimson clover (Trifolium incarnatum) into a tall fescue (Lolium arundinacea) sod and assigned randomly to 1 of 3 treatments (n = 2): 100% of N recommendation for tall fescue in a split application (100N), 50% of N recommendation (50N), and 0% of N recommendation (0N). In mid-February 2011, 6 cattle were randomly assigned to plots (1 animal/plot) for grazing until May. In late April, feces were collected from each animal, mixed and fabricated into uniformly shaped 0.5-kg (wet basis) pats, and randomly assigned for sampling at 0, 28, 56, 84 and 112 days from application onto bare ground. Soil samples (0 – 5, 5 – 10 and 10 – 20 cm depths) were also taken on each sampling day. Feces initially contained 0.68, 0.84 and 0.89% P (dry basis) for 0N, 50N and 100N, respectively. Fecal P concentration decreased (P < 0.07) from day 0 to day 28 in 0N, and was different (P < 0.036) in 50N between day 0 and days 28, 84 and 112; fecal P concentrations tended to decrease after 28 days in all treatments. Soil P concentrations were not different among treatments. Soil P concentration was greater (P < 0.0001) at 0 – 5 cm (70 mg P /kg) than 5 – 10 cm (34 mg P/kg) and 10 – 20 cm (35 mg P/kg) depths, and tended to increase from day 0 to 112 in all treatments. Results are interpreted to mean that differing temporal patterns of decrease in fecal P concentration among the N-fertilization regimes were inconsequential to soil P concentrations under winter/spring grazing.