Phosphorus fertilizer for nitrogen fertilized dairy pastures .1. Long term effects on pasture, diet and soil

The phosphorus fertilizer requirements and long term productivity of nitrogen-fertilized Gatton panic (Panicum maximum cv. Gatton) pastures, grazed by lactating dairy cows, were evaluated over 7 years. Cows grazed at 2.6 cows/ha on pastures that received annually 100 or 300 kg N/ha at each of 0, 22.5 or 45 kg P/ha. Phosphorus treatments were applied as single superphosphate, balanced for calcium by applications of gypsum. The soil had an initial available soil phosphorus content of 40 mg/kg (bicarbonate extraction). At zero P fertilizer (OF), extractable soil P declined at the rate of 1.9 mg/kg each year; at 22.5P it was maintained close to the original level while at 45P it increased at 6.6 mg/kg each year. Increased P fertilizer caused significant (P < 0.01) increases in plant P concentration from year 2 onwards. In years 6 and 7 there was significantly less green pasture and leaf on offer in 300N pastures at 0P than with 22.5P and 45P. There was no influence of rate of P fertilizer at 100N on pasture quantity on offer in any year. There were clear trends at 100N of decreasing total pasture and green dry matter (DM) on offer over the 7 years, but not at 300N. Cows at 300N consumed more leaf in the diet in autumn and winter than at 100N. Leaf was 55-60% of the diet in summer and autumn, but decreased to 21% (100N) and 37% (300N) in winter. Dead material in the diet was always higher at 100N. Pasture leaf percentage and leaf yield were the best individual predictors of leaf percentage in the diet. Diet P selected from pasture was reduced by the higher rate of N fertilizer in each season. Estimated P concentrations of the diet selected from pasture for summer, autumn and winter averaged 0.30, 0.38 and 0.28% DM for 100N and 0.19, 0.24 and 0.18% DM for 300N treatments, respectively. The response to P fertilizer was dependent on the rate of N fertilizer applied. The critical bicarbonate extractable soil P level for this soil type, below which pasture responses occurred, was 30 mg/kg at 300N. The critical level at 100N was not reached, but was <23 mg/kg P.