Between-year variation in the effects of phosphorus deficiency in breeder cows grazing tropical pastures in northern Australia

Breeder herd productivity can be severely reduced by dietary phosphorus (P) deficiency. The performance of small groups of P-deficient (P-defic) or P-supplemented (P-supp) breeder cows was studied over 5 annual cycles while grazing C4 grass-Stylosanthes pastures at a site in the seasonally dry tropics of northern Australia. Soils contained c. 4 ppm of bicarbonate extractable P. Plasma inorganic P concentrations (PIP) during the wet season indicated that the P-defic cows were deficient in P in 4 years and marginal in one year. Annual live weight (LW) changes ranged widely between annual cycles from -71 to +13 kg in P-defic cows and from +4 to +44 kg/head in P-supp cows. The LW responses to increased dietary P ranged from -9 to +115 kg, were greatest in years when LW losses by the P-defic cows were greatest, and were associated with low-rainfall years. LW gains of calves suckling P-supp cows (mean 0.86 kg/d) tended to be higher (range 0.01-0.17 kg/d; mean 0.09 kg/d) than those of calves suckling P-defic cows, but were significantly (P = 0.03) higher in only one year. Reconception appeared to be higher in P-supp than P-defic cows during the 2 years of lower rainfall. Overall, the results indicated that responses to P supplementation by breeders grazing P-deficient pastures can vary widely between years. Therefore, the response in any one year may not reliably indicate responses in the longer term