Abstract

COATES, DAVID B.; DIXON, ROB M.  and  MAYER, ROBERT J.. Between-year variation in the effects of phosphorus deficiency in breeder cows grazing tropical pastures in northern Australia. Trop. Grassl.-Forrajes Trop. [online]. 2019, vol.7, n.3, pp.223-233. ISSN 2346-3775.  https://doi.org/10.17138/tgft(7)223-233.

Breeder herd productivity can be severely reduced by dietary phosphorus (P) deficiency. The performance of small groups of P-deficient (Pdefic) or P-supplemented (Psupp) 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 Pdefic cows were deficient in P in 4 years and marginal in one year. Annual liveweight (LW) changes ranged widely between annual cycles from −71 to +13 kg in Pdefic cows and from +4 to +44 kg/head in Psupp cows. The LW responses to increased dietary P ranged from −9 to +115 kg, were greatest in years when LW losses by the Pdefic cows were greatest, and were associated with low-rainfall years. LW gains of calves suckling Psupp 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 Pdefic cows, but were significantly (P = 0.03) higher in only one year. Reconception appeared to be higher in Psupp than Pdefic 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.

Keywords : Cattle; mineral deficiency; phosphorus responses; phosphorus supplements; seasonal variation.

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