Utilising mobilisation of body reserves to improve the management of phosphorus nutrition of breeder cows

Phosphorus (P) deficiency is a major constraint to the productivity of breeder herds grazing low-P rangelands due to adverse effects on growth and fertility. However, P supplementation during the wet season, when additional dietary P is most needed, is often difficult due to practical constraints. Body P reserves in breeders can be mobilised and alleviate dietary P deficiency within an annual cycle. Approaches to estimate bone P reserves and net mobilisation or replenishment of P from the analysis of rib and hip (tuber coxae) biopsies are discussed. In at least some circumstances, breeder cows grazing P-deficient pastures mobilise bone P to alleviate the effects of diet P deficiency. Recent experiments with breeders have investigated mobilisation of body P to alleviate the adverse effects of dietary P deficiency during pregnancy and early lactation, and subsequent replenishment of body P reserves. Both mature cows and first-calf cows (FCC) calving in a high P status and fed severely P-deficient diets during lactation were able to mobilise sufficient body P reserves to provide milk for moderate calf growth (viz. 0.6–0.8 kg liveweight (LW)/day for 3 months), but this was associated with rapid cow-LW loss and markedly decreased bone P content. First-calf cows appear to have lesser capacity than mature cows to mobilise body P. FCC fed P-adequate diets during late pregnancy maintained high voluntary intakes and had higher LW and bone P reserves at calving. When fed a P-deficient diet during early lactation, these greater bone P reserves were utilised. Bone P reserves can be replenished by P-adequate diets fed post-weaning. Breeder management that relies more on mobilising body P reserves when P demands are high and on replacing these body P reserves when P demands are lower offers an opportunity to alleviate the effects of dietary P deficiencies during the early wet season in situations where P supplementation is not possible.