LAUSR

Context Lactating beef cows grazing phosphorus (P)-deficient pastures often mobilise body P to alleviate a P deficiency. Knowledge of the circumstances when body P is mobilised, and later replenished, is necessary for optimal management of the P nutrition of breeder herds. Aims To investigate mobilisation and replenishment of body P in mature Bos indicus-cross beef cows post-weaning. Methods Cows (n = 40) in their second trimester of pregnancy were individually fed ad libitum low-P (LP) or high-P (HP) diets containing moderate or high metabolisable energy (ModE-LP, HighE-LP, ModE-HP and HighE-HP) for 14 weeks. Key results Plasma inorganic P concentrations (Pi) in LP- or HP-diet cows (0.58 and 2.15 mmol/L respectively) indicated that diet P was deficient or adequate. Intakes of DM and metabolisable energy, liveweight gain, and P retention were higher (P < 0.05 to P < 0.001) in cows fed the HP diets, and were also increased in the high metabolisable-energy diets. Efficiency in use of metabolisable energy for slow growth was lower in the HighE-LP than the ModE-HP cows. The cows fed the LP diets mobilised 4–5 g body P/day. Cows fed the Mod-HP and HighE-HP diets retained 1.1 and 8.8 g body P/day, and those ModE-HP cows with low bone P reserves retained ~2.3 g P/day. Rib cortical bone P did not change in HP cows but decreased (P < 0.05) in LP cows. The HP diets increased the bone volume, mineralised bone, and the thickness of the struts in trabecular bone in the tuber coxae. Osteoid tissue decreased (P < 0.05) in HighE-HP cows. Changes in plasma concentrations of 1,25-dihydroxy vitamin D3, carboxy-terminal telopeptides of type I collagen (CTX-1) and bone alkaline phosphatase (BAP) were in accord with substantial bone mobilisation in cows fed LP diets, and bone replenishment with HP diets. Conclusions Cows that had ingested P-deficient diets during lactation were able to further mobilise body P when fed LP diets post-weaning, but cows replenished body P when fed HP diets. Implications Mature cows with low body P reserves can replenish these reserves more rapidly when consuming diets high in both P and metabolisable energy.