Bone mineral density in the tail-bones of cattle: Effect of dietary phosphorus status, liveweight, age and physiological status

In three grazing experiments in the seasonally dry tropics of Australia, growing steers (Experiment 1), first-calf cows (Experiment 2) and mature breeder cows (Experiment 3), ingested diets for 12-17 months, which were either adequate or severely deficient in phosphorus (P) (Padeq and Pdefic, respectively). Bone mineral density (BMD) at the proximal end of the ninth coccygeal vertebra (Cy9) was measured at intervals using single photon absorptiometry (SPA). Liveweight (LW) and plasma inorganic phosphorus (PIP) concentrations were monitored at intervals and rib-bone cortical bone thickness (CBT) of biopsy samples was measured at the end of Experiments 1 and 3. Measurements of LW change, PIP concentrations and CBT confirmed that diet P intakes of cattle in the Padeq treatments were adequate whereas there was severe and chronic P deficiency in the Pdefic treatments. In Experiment 1 BMD in Padeq steers increased with LW and age from ∼0.25-0.27 g/cc (8 months, 200 kg LW) to ∼0.34 g/cc (32 months, 490 kg LW), whereas in Pdefic steers BMD decreased progressively to ∼0.23-0.24 g/cc. Although BMD decreased in the Pdefic steers bone volume of Cy9 (calculated from tail-bone thickness) increased, and some net bone deposition in the Cy9 continued. Rib-bone CBT and tail-bone BMD at the end of Experiment 1 were closely correlated (r ≤ 0.93). In Experiment 2 BMD was initially 0.33 g/cc (∼25 months, 400 kg LW) and did not change through pregnancy and lactation in Padeq cows. However, in the Pdefic cows there was a gradual decline in BMD to ∼0.25 g/cc. There was no change in dimensions of the Cy9 so the decreases in BMD involved net demineralisation of bone. In Experiment 3 BMD was less responsive to P deficiency than in Experiments 1 and 2. Only after ∼11 months was BMD reduced (P < 0.05) in the Pdefic cows, and then only by 15%. In contrast, rib-bone CBT decreased by 30% due to P deficiency, and BMD was poorly correlated with CBT (r ≤ 0.4). The effects of animal weight, age and maturity on tailbone BMD of P-adequate animals, and the different responses to P deficiency observed in young growing steers, first-calf cows and mature breeders are discussed in relation to the use of SPA measured tail-bone BMD to diagnose P deficiency in grazing cattle