Phosphorus nutrition of beef cattle. 3. Effect of supplementation on the phosphorus content of blood and on the phosphorus and calcium contents of hair and bone of grazing steers

Abstract

The effect of phosphorus supplements of 35 and 70 g per head per week on the calcium and phosphorus contents of bone and hair and on plasma inorganic phosphorus levels of steers was compared with those of unsupplemented steers which grazed carpet grass (Axonopus affinis) pasture of low phosphorus content at Grafton, New South Wales. The supplements had no effect on plasma inorganic phosphorus concentration. Hair phosphorus content was not affected by either dietary phosphorus or season. Hair calcium was not influenced by pasture calcium but was related to season, with highest levels in summer and lowest in winter. Supplementation increased the amount of phosphorus in dry fat-free rib-bone (P < 0.05) but this effect was not apparent in the first three months of supplementation. The amount of calcium in bone (Y) was influenced by pasture calcium content (X1) and bone phosphorus content (X2). This relationship was described by the equation: Y = 51.0 + 140.81X1 - 3.33X2(R2 = 0.71; P < 0.001) It is suggested that bone phosphorus levels below 14.3, 13.5, and 12.7 per cent of dry fat-free bone may represent a phosphorus deficiency state in 15-27-month-old steers when pasture calcium levels are 0.18, 0.15, and 0.12 per cent respectively. Further, when estimates of the calcium status of beef steers are made from measurements of bone calcium content, corrections should be applied for bone phosphorus content. It is concluded that measurement of bone phosphorus and calcium provides the most sensitive indication of the phosphorus and calcium status of beef cattle and that when phosphorus supplementation has no effect on liveweight of beef steers its effect on mineralization of bone tissue may justify its use, particularly in areas where poor bone development and lameness occur.