Dietary phosphorus for beef cows.

Hereford heifers (48 initially) were individually fed variable amounts of dietary phosphorus (P) from weaning through their 8th gestation. During phase I, 2 groups (24 cows each) were given 20.6 to 38.1 g of P/day and 6.0 to 12.1 g of P/day (increasing as animals grew). During phase II (the 4th gestation), half of the animals from each group were maintained with the same respective diets, and the other half were given 5.1 to 6.6 g of P/day. Within 6 months, animals given 5.1 to 6.6 g of P/day gradually developed signs of P deficiency. Clinical signs of deficiency and lesions included general unthriftiness, body weight loss, reduced feed consumption, reluctance to move, abnormal stance, spontaneous bone fractures, and finally, impaired reproductive performance. Cows given 7.8 to 8.9 g of P/day in diet 2 during phase II did not develop discernible clinical signs of P deficiency. Cows that were given 5.1 to 6.6 g of P/day apparently regained their health and reproductive capability when fed 11.7 to 12.6 or 17.1 to 20.5 g of P/day. The data indicate that bones serve as an effective storage source of P and support and buffer body needs until there is serious depletion of skeletal P. These results indicate that common recommendations for dietary P, such as those outlined by the National Research Council (17.5 g of P/day over the entire year for cows weighing 450 kg), exceed the basic requirements for beef cattle; 12 g of P/day is adequate for 450-kg beef cows.