INFLUENCE OF POSTFAST DIETARY CRUDE PROTEIN AND PHOSPHORUS-CONTENT ON NITROGEN, PHOSPHORUS, CALCIUM, AND MAGNESIUM REPLETION IN SHEEP

An experiment was conducted to determine the influence of postfast dietary CP and P concentration on the repletion of N, P, Ca, and Mg lost during a 3-d fast in sheep. Four Suffolk wether lambs averaging 35 kg were used in a 4 x 4 Latin square design. Lambs were fed a control diet (700 g/d; as-fed basis) for 14 d and were then deprived of feed and water for 3 d. Lambs were then fed one of four isoenergetic realimentation diets: 1) low CP/low P, 2) low CP/high P, 3) high CP/high P, and 4) high CP/very high P. Realimentation N and Mg intakes were 9.8 and 1.1 g/d for lambs fed the low-CP diet and 18.1 and 1.7 g/d for lambs fed the high-CP diets, respectively. Realimentation P intakes were 1.40, 2.36, 2.66, and 3.82 g/d for lambs fed Diets 1, 2, 3, and 4, respectively. Nitrogen, P, Ca, and Mg apparent digestibility and balance and serum urea N, free fatty acids, P, Ca, Mg, and alkaline phosphatase were determined during the prefast, fast, and realimentation periods. Lambs fed the high-CP diets had higher (P < .05) N and P digestibility and balance than lambs fed the the low-CP diet. Increasing the dietary P content did not affect (P > .15) P balance or digestibility. In general, the realimentation diet fed did not affect (P > .15) serum concentrations of free fatty acids, alkaline phosphatase, inorganic P, Ca, or Mg.