Influence of protein concentration in prefast and postfast diets on feed intake of steers and nitrogen and phosphorus metabolism of lambs.

Two trials were conducted to determine the influence of prefast and postfast dietary protein concentration on feed intake of steers (avg wt 250 kg) and the N and P metabolism of lambs (avg wt 50 kg). In Trial 1, crossbred steers were fed prefast diets containing 8, 12 or 16% crude protein (CP) for 14 d before being deprived of feed and water for 24 h. Next, they received feed for 24 h and again were deprived for 48 h. Finally, they were fed diets containing either 10 or 15% CP. Steers fed the 16% CP prefast diet that were switched to the 10% CP realimentation diet tended to have lower feed intakes than the other fasted groups. In Trial 2, 16 crossbred lambs housed in metabolism stalls were fed prefast and postfast diets containing either 11 or 16% CP both before and after a 72-h fast in a 2 x 2 factorial arrangement. Nitrogen and P balance during realimentation tended to be affected by both the prefast and postfast dietary protein concentration. Results of this study indicate that CP content of the postfast diet alters repletion of nutrients lost during a feed and water deprivation period and can affect realimentation feed intake. However, the magnitude of this effect was dependent on protein content of the prefast diet.