NITROGEN-METABOLISM AND HORMONAL RESPONSES OF STEERS FED WHEAT SILAGEAND INFUSED WITH AMINO-ACIDS OR CASEIN

Four Holstein steers (159 kg) surgically fitted with abomasal-infusion cannulas were used in a 4 x 4 Latin square study to test amino acid ( AA) and casein (CAS) infusions on nitrogen balance and hormonal status of steers consuming vegetative wheat (Triticum aestivum L.) silage (1 2.3% CP). Treatments were 5-d infusions of 1) water (CONT), 2) arginin e (ARG; 13.69 g/d), 3) limiting amino acids (LAA, 13.69 g/d arginine 10.92 g/d histidine + 28.97 g/d lysine + 10.88 g/d methionine + 16.96 g/d threonine, and 4) Na-CAS (300 g/d). Whole blood was collected for plasma AA, growth hormone(GH), insulin, and IGF-l concentrations. Dat a were analyzed by ANOVA, and the following orthogonal contrasts were used to separate treatment means: CONT vs ARG; ARG vs LAA; and LAA vs CAS. Urinary N increased (P < .02) for CAS vs LAA. Arginine increased N retention, as did CAS, compared to LAA. Total plasma essential AA we re decreased by arginine. Mean plasma insulin concentrations were incr eased by CAS (P < .034). Arginine increased mean plasma GH levels, but not IGF-I. The CAS treatment increased (P < .015) IGF-I levels, but n ot GH. These data suggest that performance of steers fed wheat silage was limited by duodenal AA flow and that arginine was the first-limiti ng AA. Casein infusion increased plasma insulin and IGF-I, which would explain tl-ie improved growth noted in calves and lambs fed forages s upplemented with ruminally undergraded protein.