Branched-chain amino acids for growing cattle limit-fed soybean hull-baseddiets

Five ruminally cannulated Holstein steers (176 kg) were used in a 5 x 5 Lat in square to evaluate the effects of branched-chain AA supplementation on N retention and plasma AA concentrations of steers. Steers were limit-fed (3 .0 kg/d of DM) twice daily diets low in ruminally undegradable protein (12% soybean hulls, 19% alfalfa, 5% molasses, and 4% vitamins and minerals). Ac etate (400 g/d) was continuously infused into the rumen. Treatments were co ntinuous abomasal infusions of 1) 115 g/d of a mixture of 10 essential AA d esigned to exceed the steers' requirements (10AA), 2) 10AA with Leu removed , 3) 10AA with Ile removed, 4) 10AA with Val removed, and 5) 10AA with all three branched-chain AA removed. Experimental periods were 7 d, with 3 d fo r adaptation to treatments and 4 d for total fecal and urinary collections for N balance. Blood samples were collected 5 h after feeding on d 7. Retai ned N decreased in response to removal of Leu (P < 0.06), Val (P < 0.05), o r all three branched-chain AA (P < 0.05). Plasma Leu concentrations decreas ed (P < 0.05) in response to removal of Leu and all three branched-chain AA . Plasma Ile concentrations decreased (P < 0.05) in response to removal of Ile and all three branched-chain AA but increased (P < 0.05) in response to removal of Leu. Plasma Val concentrations decreased (P < 0.05) in response to: removal of Val and all three branched-chain AA but increased (P < 0.05 ) in response to removal of Leu. Responses in N balance and plasma AA conce ntrations of growing cattle limit-fed soybean hull-based diets demonstrate limitations in the basal supply of Leu and Val but not Ile provided that su pplies of all other essential AA are met.