PROTEIN AND ENERGY RELATIONSHIPS IN THE BROILER CHICKEN .1. EFFECTS OF PROTEIN QUANTITY AND QUALITY ON METABOLISM

Male broiler chickens growing from 7 to 35 d were fed on a diet contai ning 150 g crude protein (N x 6.25)/kg diet supplemented with lysine t o equal that in diets containing 166, 183 and 200 g crude protein/kg d iet (Expt 1). A second group of male broiler chickens growing over the same period were fed on a diet containing 120 g crude protein/kg supp lemented with lysine, arginine, tryptophan, threonine and isoleucine e qual to that in diets containing 144, 172 and 200 g crude protein/kg d iet (Expt 2). Growth was improved by lysine supplementation but not to the level attained by feeding 200 g crude protein/kg (Expt 1). Lysine , arginine, tryptophan, threonine and isoleucine supplementation of a low-protein diet also improved growth, but growth again fell short of that attained by feeding a diet containing 200 g crude protein/kg. Pla sma insulin-like growth factor-1 and thyroxine concentrations increase d and triiodothyronine decreased as the crude protein level increased from 150 to 200 g/kg diet. Supplemental lysine did not affect plasma l evels of these hormones. Although dietary crude protein levels noticea bly changed rates of in vitro lipogenesis, changing either the level o f a single limiting amino acid or the levels of several limiting amino acids did not change lipogenesis.