Effects of Dietary Excess Tyrosine and Level of Feed Intake on Nitrogen and Energy Utilization in Chicks

Abstract

The present study using 8-day-old White Leghorn male chicks, was made to determine whether retarded growth of chicks fed a tyrosine excess (Tx) diet ad lib was due to the depression in feed intake or the reduced efficiency of feed utilization. A Tx diet was prepared by supplementing a 20% crude protein basal diet with 5% L-tyrosine. The levels of feed intake prepared for 10 days were as follows: 160 (corresponding to the amounts of the basal diet the chick would consume ad lib), 140, 120 (corresponding to the amounts of the Tx diet the chick would consume ad lib), 100, and 80 g per bird. Body weight gain, gainrfeed ratio, and energy retention rate of chicks fed the basal or Tx diet decreased proportionally as the level of feed intake was decreased. The addition of Tx to the basal diet significantly reduced the body weight gain, gain:feed ratio, metabolizable energy (ME) value, and nitrogen and energy retention rates at all levels of feed intake. Body composition was not significantly affected by the addition of Tx to the basal diet at all levels of feed intake. The plasma free tyrosine concentration was markedly increased by the addition of Tx to the basal diet and also with increments in feed intake. Dietary Tx per se had a decreasing effect on body weight gain, efficiencies of dietary nitrogen, energy utilization, and ME value. The growth retardation of chicks fed a Tx diet ad lib was due to the combined effects of the depression in feed intake and the reduced efficiency of feed utilization.