Experiment one was a factorial arrangement of diets containing two levels of protein (P) (100% (HP) or 70% (LP) of National Research Council (1984) (N.R.C.)) and two levels of energy (E) (100% (HE) or 90% (LE) of N.R.C.), which were fed ad libitum to Nicholas toms from 10 days to 6 weeks (wks) of age. Subsequently, the four treatment groups were fed diets containing either 4% or 8% fat (F) to 20 wks of age. LP or LE reduced weight gain (WG) and increased feed/gain (F/G) until 6 weeks (P \u3c.005). After 6 wks of age, LP toms had consistently lower body weight (BW) and F/G than HP toms until 20 wks (P \u3c.005). In comparison, LE and HE toms had the same BW at 20 wks. WG increased and F/G decreased as the level of F increased (P \u3c.005), irrespective of P and E. Percentage carcass fat at 6 wks was increased by LP, and decreased by LE (P \u3c.005). LP also increased leg length relative to 6-wk BW and reduced breast meat yield. Carcass composition and structural proportionality of LP and LE toms were restored at 12 and 20 wks with the exception of breast yield, which was still adversely affected by LP at 12 wks. At 12 and 20 wks, % carcass fat increased as the level of F increased. Results show that toms responded more favorably to early P than E restriction, regardless of fat content of the realimentation diet;In experiment two, four levels of dietary protein (P) (100%, 80%, 70%, or 60% of N.R.C.) were fed to toms from 1-6 wks and subsequently the toms were fed according to N.R.C. until 20 wks of age. As the level of P decreased 6-wk BW decreased and 1-6 wk F/G increased. P did not influence 6-20-wk WG, but F/G was improved (P \u3c.005). Furthermore, 60-70% P reduced the incidence of severe leg weakness observed at 18 wks by more than 60%. Percentage carcass fat at 6 wks increased linearly and breast and thigh meat yields decreased as P decreased, but other carcass components were unaffected by P. Carcass composition and meat yield were restored to normal proportions at 12 and 20 wks, irrespective of the level of P; however, breast yield at 20 wks was reduced by 60% P. Results show that large toms do not exhibit compensatory WG, irrespective of the level of P; however, significant savings in feed costs and reduced incidence of leg weakness without adverse effects on carcass quality may be economically beneficial if toms are fed at 70-80% of N.R.C. protein recommendations from 1-6 wks of age;Reference. National Research Council. 1984. Nutrient Requirements of poultry. 8th ed. Natl. Acad. Sci., Washington, DC
