Nitrogen metabolism in genetically fat and lean chickens.

Two experiments were conducted to investigate the effect of dietary protein content (170 to 230 g/kg in Experiment 1 and 131 to 251 g/kg in Experiment 2) and initial growth rate, estimated from live body weight at 4 wk of age, on growth rate from 4 to 7 wk of age, nitrogen retention, energy metabolism, and amino acid catabolism in genetically fat (FL) and lean (LL) lines of chickens. There was no difference between lines in energy utilization either in metabolizability or in expenditure (basal metabolic rate, maintenance, or diet-induced thermogenesis). The only divergence between lines was in the partition of a similar amount of retained energy between lipid and protein deposition. In both experiments, LL chickens showed greater protein retention efficiency than FL birds; moreover, the LL line did not appear more sensitive to low dietary protein contents than the FL line. Selecting birds on their growth rate or live body weight at 4 wk of age resulted in the selection of different rates of fattening. Indeed, the slow-growing FL and LL chickens differed less in their nitrogen metabolism than did fast-growing birds, which were also fatter. The lower protein retention efficiency observed in FL chickens was related to an increase of dietary amino acid degradation as revealed by a greater rate of uric acid excretion in the fed state.