EFFECT OF DIETARY-PROTEIN QUALITY ON ENERGY-METABOLISM IN GROWING CHICKENS

A slaughter experiment was carried out to study the effect of dietary protein quality on maintenance energy requirements and energy costs fo r protein accretion and fat deposition in fast-growing broiler-type ma le chickens. Three isonitrogenous (200 g crude protein/kg DM) acid iso energetic (14 kJ metabolizable energy (ME)/g DM) semipurified diets ba sed on soyabean meal unsupplemented (diet S) or supplemented with 20 g L-lysine/kg (diet SL) or 2 g DL-methionine/kg (diet SM), in order to promote a decrease or an increase in growth rate respectively, were se lected and given at four feeding levels (ad lib, or restricted to 40, 28 and 18 g DM/d, on average) to 10-d-old fast-growing male broiler-ty pe chicks for 2 weeks. Both the efficiency with which ME was used to s upport growth (k(g)) and the maintenance requirements (ME(m)) signific antly decreased inversely to the biological value of the dietary prote in (k(g) = 0.660, 0.600 and 0.572; ME(m) = 597, 522 and 464 kJ/kg W-0. 75 per d, for diets SL, S and SM respectively). The partial efficienci es of use of ME for protein accretion (k(f)) or fat deposition (k(i)) were also inversely related, the former increasing with the quality of the protein offered. An alternative procedure was used to try to over come the statistical problems inherent in the partition of ME between fat and protein.