EFFECTS OF DIETARY-PROTEIN CONTENT, ADDITION OF NONESSENTIAL AMINO-ACIDS AND DIETARY METHIONINE TO CYSTEINE BALANCE ON RESPONSES TO DIETARYSULFUR-CONTAINING AMINO-ACIDS IN BROILERS

1. Two experiments were conducted with male broiler chicks from 2 to 5 weeks of age to determine the effect of dietary protein content and a mino acid balance on the response to dietary sulphur-containing amino acids (SAA) in terms of performance and carcase quality. 2. In experim ent 1, 5 graded amounts of a DL-methionine and L-cysteine (1:1 by weig ht) mixture were added to basal diets containing 197 or 233 g crude pr otein/kg. The diets containing 197 g protein/kg were fed with or witho ut the further addition of 36 g crude protein/kg from nonessential ami no acids. The amino acid balance of all diets was kept constant for al l essential amino acids except the SAA. In experiment 2, 5 graded amou nts of SAA from either a crystalline source (DL-methionine or a mixtur e of DL-methionine and L-cysteine) or from intact proteins were added to a diet containing 208 g protein/kg. 3. At each protein concentratio n there were significant responses to the SAA addition in weight gain, food conversion efficiency, and carcase quality. Non-linear exponenti al repression analyses were used to describe bird responses to SAA con centration. The broiler chick's requirement for SA increased with incr easing dietary protein concentrations ranging from 197 to 259 g protei n/kg. 4. The utilisation of SAA differed also with differences in orig in (crystalline or peptide-bound), and methionine:cysteine balances. C ompared to DL-methionine, a 1:1 mixture of DL-methionine and L-cystein e was only 81% or 86% as effective in supporting growth or food conver sion, respectively. SAA from added protein was even less effectively u tilised. 5. The addition of nonessential amino acids tended to decreas e food intake without affecting SAA utilisation. 6. Slaughter yield an d breast meat yield were clearly increased while fat deposition was cl early decreased, by SAA addition. The response in breast meat yield su ggested an important economic benefit for further meat processing. Nit rogen retention was significantly enhanced by SAA supplementation from crystalline sources, and this led to reductions of up to 30% in the a mount of nitrogen excreted per kg weight gain.