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
amino acid balance on the response to dietary sulphur-containing
amino acids (SAA) in terms of performance and carcase quality. 2. In experiment 1, 5 graded amounts of a DL-
methionine and
L-cysteine (1:1 by weight) mixture were added to basal diets containing 197 or 233 g crude
protein/kg. The diets containing 197
g protein/kg were fed with or without the further addition of 36 g crude
protein/kg from nonessential
amino acids. The
amino acid balance of all diets was kept constant for all
essential amino acids except the SAA. In experiment 2, 5 graded amounts of SAA from either a crystalline source (DL-
methionine or a mixture of DL-
methionine and
L-cysteine) or from intact
proteins were added to a diet containing
208 g protein/kg. 3. At each
protein concentration there were significant responses to the SAA addition in
weight gain, food conversion efficiency, and carcase quality. Non-linear exponential regression analyses were used to describe bird responses to SAA concentration. The broiler chick's requirement for SAA increased with increasing
dietary protein concentrations ranging from 197 to 259
g protein/kg. 4. The utilisation of SAA differed also with differences in origin (crystalline or
peptide-bound), and
methionine:
cysteine balances. Compared to DL-
methionine, a 1:1 mixture of DL-
methionine and
L-cysteine was only 81% or 86% as effective in supporting growth or food conversion, respectively. SAA from added
protein was even less effectively utilised. 5. The addition of nonessential
amino acids tended to decrease food intake without affecting SAA utilisation. 6. Slaughter yield and breast meat yield were clearly increased while fat deposition was clearly decreased, by SAA addition. The response in breast meat yield suggested an important economic benefit for further meat processing.
Nitrogen retention was significantly enhanced by SAA supplementation from crystalline sources, and this led to reductions of up to 30% in the amount of
nitrogen excreted per kg
weight gain.