Maintenance sulfur amino acid requirements of young chicks and efficiency of their use for accretion of whole-body sulfur amino acids and protein.

Peterson x Hubbard male chicks were used in two bioassays conducted to determine the maintenance requirement and efficiency of utilization of dietary Met and Cys in young chicks. In each assay, chicks were given free access for 10 d to crystalline amino acid (AA) diets containing graded levels of DL-Met (Assay 1) or graded equal levels of DL-Met and L-Cys (Assay 2). Doses of Met represented 5, 10, 40, 55, 70, and 95% of its ideal level in Assay 1, with all other AA maintained at minimized excess levels that were 15% (of ideal) above the various doses of Met, except for Cys, which was maintained at 100% of its ideal level for all treatments. For example, when Met was fed at 40% of its ideal level, all other AA were fed at 55% of their ideal levels, and Cys was fed at 100%. In Assay 2, Met and Cys were fed at equal levels representing 5, 10, 40, 55, 70, and 95% of ideal with all other AA maintained at minimized excess levels that were 15% (of ideal) above the various doses of Met + Cys. After 24 h of feed deprivation, chicks were killed for whole-body protein and AA analysis. In Assay 1, Met accretion was a linear (P < 0.01) function of Met intake (r2 = 0.97). The slope of the Met accretion regression line indicated that 68% of the crystalline Met ingested (above maintenance) was retained. In Assay 2, increases in whole-body protein and whole-body TSAA were linear (P < 0.01) between dosage levels of 5 and 70% of the ideal TSAA level. Slope of the TSAA accretion line between these dose levels indicated that 52% of the TSAA was retained. The TSAA requirement for zero protein accretion was calculated to be 3.2 mg/d or 9.4 mg/d per kg3/4, whereas the TSAA required for zero TSAA accretion was 5.3 mg/d or 15.3 mg/d per kg3/4.