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 dete rmine the maintenance requirement and efficiency of utilization of dietary Met and Cys in young chicks. In each assay, chicks were given free access f or 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 maintaine d 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 m aintained at minimized excess levels that were 15% (of ideal) above the var ious 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 li near (P < 0.01) function of Met intake (r(2) = 0.97). The slope of the Met accretion regression line indicated that 68% of the crystalline Met ingeste d (above maintenance) was retained. In Assay 2, increases in whole-body pro tein 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 t hese dose levels indicated that 52% of the TSAA was retained. The TSAA requ irement for zero protein accretion was calculated to be 3.2 mg/d or 9.4 mg/ d per kg(3/4), whereas the TSAA required for zero TSAA accretion was 5.3 mg /d or 15.3 mg/d per kg(3/4).