Microbial phytase does not improve protein-amino acid utilization in soybean meal fed to young chickens

Three growth trials were conducted with young chicks to evaluate crude prot ein (CP) utilization in soybean meal (SBM) as affected by dietary addition of microbial phytase. In assay 1, chicks were fed two CP-deficient (50 or 1 50 g CP/kg) levels of dehulled SBM, and each SBM level was then supplemente d with equimolar amounts of cystine or methionine (Met) or with 1200 U phyt ase/kg. At 50 g CP/kg, cystine or Met supplementation improved (P < 0.05) m easures of growth performance, but when 150 g CP/kg from SBM was fed, only Met addition improved (P i 0.05) weight gain, food efficiency and protein e fficiency ratio (PER). Thus, Cys was more limiting than Met in the diet tha t contained 50 g CP/kg, but Met was clearly first-limiting in the diet that contained 150 g CP/kg. Phytase supplementation did not improve (P > 0.10) chick performance at either level of CP. Chicks in assay 2 were fed 100 g C P/kg furnished by SBM, casein or corn gluten meal in the absence and presen ce of 1200 U phytase/kg. Weight gain, gain/food and PER values were greater (P i 0.05) in chicks fed SBM than in those fed casein, and greater (P < 0. 05) in chicks fed casein than in those fed corn gluten meal. Phytase supple mentation had no effect (P > 0.10) on any measure of chick performance, reg ardless of the protein source fed. In assay 3, three deficient levels of CP (50, 100 and 150 g/kg) from SBM were fed in the absence and presence of 12 00 U dietary phytase/kg. Weight gain, food efficiency and protein accretion increased linearly (P < 0.05) as a function of protein intake, but phytase supplementation had no effect (P > 0.10) on slopes of the body weight and protein accretion curves. Likewise, phytase addition did not affect (P > 0. 10) measures of protein utilization, i.e., weight gain/protein intake and p rotein gain/protein intake at any of the CP levels that were fed. Because s ulfur amino acids are the growth-limiting factors when protein-deficient le vels of SBM are fed to young chicks, we conclude that dietary addition of p hytase does not improve sulfur amino acid utilization in SBM.