DL-Methionine is as efficacious as L-methionine, but modest L-cystine excesses are anorexigenic in sulfur amino acid-deficient purified and practical-type diets fed to chicks.

Three bioassays were conducted to elucidate the effects of dietary cystine on the efficacy of L-Met and DL-Met fed to chicks. In assay 1, a purified diet markedly deficient in Met (0.12%) and cyst(e)ine (0.05%) was used to compare the relative effectiveness of L-Met and DL-Met in the presence of graded levels of dietary cystine. Addition of 0.05% Met improved (P < 0.01) weight gain when added to diets with 0 or 0.07% added L-cystine, but weight gain decreased linearly (P < 0.01) with greater cystine supplementation up to 0.35%, regardless of Met supplementation. There were no differences in growth performance due to supplementation of L-Met vs. DL-Met. In assay 2, a corn-soybean meal-peanut meal diet (0.25% Met and 0.25% cyst(e)ine) was supplemented with 0, 0.025, 0.05, or 0.075% L-Met plus 0 or 0.2% added L-cystine. Supplemental Met improved (P < 0.01) growth performance, but weight gain and feed intake were depressed (P < 0.01) by cystine supplementation. Whereas 0.2% added L-cystine depressed feed intake 6.9%, weight gain was reduced only 3.6%. Thus, cystine supplementation actually improved (P < 0.01) gain:feed. In assay 3, the corn-soybean meal-peanut meal diet was supplemented with 0 or 0.03% L-Met or DL-Met in the absence or presence of 0.2% added L-cystine. Again, Met supplementation improved (P < 0.01) growth performance, whereas supplemental cystine reduced (P < 0.05) weight gain and feed intake, but increased (P < 0.01) gain:feed. From these bioassays, it may be concluded that there is no evidence to suggest differences in effectiveness between L-Met and DL-Met in purified or practical-type low-protein diets of varying sulfur amino acid (SAA) content fed to chicks from 8 to 20 d of age. However, supplemental cystine has a negative impact on voluntary feed intake when supplemented in diets markedly deficient in SAA. This effect is evident with minimal SAA intake and when greater than 50% of SAA intake is provided by cyst(e)ine, rather than Met.