Efficacy of Cysteine in Replacing Methionine in the Immune Responses of Broiler Chicks

: Two trials were conducted to compare the effects of supplements of methionine and cysteine on the growth and immune responses of broiler chicks fed corn-soy diets. The basal diet contained 21% crude protein, 3,255 kcal metabolizable energy/kg diet, .35% methionine, .37% cysteine, and .13% choline. Additions to the basal diet were methionine (.063, .25, .85, and 1.45%), or cysteine (.203%), or a combination of methionine (.063%) and cysteine (.153%). Total antibody and 2-mercaptoethanol-resistant antibodies, immunoglobulin G (IgG), were determined in chicks inoculated intraperitoneally at 14 days of age and serially bled at 4, 7, and 10 days postinoculation. Thymus-derived (T)-cell-dependent in vivo mitogen response to phytohemagglutinin-P (PHA-P) was assessed via wing web swelling. The methionine requirement for growth (0 to 3 wk of age) was found to be no more than .413% of the diet (.35% in the basal diet plus .063% added). Addition of 1.45% methionine to the basal diet resulted in significant depression (P less than .05) in growth. The antibody responses generally peaked at 7 days postprimary inoculation. Both methionine and cystine supplementation at low levels resulted in improvement in the cell mediated PHA-P responses as well as in the IgG (T-cell-dependent) responses. High supplemental methionine (1.45%), however, caused significant (P less than .05) depressions in both responses. Equimolar additions of methionine and cysteine (16.8 mmol/kg diet) showed that cysteine was about 84 and 70% as efficacious as methionine in the IgG and the PHA-P stimulation (PHA-I), respectively, in healthy chicks.(ABSTRACT TRUNCATED AT 250 WORDS)