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.