Growth, immunoresponsiveness, and disease resistance of diverse stocks of chickens reared under two nutritional regimens

Growth, immunocompetence, and disease resistance were measured in a commercial broiler stock (BC), a commercial white-egg-layer stock (LC), and a White Plymouth Rock line selected for high juvenile BW (HW) under dietary regimes differing in protein and energy content. Diet E had 20% more protein and 17% more ME than Diet A. Stock by sex interactions were significant for BW at 28 and 36 d of age because sexual dimorphism occurred at younger ages in the meat than in the layer stocks. When inoculated with 0.1 mL of a 0.25 or 2.50% suspension of SRBC, diet by stock interactions were significant for antibody titers 6 d postinoculation (PI) at the lower dosage. Interactions were not present at the higher dosage or for either dosage 13 d PI. Diet by stock interactions were significant at both dosages for change in SRBC titers from 6 to 13 d PI. The interactions resulted from a significantly smaller decline in BC than HW or LC chicks fed Diet A, whereas all stocks responded similarly when fed Diet E. A significant decline in rate of BW gain was observed in chicks inoculated with marble spleen disease virus (MSDV), with the increase in relative weight of spleen significantly greater for LC than HW or BC chicks 6 d PI. When heterophil to lymphocyte ratios were used as an assay of response to MSDV challenge, ratios were significantly higher for HW chicks fed Diet A than for chicks fed Diet E and for LC chicks fed Diet E than for chicks fed Diet A, but not different for BC chicks on the two diets