Effect of Protein Source (Casein vs. Soy) on Growth in Antigen Exposed Pigs

and Implications Thirty sets of four littermate pigs were reared via a segregated early weaning (SEW) scheme, individually penned, and allowed to consume one of four diets ad libitum from body weights of 6 to 25 kg. The dietary protein sources (29% total diet) consisted of casein-starch (Cas) and isolated soy (Soy) that provided, respectively, 100/0, 66/33, 33/66, and 0/100% of the digestible lysine in the supplemental protein mix in the four diets. The Cas and Soy contained 0 and 604 ppm of isoflavones, an inhibitor of cytokine synthesis and immune cell reactivity. Eight days post-allotment, pigs in one-half of the litters were exposed to an antigen, 1ml of 10 attenuated porcine reproductive and respiratory syndrome (PRRS) virus cells administered intramuscularly. Exposure to the nonreplicating, attenuated virus resulted in elevated serum acute phase protein (AGP) concentrations (581 vs. 668 μg/mL) pooled across time. The magnitude of the response was greater in the initial 12 days of the study. Antigen exposure (AE) also resulted in lower daily gains (665 vs. 644 g) and gain:feed ratios (.719 vs. .713) over the duration of the study. As dietary Soy content increased, serum concentrations of the AGP in the AE pigs were decreased (669, 722, 637, 626 μg/ml). Similarly, greater Soy inclusion depressed gain:feed ratios in the AE pigs (.730, .731, .699, .693), but not in the non-AE pigs (.708, .728, .723, .716), resulting in a protein source by AE interaction. Greater Soy inclusion did not alter daily gains in either AE group. Based on these data, the response of pigs to dietary protein source (Cas vs. Soy) is dependent on the pig’s level of antigen exposure.