Abstract Two experiments were conducted to test the hypothesis that reducing the inclusion of soybean meal (SBM) and increasing crystalline amino acids (AA) in corn-SBM diets may impact nitrogen balance, growth performance, carcass traits, and immune responses of growing pigs, and that supplementation with soy isoflavones may improve the immune system. Six experimental diets were formulated. A high-protein diet containing no crystalline AA, a medium-protein diet containing less SBM with Lys, Met, and Thr, and four low-protein diets containing crystalline Lys, Met, Ile, Thr, Trp, and Val, in a 2 × 2 factorial arrangement with 0 or 0.4% soy isoflavones and 0 or 2.0% glutamic acid as an additional nitrogen source were formulated. In Exp. 1, a total of 240 pigs (initial weight: 9.65 ± 0.87 kg) were allotted to the six diets in a randomized complete block design with eight replicate pens per treatment and four pigs per pen. Pigs were fed in a 5-phase feeding program (11 to 125 kg), and growth performance, carcass characteristics, and blood parameters were determined. In Exp. 2, 60 pigs (initial weight: 17.8 ± 0.9 kg) were allotted to the six diets of phase 1 from Exp. 1, in a randomized complete block design and housed in metabolism crates for urine and fecal collection for 5 days. In both experiments, data were analyzed using the MIXED procedure of SAS with diet as the fixed effect and block as the random effect. Results of Exp. 1 indicated that average daily gain, average daily feed intake, and gain to feed ratio for the entire experimental period were not affected by diet (Table 1). Hot carcass weight was greater (P 0.05) for pigs fed the medium-protein diet compared with pigs fed the low-protein diet containing soy isoflavones and glutamate, but no differences were observed among the other diets. Carcass backfat tended to increase (linear, P = 0.075) in pigs fed the low protein dites. Plasma urea nitrogen decreased (linear, P 0.001) as protein was reduced. In Exp. 2, daily nitrogen intake, urinary nitrogen excretion, absorbed nitrogen, and retained nitrogen (g/d) were reduced (linear; P 0.001) as dietary protein decreased, but the digestibility of nitrogen was not affected (Table 2). In conclusion overall growth performance, immune responses, and gene abundance of gut-protective proteins were not affected by dietary protein level, but, the carcass was less lean, and backfat was thicker in pigs fed a diet containing less protein than in pigs fed the high-protein diet. Additionally, no effects of adding soy isoflavones or extra nitrogen on the performance of pigs were observed. Reducing dietary protein also reduced daily protein retention.
Ibagon et al. (Wed,) studied this question.
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