Feeding intact protein from soybean meal instead of corn and synthetic amino acids does not affect growth performance, carcass composition, blood cytokines, or mRNA abundance of intestinal amino acid transporters in growing pigs, but net energy tends to be greater in diets with soybean meal.

The objective was to test the hypothesis that reducing dietary crude protein in corn-soybean meal (SBM) diets will not increase dietary net energy (NE) and will not affect growth performance, carcass composition, nutrient deposition, intestinal morphology, blood cytokine concentrations, or the mRNA abundance of intestinal amino acid (AA) transporters. A corn-SBM-based diet was prepared, and three additional diets were formulated by reducing SBM inclusion, increasing corn, and adding three, four, or five synthetic AA (Lys, Met, Thr, Trp, and Val), resulting in diets with protein levels of 20.0, 16.4, 15.4, and 13.4%, respectively. All diets were formulated to meet requirements for standardized ileal digestible indispensable AA. A total of 176 pigs (initial weight: 32.2 ± 4.2 kg) were used. On day 1, 16 randomly chosen pigs were euthanized, and body nutrient composition was determined. The remaining 160 pigs were allotted to the four experimental diets with four pigs per pen and 10 replicate pens per diet. Diets were provided for 28 d. One pig per pen was slaughtered on day 29, and blood, carcass, and viscera were collected and analyzed for nitrogen, fat, and energy to calculate nutrient deposition. Samples of blood were also analyzed for total protein, albumin, plasma urea nitrogen, and cytokines. Samples of ileal mucosa, ileum and colon tissue, and ileum and colon digesta were collected, and tissue morphology, and mRNA abundance of AA transporters were determined. Results indicated that average daily gain, average daily feed intake, gain-to-feed ratio, carcass characteristics, and protein, lipid, and energy depositions were not affected by reducing dietary crude protein, but NE in diets tended to decrease (linear, P = 0.051). Blood urea nitrogen was reduced (linear, P < 0.001) as dietary protein levels were reduced, but blood total protein or albumin was not affected by dietary protein levels. Blood cytokines, jejunal and ileal morphology, ammonia in ileal and colon digesta, and mRNA abundance of AA transporters in ileal mucosa were also not affected by the treatments. Bacterial protein in colon digesta decreased (linear, P = 0.030) by reducing dietary protein levels. In conclusion, reducing dietary protein levels did not affect growth performance, carcass composition, nutrient deposition, intestinal morphology, blood cytokines, or mRNA abundance of AA transporters in growing pigs, but NE of diets tended to reduce as dietary protein levels were reduced.