Replacement of soybean meal in diets for growing pigs with corn and synthetic amino acids results in reduced energy and nitrogen digestibility and reduced daily nitrogen retention, but metabolizable energy is not changed

The objective was to test the hypothesis that feeding intact protein from soybean meal (SBM) to growing pigs instead of a combination of corn, SBM and synthetic amino acids (AA) results in greater nitrogen retention and digestible energy (DE) without affecting metabolizable energy (ME) in the diet. A control corn-SBM diet and 3 diets, in which the inclusion of SBM was reduced and 3, 4, or 5 synthetic AA (i.e., Lys, Met, Thr, Trp, and Val) were added, were formulated. The concentration of standardized ileal digestible indispensable AA was at or above requirements for growing pigs in all diets, but dietary crude protein was reduced as synthetic AA were added. Forty pigs (initial body weight: 20.5 ± 2.4 kg) were allotted to the 4 diets using a randomized complete block design with 2 blocks of 20 pigs and 5 pigs per diet in each block. Pigs were housed in metabolism crates containing fully slatted floors, screens, and urine pans that allowed for quantitative collection of feces and urine for 4 d after 5 d of adaptation. Samples of diets, feces, and urine were analyzed for dry matter (DM), gross energy (GE), and nitrogen. The statistical model included diet as the fixed effect and block and replicate within the block as random effects. Polynomial contrast coefficients were used to determine linear and quadratic effects of reducing dietary protein. Results indicated that apparent total tract digestibility (ATTD) of DM was reduced (quadratic, P = 0.027), and ATTD of GE also was reduced (linear, P = 0.046) as SBM inclusion was reduced in diets. Absorbed nitrogen, retained nitrogen (g/d), and ATTD of nitrogen were reduced (linear, P < 0.001) as SBM inclusion decreased, but retention of nitrogen, calculated as percent of intake or percent of absorbed nitrogen, increased (linear, P < 0.001) as dietary SBM was reduced. The DE in diets decreased (linear, P = 0.007) as SBM inclusion was reduced, whereas SBM inclusion had no effect on ME. Reducing SBM inclusion tended to increase (quadratic, P = 0.096) ME to GE ratio and increased (linear, P = 0.008) ME to DE ratio. In conclusion, diets containing intact protein from SBM had greater ATTD of GE and nitrogen, and greater DE, whereas ME was not changed when compared with diets containing synthetic AA. Daily protein retention decreased when synthetic AA rather than SBM were used to furnish the digestible AA in diets.