PS10-15. Nitrogen Balance in Growing Pigs Fed Diets Containing Varying Levels of Standardized Ileal Digestible Threonine and Soybean Hulls.

Abstract Fiber-rich feedstuffs, including soybean hulls (SBH), can reduce nutrient digestibility and increase endogenous losses of threonine (Thr), resulting in a greater Thr requirement for protein deposition. A nitrogen (N)-balance study was conducted to determine whether supplementing standardized ileal digestible (SID) Thr above the estimated NRC (2012) requirements would improve body protein deposition when pigs are fed diets with SBH. Four growing barrows (Large White x Landrace x Duroc; 37.73 ± 1.22 kg initial BW) were allotted to a 4 × 4 Latin square design with 4 diets and 4 periods. Corn-soybean meal-based diets were used as treatments, which were arranged in a 2 × 2 factorial with 2 levels of SID Thr (0.59 or 0.70%; 100 and 119% of NRC requirements, respectively) and 2 levels of SBH (0 or 15%). Each period lasted 12 d, which comprised 7 d of adaptation to the dietary treatment followed by 5 d of total fecal and urine collection. Daily feed allowance during the collection periods was calculated based on metabolic BW (BW0.75). Feed was offered daily in 2 meals, with 60% provided in the morning (0730 h) and 40% in the afternoon (1600 h), with ad libitum access to water. All data were analyzed using the MIXED procedure of SAS. No SBH × SID Thr interactions were observed for N intake, fecal N output, and apparent total tract digestibility (ATTD) of N. Regardless of SID Thr level, pigs fed SBH diets had greater N intake (P = 0.01) and fecal N output (P < 0.001), but lower ATTD of N (P = 0.001) compared with pigs fed diets without SBH. There was a SBH × SID Thr interaction (P = 0.03) for urinary N output, such that N output in the urine was decreased (P < 0.05) in diets with no added SBH combined with 0.70% SID Thr level. There were no SBH or SID Thr main effects or interactions for N absorbed, N retained, and protein deposition. In conclusion, combining the effects of dietary SBH inclusion and SID Thr level at 119% of NRC-estimated requirements did not increase protein deposition in growing pigs and further indicated that feeding SBH reduced N digestibility without compromising N retention and protein deposition efficiency.