PSXIII-3 Effects of supplementing threonine above the nutritional requirements in diets with soybean hulls on growth performance and digesta transit of growing pigs.

Abstract Soybean hulls (SBH) are a low-cost, abundantly available fiber-rich coproduct used in pig diets to reduce feed costs and environmental footprints, thus contributing to more sustainable swine production. However, fiber-rich diets can increase endogenous losses of threonine (Thr), thereby influencing the requirements for standardized ileal digestible (SID) Thr in the diet. A total of 64 barrows (Large White x Landrace x Duroc; 28.3 ± 3.19 kg initial BW) were used in a 28-d growth trial to evaluate the effects of diets at 100 or 119% of the National Research Council (NRC, 2012) SID Thr recommended levels with or without SBH on growth performance and total retention time of digesta in the gastrointestinal tract. Pigs were blocked by initial BW and randomly assigned to 1 of 4 treatments with 2 pigs per pen and 8 replicate pens per treatment. Dietary treatments were arranged as a 2 × 2 factorial with two levels of SBH (0 or 15%) and two levels of SID Thr (0.59 or 0.70%; 100 and 119% of NRC requirements, respectively) in corn-soybean meal-based diets. Pig had ad libitum access to feed and water. Pig BW and pen feed intake were recorded at 14-d intervals. Total tract retention time was measured by adding 0.1% ferric oxide in the morning feed on d 24 and recording the time of appearance of red-colored feces. All data were analyzed using the MIXED procedure of SAS. There were no SBH × SID Thr significant interactions for BW, ADG, and ADFI. Pig growth was not affected by the dietary levels of SID Thr. Pigs fed SBH-diets had decreased (P = 0.04) ADFI from d 1 to 14 and tended to have decreased ADFI from d 15 to 28 (P = 0.09) and overall (P = 0.06). There were significant interactions between SBH and SID Thr levels (P < 0.02) for G:F from d 1 to 14, d 15 to 28, and overall, but no differences between SID Thr levels were detected within each SBH level. From d 1 to 14, within both SID Thr levels, pigs fed 15% SBH had greater G:F than pigs fed diets without SBH. Conversely, from d 15 to 28 and overall, pigs fed 15% SBH had greater G:F than those not receiving SBH when SID Thr level was 0.70%, but no SBH effects were observed when SID Thr was fed at 0.59%. There were no SBH or SID Thr main effects or interactions for total-tract retention time. In conclusion, increasing SID Thr above the current NRC requirements was only justified when 15% SBH was added to growing pig diets, as including 0.70% SID Thr to diets without SBH caused adverse effects on feed efficiency.