Impact of dietary fiber and immune system stimulation on threonine requirement for protein deposition in growing pigs.

High dietary fiber (DF) and immune system stimulation (ISS) are thought to limit amino acid availability for protein deposition (PD) in growing pigs. A nitrogen-balance study was conducted to determine Thr requirement for optimal PD when DF and ISS were present alone and in combination. A total of 90 barrows (20.5 kg initial BW; SD = 0.75 kg) were randomly assigned to one of 10 dietary treatments (n = 9) in nine blocks. Diets consisted of a low fiber (LF; 12.5% total DF) or high fiber (HF; 18.5% total DF by adding 10% sugar beet pulp and 5% wheat bran to the LF diet) with graded levels of Thr (0.49%, 0.57%, 0.65%, 0.73%, and 0.81% standardized ileal digestible [SID]) fed at 2.2 × maintenance ME requirements. After an 8-d adaptation, two 4-d nitrogen balance collection periods (pre-ISS and ISS) were conducted. ISS was induced by repeated injections (i.m.) of increasing doses of Escherichia coli lipopolysaccharide. Blood samples were taken during both periods to assess acute phase proteins and complete blood cell count. Data were analyzed by PROC MIXED with fixed effects of period, Thr, fiber, and their interactions, with block as a random effect. Nitrogen balance was analyzed separately for each period. Threonine requirement was estimated using PROC NLIN quadratic break-point model. Serum concentration of albumin, haptoglobin, fibrinogen, whole blood white blood cell, and platelet count were affected by ISS (P < 0.001) confirming successful ISS. During pre-ISS, PD increased linearly (P < 0.01) as Thr concentration in the diet increased, with a significant interaction (P < 0.05) between fiber and Thr. During ISS, PD increased linearly (P < 0.05) as Thr concentration in the diet increased. Quadratic break-point model estimated SID Thr required to maximize PD of pigs fed LF and HF diets during pre-ISS period was 0.68% (R2 = 0.88) and 0.78% (R2 = 0.99), respectively. During ISS, the SID Thr requirement was estimated at 0.76% (R2 = 0.76) for LF diet and 0.72% (R2 = 0.95) for HF fed pigs. HF and ISS independently increased Thr requirement for maximum PD, but these effects were not additive. HF may therefore mask the effects of ISS on Thr requirement for immune response and PD.