Effects of dietary soybean meal concentration on growth performance and immune response of pigs during a porcine reproductive and respiratory syndrome virus challenge

Abstract Text: Soybean meal (SBM) contains naturally-occurring isoflavones that may exert anti-viral activity.  Thus, the effects of dietary SBM concentration on the growth performance and immune response of pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV) were evaluated.  Four experimental treatments included a 2 × 2 factorial arrangement of 2 dietary SBM concentrations, 17.5% (LSBM) and 29% (HSBM), and 2 PRRSV infection states, uninfected and PRRSV-infected.  The 2 experimental diets were formulated to be isocaloric and contain similar digestible concentrations of Lys, Met, Trp, Thr, and Val.  Total isoflavone contents of the LSBM and HSBM diets were 700 and 1,246 mg/kg, respectively.  Weanling pigs (32 barrows and 32 gilts, 7.14 ± 0.54 kg) were individually housed and provided a common diet for 1 wk before being allotted to 4 treatment groups (n = 16).  After receiving experimental diets for 1 wk, pigs received a sham inoculation (sterile PBS) or a 1 × 10⁵ 50% tissue culture infective dose of PRRSV at 35 d of age (0 d post-inoculation, DPI).  Growth performance was recorded weekly, and rectal temperatures were measured daily beginning on 0 DPI.  Blood was collected on 0, 3, 7, and 14 DPI for determination of blood cell counts, serum PRRSV load, and haptoglobin and cytokine concentrations.  Infection with PRRSV increased (P < 0.01) rectal temperatures and suppressed (P < 0.01) the growth of pigs.  From 7 to 14 DPI, ADG of pigs fed LSBM was greater (P < 0.05) than that of pigs fed HSBM in the uninfected group, but in the PRRSV-infected group, ADG of pigs fed HSBM was greater (P < 0.05) than that of pigs fed LSBM.  Overall (0 to 14 DPI) ADG was similar for pigs fed HSBM (576 g/d) and LSBM (608 g/d) in the uninfected group, and tended to be greater (P = 0.06) for pigs fed HSBM (374 g/d) than for pigs fed LSBM (314 g/d) in the PRRSV-infected group.  At 14 DPI, PRRSV-infected pigs fed HSBM had higher (P < 0.05) hematocrit values and a tendency for lower (P = 0.06) serum PRRSV load compared with pigs fed LSBM.  Serum haptoglobin and tumor necrosis factor-α concentrations were lower (P< 0.05) at 3 and 14 DPI, respectively, in PRRSV-infected pigs fed HSBM compared with LSBM.  In conclusion, increasing the dietary SBM concentration modulated the immune response and tended to improve the growth performance of pigs during a PRRSV infection.

Keywords: pig, PRRS, soybean meal