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.  An experiment was conducted to determine the effects of dietary SBM concentration on the growth performance and immune response of pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV).  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 in disease containment chambers and provided a common diet for 1 wk before being allotted to 4 treatment groups (n = 16).  Pigs were fed experimental diets for 1 wk before receiving either a sham inoculation (sterile PBS) or a 1 × 10⁵ 50% tissue culture infective dose of PRRSV at 35 d of age (0 d postinoculation, 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 differential complete 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 0 to 14 DPI.  In the uninfected group, growth performance of pigs fed LSBM was generally greater than pigs fed HSBM.  However, in the PRRSV-infected group, pigs fed HSBM had improved (P < 0.05) ADG compared with pigs fed LSBM from 7 to 14 DPI.  Average daily gain of pigs fed LSBM and HSBM from 0 to 14 DPI was 608 and 576 g/d for uninfected pigs, and 314 and 374 g/d for PRRSV-infected pigs, respectively.  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 than in pigs fed LSBM.  In conclusion, increasing the dietary SBM concentration modulated the immune response and improved the growth performance of weanling pigs during a PRRSV infection.

Keywords: pig, PRRS, soybean meal