Impact of dietary soybean hulls or withdrawal prior to market on growth performance, nutrient digestibility, and carcass characteristics of grow-finish pigs

In two experimental approaches, this study aimed to evaluate the effects of increasing levels of soybean hulls (SBH) (Exp. 1) and the withdrawal of high SBH from diets prior to marketing (Exp. 2) on growth performance, carcass characteristics, and apparent total tract digestibility (ATTD) of nutrients in grow-finish pigs. In Exp. 1 (n = 12 pens/treatment), 192 pigs (initial BW 21.99 ± 2.75 kg) were fed a corn-soybean meal-based diet containing 0% (0SBH), 7.5% (7.5SBH), 15% (15SBH), or 22.5% (22.5SBH) SBH. In Exp. 2 (n = 15 pens/treatment), 240 pigs (initial BW 26.5 ± 3.03 kg) were fed either 0SBH or 22.5SBH continuously until marketing (-0d) or switched from 22.5SBH to 0SBH either 27 (-27d) or 13 (-13d) days before marketing. Pig body weight (BW) and feed disappearance were measured to calculate average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G:F). A balance study was conducted on 32 gilts (n = 8 pigs/treatment) of Exp. 1 beginning on d 56 to determine ATTD of nutrients. Hot carcass weight (HCW), back fat depth (FD), loin depth (LD), and carcass yield were recorded at the processing plant. Data were analyzed using ANOVA, with linear and quadratic contrasts applied for Exp. 1. Pen served as the experimental unit for live performance and carcass characteristics and pig as the experimental unit for ATTD analyses in Exp. 1. In Exp. 1, increasing SBH levels linearly decreased G:F (<i>P</i> < 0.05) and tended to reduce ADG (<i>P</i> = 0.059), without affecting ADFI (<i>P</i> > 0.10), resulting in a 4 kg reduction of final BW (<i>P</i> = 0.067). Inclusion of SBH up to 22.5% decreased HCW, yield, FD, and LD (<i>P</i> < 0.05). In Exp. 2, although 22.5SBH had no effect on ADG and ADFI (<i>P</i> > 0.10), pigs fed 22.5SBH (-27d, -13d, or -0d) had a 6.5% reduction in G:F compared to 0SBH (<i>P</i> < 0.05). Pigs in the -13d and -0d treatment groups had reduced HCW compared to 0SBH (<i>P</i> < 0.05), while yield was reduced by 2% and 5% in the -27d and -0d groups, respectively (<i>P</i> < 0.05). Pigs fed 22.5SBH (-0d) had a 30.7% reduction in FD and a 6.4% reduction in LD compared to 0SBH (<i>P</i> < 0.05). These data suggest that increasing SBH levels in the diet of grow-finish pigs negatively affects feed efficiency, carcass weight and yield, fat and loin depth. However, carcass yield, HCW, FD, and LD loss may recover if pigs are transitioned back to a diet without soybean hulls, depending on the timing of withdrawal before marketing.