Enzymatic Soybean as Milk Protein Substitute in Formula for Ultra-Early-Weaning Piglets: Effects on Intestinal Health and Stress During Dietary Transition

Milk formulas for piglets predominantly rely on cow-derived proteins, which are costly and raise biosecurity concerns. As a highly digestible plant protein source, enzymatic soybean (ESB) may offer a viable alternative. This study evaluated the effects of partially replacing whole milk powder (WMP) with ESB in formulas on growth performance, intestinal health, and stress indicators of ultra-early-weaning piglets, followed through the nursery stage on the solid diet. Eighty-four male piglets, weaned on postnatal day 14 (PND 14), were randomly assigned to one of three isonitrogenous milk formulas until PND 24: a milk protein-based formula (MF) or the same formula in which ESB replaced either 20% (PF20) or 40% (PF40) of WMP. Eight piglets per group were sacrificed for sampling on PND 24. Thereafter, the remaining piglets from the MF and PF20 groups, together with twenty additional age-matched, sow-nursed (SN) piglets, received the same solid nursery diet from PND 24 to 52. The results indicated that both PF20 and PF40 piglets exhibited lower ADG, and ADFI than MF piglets, with PF40 piglets showing a greater reduction (approximately 70 g/d in ADG) and higher F/G ratios, along with lower carcass weight, fat percentage, and body mass index (p < 0.05). Moreover, PF40 piglets showed lower sucrase activity and higher mRNA expression of ileal inflammation- and immune-related genes on PND 24. During the nursery phase, intriguingly, MF and PF20 piglets exhibited higher ADG and lower diarrhea index between PND 24 and 31 compared with SN piglets (p < 0.05), which could be associated with lower plasma cortisol and C-reactive protein concentrations in MF and PF20 piglets after the dietary transition on PND 28, with plasma haptoglobin levels being lowest in the PF20 piglets. In conclusion, partial replacement of WMP with ESB in formulas induced early growth retardation and altered intestinal morphology and function in ultra-early weaning piglets. As compared with sow rearing, however, formula-fed piglets showed favorable performance during adaptation to solid nursery feed. Moreover, piglets fed the formula containing a moderate level of ESB exhibited lower stress responses, which indicates that ESB may serve as a feasible protein source in milk formulas for ultra-early-weaned piglets.