Effects of Dietary Glycine Supplementation on Reproductive Outcomes and Early Piglet Growth in Local Black Gilts: Associations with Placental Traits and Maternal Gut Microbiota–Metabolome Profiles

Dietary glycine may influence reproductive outcomes, placental development, and early piglet growth in gilts. Sixty local black gilts were randomly assigned to a basal diet (CON), an isonitrogenous control supplemented with 1.19% L-alanine (ALA), or 1.0% glycine (GLY; n = 20 per group) from 30 d before mating. After pregnancy confirmation, 10 pregnant gilts per treatment were randomly selected according to the prespecified design for follow-up to farrowing. Pregnancy rate differed among treatments (p = 0.004) and was higher in GLY than in CON. Compared with both controls, GLY had greater live-born litter weight at birth (p < 0.001), a lower proportion of low-birth-weight piglets (p = 0.010), and greater litter weight at 28 d (p < 0.001). GLY also had greater placental vascular densities and higher expression of KDR, NOS3, and several nutrient-transport-related genes. At gestation day 109, GLY was associated with treatment-related gut microbial differences and significant enrichment of fecal purine metabolism, with higher normalized abundances of putatively annotated adenosine, guanosine, and cGMP than in both controls. Overall, glycine supplementation was associated with improved selected reproductive and early piglet growth outcomes and altered placental and metabolic features.