Effects of 25-Hydroxyvitamin D3 Combined with Phytase and Probiotic on Calcium–Phosphorus Metabolism, Bone Development, and Growth Performance in Weaned Piglets

Background/Objectives: Calcium–phosphorus metabolism is critical for skeletal development in weaned piglets. This study evaluated the effects of dietary 25-hydroxyvitamin D3 (25-OH-VD3) in combination with phytase and probiotics on mineral metabolism, bone development, and related molecular mechanisms in weaned piglets. Methods: Sixty 28-day-old weaned piglets (7.1 ± 1.30 kg) were randomly assigned to four dietary treatments for 31 days (including 3 days of acclimation): CON (basal diet + 50 µg/kg 25-OH-VD3), HI (CON + 50 mg/kg phytase), CY (CON +10 mg/kg probiotics), HICY (CON + 50 mg/kg phytase + 10 mg/kg probiotics). Apparent calcium digestibility, serum biochemical indices, bone mineral density (BMD), and mRNA and protein expression of calcium–phosphorus transport- and metabolism-related genes in jejunal mucosa and kidney were assessed. Results: Compared with CON, piglets in the HI, CY, and HICY groups showed higher apparent calcium digestibility (p < 0.05). Serum transforming growth factor-β was elevated in CY and HICY (p < 0.05). HI enhanced metatarsal and toe BMD (p < 0.05) and upregulated jejunal solute carrier family 34, member 2 (SLC34A2) and SLC34A3 mRNA expression (p < 0.05). In contrast, HICY reduced mRNA expression of transient receptor potential cation channel subfamily V member 6 and calcium-binding protein D28k, as well as of calcium-binding protein D9k and cytochrome P450 27B1 in the kidney (p < 0.05). Renal calcium-sensing receptor protein abundance increased in CY (p < 0.05). Conclusions: Supplementation of 25-OH-VD3 with phytase and/or probiotics improved calcium utilization and modulated key transport pathways, contributing to enhanced bone development in weaned piglets. These findings highlight coordinated nutritional regulation of mineral metabolism during early post-weaning growth.