Improved Vitamin D Status in Post-weaning Piglets Increased Calcium Digestibility and Modulated the Immune Response Independently of the Husbandry Conditions

Post-weaning period is one of the most stressful phases in a piglet’s life. Poor husbandry practices, such as dirty pens and suboptimal temperatures, can intensify stress, impair growth, and compromise gastrointestinal and immune function (Jayaraman & Nyachoti, 2017). Piglets are born with the lowest plasma 25-OH-D3 levels among livestock species, and vitamin D transfer via colostrum and milk is insufficient to meet their needs (Matte & Auder, 2020). Beyond its role in mineral metabolism, the active form of vitamin D (1,25-dihydroxyvitamin D) modulates immune responses and contributes to robustness. This study evaluated the impact of 35-day supplementation with either vitamin D3 or 25-OH-D3 in weaned piglets raised under two husbandry conditions. A total of 384 piglets (21 days old, 6.7 ± 0.6 kg) were assigned to a 2x2 factorial design with four treatments: T1: Control Husbandry conditions (CH; Clean pens and standard temperature: d0-14 at 24 °C, d15-28 at 22 °C, d 29-35 at 20ºC) + 2000IU/kg Vitamin D3; T2: CH + 50mcg 25-OH-D3; T3: Poor Husbandry condition (PH; Dirty pens and 4 °C lower temperature than CH in each period) + 2000IU/kg Vitamin D3; T4: PH + 50mcg 25-OH-D3. Body weight and feed intake were recorded on days 0, 14, and 35. Diarrhea scores and laying posture were monitored biweekly. Blood samples were collected in days 0, 12, and 32, and were analyzed for vitamin D status, hematology, and immune markers. Fecal samples collected day 32 were used to assess digestibility of dry matter, protein, calcium, and phosphorus. Data was analyzed using a 2 × 2 factorial design in JMP 19 (SAS Institute, Cary NC), applying ANOVA to assess main effects and interactions. No interaction between the 2 main variables was detected in any of the parameters studied. PH conditions reduced (P < 0.01) average daily gain and feed intake, increased feed conversion ratio in the pre-started period (d1-14), worsened diarrhea scores and lowered laying posture scores (P < 0.001) during the whole trial. PH also altered immune and bone biomarkers in plasma, including decreased eosinophils (P < 0.001) and increased albumin, IgA (P < 0.01), and calcium (P < 0.05). At day 12, ALT (P < 0.001), urea nitrogen (P < 0.01), and osteocalcin (P < 0.05) were reduced; at day 32, CTX1 decreased, and SOD increased (P < 0.05). Supplementation with 25-OH-D3 improved plasma vitamin D status (P < 0.001) and calcium digestibility (P < 0.02) compared to vitamin D3. IgG and hemoglobin tended to increase day 12 (P = 0.06), while IgG decreased (P < 0.01) and IgA tended to increase (P = 0.07) day 32. In conclusion, PH negatively affected growth performance, blood biomarkers indicative of health status, and bone development, triggering immune and antioxidant responses. Supplementation with 25-OH-D3 enhanced vitamin D status, calcium digestibility, and immune modulation, regardless of husbandry conditions.