Dietary Calcium-to-phosphorus Ratio and Vitamin 25-OH-D3 Effects on Growth and Bone Health in Developing Gilts

The objective of this study was to evaluate two calcium-to-phosphorus ratios and two levels of vitamin 25-OH-D3 on bone health in replacement gilts. In a randomized complete block design, 432 Duroc × Whiteline (Landrace and Large White composite) gilts housed 9 per pen were randomly allocated to be fed one of four diets from 100 to 172 days of age with 12 replicate pens per treatment. The corn-soybean meal diets consisted of two levels of 25-OH-D3 supplementation (50 μg (2,000 IU) 25-OH-D3/kg as HyD®, DSM Nutritional Products or no HyD®) and two calcium-to-phosphorus (Ca/P) ratios (1.5 STTD Ca/STTD P or 2.4 STTD Ca/STTD P). All diets contained 800 IU/kg vitamin D3, 0.28% STTD P, and no phytase. The gilts were housed in a curtain-sided, naturally ventilated barn on slatted concrete flooring and had ad libitum access to feed and water. All gilts were individually weighed at 100 and 172 days of age, and ADG was calculated on a pen basis. To minimize variation in vitamin 25-OH-D3 status from environmental factors, gilts selected for bone health analysis were housed in the two middle rows of pens within the four-row barn, which minimized their exposure to sunlight. At 178 days of age, a subset of gilts (n = 60), composed of littermates across dietary treatments, was harvested at a commercial abattoir. Sections of the 10th and 11th rib were collected for structural and histological analysis. For structural analysis (10th rib), peak force, bending moment, and bone diameter were measured using a Texture Analyzer TA-HD Plus, with a 250kg load cell. Bones were held by two supports spaced 75mm apart for the strength tests. Histological grading (11th rib) assessed physis architecture and infarctions/fracture lines. The physis grading (scores 0–3) evaluated cartilage growth plate architecture, ranging from no findings (0) to extensive areas of the zone of hypertrophy that were expanded and extended down into the primary and secondary spongiosa (3). The infarctions/fracture scoring (scores 0–3) quantified bone lesions as no infarctions (0), small infarctions (< 50% of bone diameter) (1), large infarctions ( >50%) (2), or a cortical fracture (3). Data were analyzed as a 2 × 2 factorial arrangement with Ca/P and HyD® as fixed effects. Models for bone health traits also included litter of origin as a random effect. Results (Table 1) demonstrated that gilts fed the higher Ca/P ratio and HyD® had greater (P < 0.02) peak force and bending moment. Conversely, gilts fed HyD® tended to have lower (P = 0.05) bone diameter. Furthermore, gilts fed the greater Ca/P ratio tended (P = 0.08) to have lower infarction scores. These results suggest increasing the Ca/P ratio and enhancing 25-OH-D3 serum levels via HyD® supplementation may be a strategy to increase bone strength in developing gilts.