Increasing calcium from deficient to adequate concentration in diets for gestating sows decreases digestibility of phosphorus and reduces serum concentration of a bone resorption biomarker.

The objective of this experiment was to test the hypothesis that the concentration of Ca in diets fed to late gestating sows affects the apparent total tract digestibility (ATTD) and retention of Ca and P, serum concentrations of Ca and P, hormones, and blood biomarkers for bone formation and resorption. Thirty-six sows (average parity = 2.8) were housed in metabolism crates from day 91 to day 104 of gestation and fed 1 of 4 experimental diets containing 25, 50, 75, or 100% of the requirement for Ca. All diets met the requirement for P. The initial 5 d of each period were the adaptation period, which was followed by 4 d of quantitative collection of feces and urine. At the end of the collection period, a blood sample was collected from all sows. Results indicated that feed intake, weights of dried fecal and urine samples, and the ATTD of DM were not affected by dietary Ca, but ATTD of Ca increased (quadratic, P < 0.05) as Ca in diets increased. Urine Ca output was not affected by dietary Ca, but Ca retention increased (quadratic, P < 0.05) as Ca intake increased. Fecal P output increased (linear, P < 0.001) as dietary Ca increased, which resulted in a linear decrease (P < 0.001) in the ATTD of P. Urine P output also decreased (linear, P < 0.001) as dietary Ca increased, but P retention increased (linear, P < 0.05). Regressing the apparent total tract digestible Ca against dietary Ca intake resulted in a regression line with a slope of 0.33, indicating that true total tract digestibility of Ca in calcium carbonate was 33%. Serum concentrations of Ca and P and estrogen, calcitonin, and parathyroid hormone were not affected by dietary Ca. Serum concentration of carboxyterminal cross-linked telopeptide of type I collagen (CTX-I) decreased (linear, P < 0.05) as dietary Ca increased, which is a result of reduced bone resorption as dietary Ca increased. Serum bone-specific alkaline phosphatase tended to decrease (linear, P < 0.10) as Ca in diets increased, but the concentration of osteocalcin (OC) in serum was not affected by dietary Ca. The ratio between OC and CTX-I tended to increase (P < 0.10) as dietary Ca increased, which indicated that there was more bone formation than resorption in sows as dietary Ca increased. In conclusion, P digestibility in late gestating sows decreased, but retention of P increased, as dietary Ca increased from inadequate to adequate levels and blood biomarkers for bone resorption changed as Ca and P retention increased.