Current recommendations for Ca and P in sow diets rely on digestibility data from growing pigs. Compared with growing pigs, however, sows absorb a smaller proportion of the Ca and P they consume, retain less of both minerals, and lose more Ca and P through basal endogenous losses. Recent factorial calculations that used digestibility values obtained in sows indicate that the Ca and P requirements for gestating and lactating sows are greater than those recommended by NRC (2012). However, these estimates have not yet been evaluated in sows.
Vitamin D3 is needed for Ca and P absorption, but sows appear to be inefficient in utilizing dietary vitamin D3. However, the vitamin D metabolite 25-hydroxyvitamin D3 (25OHD3) may be used as an alternative source of vitamin D3 and in a recent experiment, the digestibility of Ca and P in late-gestating sows increased when diets were supplemented with 25OHD3. Whether or not this benefit continues into lactation, and whether or not it changes when sows receive more Ca and P than NRC recommends, has not been investigated. An experiment was, therefore, conducted to test the hypothesis that addition of 25OHD3 to diets for lactating sows positively impacts Ca and P digestibility, sow and litter performance, milk composition, bone biomarkers and vitamin D status of sows and their pigs.
Fifty-six sows (parity 2 to 6) were allotted to four dietary treatments (14 sows per treatment) from day 75 of gestation until weaning on day 21 of lactation. Corn, soybean meal and soybean hulls were the main ingredients in gestation diets, whereas lactation diets were based on corn and soybean meal. The four treatment diets were arranged in a 2 × 2 factorial and included two levels of Ca and P, either following NRC (2012) or following recent estimates based on factorial calculations. Each diet was formulated both without 25OHD3 and with 50 µg/kg of 25OHD3 (Hy-D®; dsm-firmenich Animal Nutrition &Health, Pendergrass, GA, USA). In late gestation, the NRC-based diets contained 0.72% Ca and 0.55% P, and the diets based on factorial calculations contained 1.17% Ca and 0.70% P, resulting in Ca:P ratios of 1.31:1 and 1.67:1, respectively. In lactation, the corresponding values were 0.68% Ca and 0.60% P (1.13:1) and 1.36% Ca and 0.98% P (1.39:1). All diets contained 1,660 IU/kg of vitamin D3, and titanium dioxide was added as a marker for digestibility calculations. Feces were collected on days 91 to 95 of gestation and on days 12 to 16 of lactation. Litter size and litter weight were recorded at farrowing, and cross-fostering was conducted only within treatment. Milk samples were collected on days 12 to 14 of lactation and urine samples were collected on days 17 to 19. On the day of weaning, all sows and pigs were weighed and blood was collected from each sow and from one female pig per litter for analysis of plasma 25OHD3.
Results indicated that supplementing diets fed in late gestation with 25OHD3 increased (P < 0.05) the standardized total tract digestibility of both Ca and P (Figure 1), which is in agreement with results of two previous experiments. Increasing the Ca:P ratio also increased (P < 0.05) the STTD of Ca, but reduced (P < 0.05) the STTD of P, which is also in agreement with previous data demonstrating that dietary Ca may chelate with P and reduce digestibility.
