URBANA, Ill. – Swine diets are primarily formulated and tested in growing pigs. Studies show the ideal ratio of nutrients, specifically the ratio of calcium to phosphorus, changes slightly but predictably as pigs progress through the growing stage. What’s not well-understood is how calcium requirements change throughout gestation. A recent publication in the Journal of Animal Sciences reveals an important pattern of lower digestibility in gestating sows.

“We saw in an earlier experiment that sows had much lower calcium digestibility than growing pigs. The sows we used in that experiment were in the middle of gestation. Then the question came: Was that because digestibility of calcium during that specific period is particularly low, or is digestibility of calcium low all the time during gestation? That was the background for this work,” says Hans H. Stein, professor in the Department of Animal Sciences at the University of Illinois and corresponding author on the study.

Stein and graduate researchers Su Lee and Vanessa Lagos fed four experimental diets to 36 gestating sows at three stages of gestation: early (days 7 to 20), mid (days 49 to 62), and late (days 91 to 104). All the diets were based on a standard corn diet. Two contained calcium carbonate; one with 500 grams of phytase, and one with no added phytase. The other two diets contained no calcium source, but were also formulated with and without added phytase. All vitamins and minerals, except calcium in the calcium-free diets, were provided at the required rates for normal development.

Urine and feces were collected from each sow during each phase of the experiment and analyzed for apparent total tract digestibility (ATTD), standardized total tract digestibility (STTD), and basal endogenous loss of calcium. These measurements indicated how much of the calcium in the diet was absorbed by the pig before being excreted. 

“For digestibility of calcium and phosphorus, we saw relatively low values in early and mid-gestation, but much higher values in late gestation. We also saw much greater endogenous losses throughout gestation than during the growing stage, though this, too, varied across the gestation period,” Stein says. “We don’t know the exact reason, but the bottom line is that sows in late gestation have much greater digestibility of calcium than sows in earlier periods of gestation. That makes it more complicated to formulate diets. This is the first work in this area to look at that.”

Phytase, which is added in diets for growing pigs to predictably increase calcium and phosphorus digestibility, had a more variable effect in sows than what is usually observed in growing pigs, further complicating the message for the swine nutrition industry.

Stein notes that although this is his second study to quantify calcium digestibility in gestating sows, he has only scratched the surface. Ultimately, he hopes to provide an optimal calcium-to-phosphorus ratio to the swine nutrition industry for each stage of gestation and into the lactation period.

As for why digestibility increases in late gestation, Stein says, “It’s not surprising that nature has made it such that the mother can extract more nutrients from food when the babies start growing. She has to make sure they get enough. I think it’s just another one of nature’s wonders.”  

The article, “Basal endogenous loss, standardized total tract digestibility of calcium in calcium carbonate, and retention of calcium in gestating sows change during gestation, but microbial phytase reduces basal endogenous loss of calcium,” is published in the Journal of Animal Science [DOI: 10.1093/jas/skz048]. Authors include Su Lee, Vanessa Lagos, Carrie Walk, and Hans Stein. The work was supported by AB Vista, Marlborough, UK. The Department of Animal Sciences is in the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois.