WHEN formulating diets for pigs, it is more accurate to use values for standardized or true nutrient digestibility than values for apparent nutrient digestibility because the former are additive in mixed diets.
Research at the University of Illinois, led by professor of animal sciences Hans Stein, examined the true digestibility of calcium in swine diets.
"We know that there are endogenous losses of calcium in cattle and chickens, and our hypothesis was that the same is true for pigs," Stein said. "We also wanted to determine if adding microbial phytase to the diets would affect endogenous losses of calcium."
Stein's team set out to determine the apparent total tract digestibility (ATTD) and true total tract digestibility (TTTD) of calcium in canola meal without and with microbial phytase.
They fed growing pigs four diets containing 0.08%, 0.16%, 0.24% or 0.32% calcium. All of the calcium in the diets came from canola meal, which is one of the few ingredients that contain both phytate and appreciable amounts of calcium. In addition, they fed four other diets that were identical to the first four except that they also contained 1,500 units per kilogram of microbial phytase.
In diets both with and without added phytase, the ATTD of calcium increased as the calcium level in the diets increased. This indicated that there was endogenous loss of calcium.
Using regression equations, the researchers estimated that the total endogenous loss of calcium was 0.160 g/kg of dry matter intake (DMI) for pigs fed diets with no added phytase and 0.189 g/kg of DMI for pigs fed diets containing microbial phytase. These values were not statistically different, demonstrating that phytase doesn't affect endogenous loss of calcium, Stein said.
Next, values for the TTTD of calcium were calculated by correcting the ATTD for total endogenous losses. Unlike ATTD, the TTTD of calcium was not affected by the level of calcium in the diet. The ATTD and TTTD of calcium were both greater in diets containing added phytase than in diets with no phytase added.
Stein said the results bore out his team's hypothesis.
"There is a measurable loss of endogenous calcium from the gastrointestinal tract of pigs," he said. "The fact that TTTD values are unaffected by dietary calcium levels indicates that the only reason ATTD increases as dietary calcium increases is because of the reduced contribution of endogenous calcium to the total output.
"Because TTTD values for calcium were not influenced by the level of calcium in the diets, we expect these values to be additive in mixed diets," he added. "It is, therefore, necessary to take endogenous losses of calcium into account when formulating diets for pigs."
Future work will focus on determining the digestibility of calcium in additional feed ingredients.
The study, "Endogenous Intestinal Losses of Calcium & True Total Tract Digestibility of Calcium in Canola Meal Fed to Growing Pigs," was published recently in the Journal of Animal Science. It was co-authored by Caroline Gonzalez-Vega and Yanhong Liu of the University of Illinois and Carrie Walk of AB Vista Feed Ingredients.