Calcium supplementation is important for swine diets because most commonly used feed ingredients have low concentrations of calcium. In a typical corn-soybean meal diet for a growing pig, the corn and soybean meal contribute only about 16% of the total calcium, with the rest coming from supplements. Apparent total tract digestibility (ATTD) values for calcium have not been reported for many common ingredients, and no values for the standardized total tract digestibility (STTD) of calcium have been reported. Therefore, an experiment was conducted to determine the ATTD and STTD of calcium in five calcium supplements.

An additional objective was to test the hypothesis that inclusion of microbial phytase in the diets increases the ATTD and STTD of calcium. Results of previous research has indicated that inclusion of microbial phytase in swine diets often increases the digestibility of calcium, but the effect of phytase on the STTD of calcium in individual ingredients has not been reported.

Five calcium supplements were used in the experiment. L. calcareum calcium is derived from the red seaweed Lithothamnium calcareum, and is sold commercially under the name Vistacal. Sugar beet coproduct is the "spent lime" left over after calcium carbonate is used to precipitate out impurities in sugar beet juice. It is sold under the name LimeX. Monocalcium phosphate (MCP), dicalcium phosphate (DCP), and calcium carbonate were also tested.

A total of 104 growing barrows with an average initial body weight of 17.73 kg were fed one of 13 diets. The basal diet contained 0.33% calcium and 0.44% total phosphorus, and consisted of corn, cornstarch, potato protein isolate, soybean oil, calcium carbonate, monosodium phosphate (MSP), vitamins, and minerals. For the MCP diet, MCP was added to the basal diet and MSP was eliminated to yield a diet containing 0.63% calcium and 0.66% total phosphorus. Four other diets contained approximately 0.80 % calcium and 0.40 to 0.67% total phosphorus, and were formulated by adding DCP, calcium carbonate, L. calcareum calcium, or sugar beet coproduct to the basal diet at the expense of cornstarch and adjusting the inclusion of MSP.

An additional six diets were formulated by adding 500 units per kg of microbial phytase to each of the previous six diets. The thirteenth and final diet contained no calcium and was used to measure basal endogenous losses of calcium.

Regardless of whether or not microbial phytase was added to the diets, the ATTD and STTD of calcium was greatest (P < 0.05) in MCP. The ATTD and STTD of calcium in DCP was greater (P < 0.05) than in calcium carbonate, L. calcareum calcium, or sugar beet coproduct, which did not differ from each other.

When the mean of all ingredients was analyzed, no interaction between ingredient and response to the addition of microbial phytase was observed. However, when ingredients were analyzed individually, the addition of phytase increased (P < 0.05) the ATTD and STTD of calcium in calcium carbonate.

• Values for ATTD and STTD of calcium were the greatest for MCP, followed by DCP. ATTD and STTD values for calcium in calcium carbonate, L. calcareum calcium, and sugar beet coproduct did not differ.
• The inclusion of microbial phytase increased the ATTD and STTD of calcium in calcium carbonate, but not in any of the other calcium supplements tested.
• Because values for STTD of calcium differ, it is important for diet formulation to use values for STTD of calcium for each source.

Table 1. Apparent and standardized total tract digestibility of calcium in calcium supplements without or with microbial phytase

This report is based on unpublished data by Caroline González-Vega, Carrie L. Walk,and Hans H. Stein.