The presence of phytate in swine diets reduces the digestibility of calcium because phytate is able to bind calcium from organic sources and some inorganic sources, making it inaccessible to the pig. Microbial phytase breaks down phytate and increases the availability of calcium. An experiment was conducted to test the hypothesis that inclusion of microbial phytase increases the apparent (ATTD) and standardized (STTD) total tract digestibility of calcium in fish meal in diets containing phytate from corn and corn germ.

Besides phytate, corn and corn germ also add fiber and fat to diets, so it is important to know how fiber and fat affect calcium digestibility. Therefore, a second experiment was conducted to test the hypothesis that the values of ATTD and STTD of calcium obtained from cornstarch and corn based diets may differ, and to determine the effect of dietary fiber and fat on the ATTD and STTD of calcium in fish meal.

The first experiment was designed to determine the effect of phytase on the ATTD and STTD of calcium in fish meal in diets that either contained phytate or did not contain phytate. Forty growing pigs with an average initial body weight of 19.16 kg were fed one of five diets. The fish meal and cornstarch based diet contained no phytate, whereas the fish meal and corn-corn germ based diet contained 0.7% phytate. Two levels of microbial phytase (0 and 500 FTU/kg) were added to each diet, for a total of four experimental diets. Fish meal was the only source of calcium in these diets. A calcium-free diet was also used to determine basal endogenous losses of calcium.

The ATTD and STTD values for calcium in pigs fed the corn-corn germ based diet were greater (P < 0.001) than the ATTD and STTD in pigs fed the cornstarch based diets (Table 1). The addition of phytase increased (P < 0.05) the ATTD and STTD of calcium in the corn-corn germ based diet, but did not affect the ATTD and STTD of calcium in the cornstarch based diets.

The second experiment used fifty growing pigs with an average initial body weight of 19.36 kg were used for this experiment. Two diets were formulated based on fish meal and cornstarch. Solka floc was added to one of the diets to evaluate the effect of fiber. Two additional diets were formulated based on fish meal and corn, with soybean oil added to one of the diets to evaluate the effect of fat. As in Experiment 1, a calcium-free diet was also used.

The ATTD and STTD values for calcium were greater (P < 0.05) in pigs fed corn-based diets than in pigs fed cornstarch based diets (Table 2). The addition of fiber to cornstarch diets increased (P < 0.001) the digestibility of calcium. The level of fat in the diets did not affect the ATTD or STTD of calcium.

• ATTD and STTD of calcium were greater for pigs fed diets based on corn and corn germ than for pigs fed a cornstarch based diet.
• Adding phytase to diets increased the ATTD and STTD of calcium when phytate was present in cornstarch based diets.
• Addition of fiber to diets increased ATTD and STTD of calcium, but addition of fat did not affect calcium digestibility.
• The increase in ATTD and STTD of calcium in corn-corn germ based diets relative to values in cornstarch-based diets may be due to the greater fiber concentration in corn and corn germ.
• The data indicate that the digestibility of calcium in synthetic diets based on cornstarch is not always representative of the digestibility in practical diets.

Table 1. Apparent and standardized total tract digestibility of calcium in diets containing fish meal without or with phytate and without or with microbial phytase

Table 2. Apparent and standardized total tract digestibility of calcium in diets containing fish meal and either cornstarch or corn, and differing levels of fiber and fat

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