Standardized total tract digestibility (STTD) of Ca has been determined for most Ca containing ingredients fed to growing pigs, but the basal endogenous loss of Ca is greater and the STTD of Ca in sows in mid-gestation are less than in growing pigs. As a consequence, if diets for gestating sows are formulated using STTD values determined in growing pigs, the provisions of digestible Ca may be less than anticipated.

The efficacy of phytase to release Ca and P is believed to be influenced by the physiological state of pigs with phytase fed to sows in mid-gestation releasing less Ca and P compared with growing pigs or sows in late-gestation. However, it is not known if values for STTD of Ca or retention of Ca and P that are measured in one period of gestation are representative of the entire gestation period.

Therefore, an experiment was conducted to test the hypothesis that the STTD of Ca from calcium carbonate and the response to microbial phytase on STTD of Ca and apparent total tract digestibility (ATTD) of P in corn-based diets fed to gestating sows are constant throughout the gestating period for sows. The second objective was to test the hypothesis that retention of Ca and P does not change during gestation.