Two experiments were conducted to determine endogenous losses of Ca and determine the effects of phytase, phytate, source of dietary Ca, and Ca level on Ca digestibility. The first objective of Exp. 1 was to test the hypothesis that endogenous Ca is lost from the gastrointestinal tract of growing pigs, and that values for true total tract digestibility (TTTD) of Ca are different from values for apparent total tract digestibility (ATTD) of Ca. The second objective was to determine the effect of microbial phytase on ATTD and TTTD of Ca in canola meal. Forty eight growing barrows (initial BW: 16.7 ± 2.5 kg) were allotted to a randomized complete block design with 8 dietary treatments and 6 pigs per treatment. Diets were formulated to contain 0.08, 0.16, 0.24, or 0.32% Ca from canola meal and 0 or 1,500 units per kg of microbial phytase (Quantum Blue, AB Vista Feed Ingredients, Marlborough, UK). Feces and urine samples were collected from d 6 to d 11. Total endogenous losses of Ca were determined using the regression procedure. Results indicate that the estimated total endogenous losses of Ca were 0.160 and 0.189 g/kg DMI for canola meal without and with microbial phytase, respectively, and these values were not different. The ATTD of Ca increased (P < 0.05) if dietary Ca increased; however, the TTTD of Ca was not affected by dietary Ca. The ATTD and TTTD of Ca increased (P < 0.01) if phytase was added to the diets. The first objective of Exp. 2 was to test the hypothesis that standardized duodenal digestibility (SDD), standardized ileal digestibility (SID), and standardized total tract digestibility (STTD) of Ca in calcium carbonate and Vistacal are not different at 2 different Ca levels, and that phytate affects digestibility of Ca in these 2 ingredients to the same degree. The second objective was to determine where in the intestinal tract Ca absorption takes place and if measurable quantities of basal endogenous Ca are lost in the stomach, small intestine, or large intestine. Nine growing pigs (initial BW: 23.8 ± 1.3 kg) were surgically equipped with a T-cannula in the duodenum and another cannula in the distal ileum and were allotted to a 9 × 6 incomplete Latin square design with 9 diets and 6 periods. Diets contained calcium carbonate or Vistacal as the sole source of Ca, 0 or 1% phytate, and 0.4 or 0.8% Ca. A Ca-free diet was also formulated and used to measure endogenous losses of Ca. Fecal, ileal, and duodenal samples were collected on d 5 and 6, 7 and 8, and 9 and 10, respectively. Results indicated that duodenal endogenous losses of Ca (1.03 g/kg of DMI) were greater (P < 0.05) than ileal (0.42 g/kg of DMI) and total tract endogenous losses of Ca (0.67 g/kg of DMI). Standardized digestibility of Ca was not affected by level of phytate, but decreased (P < 0.05) as Ca level increased in Vistacal diets; standardized digestibility of Ca did not decrease as Ca increased if calcium carbonate was the source of Ca (interaction, P < 0.05). The SDD, SID, and STTD of Ca were not different if calcium carbonate was the source of Ca. However, the SID and STTD of Ca in Vistacal was greater (P < 0.05) than the SDD of Ca, but no differences were observed between SID and STTD of Ca (interaction, P < 0.05). The SDD, SID, and STTD of Ca in calcium carbonate were greater (P < 0.05) than in Vistacal. It was concluded that endogenous Ca is lost from the intestinal tract of pigs. As a consequence, values for STTD or TTTD of Ca need to be determined in feed ingredients fed to pigs. For formulation of diets fed to pigs, Ca source, dietary Ca level, and inclusion of microbial phytase to the diets are factors to consider because they influence Ca digestibility.
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