In series of in vitro trials were conducted to evaluate the influence of limestone, dicalcium phosphate, phytase, and the digestion phase on calcium and phosphorus solubility. Experimental samples were arranged as a 2 × 2 × 2 × 2 factorial and contained corn (experiment 1) or soybean meal (experiment 2) plus limestone, dicalcium phosphate, phytase, and all combinations. Calcium and available phosphorus in the samples were maintained at 1.0% with limestone and 0.45% with dicalcium phosphate, respectively. Phytase was added to the samples at 1,000 FTU/kg. Samples were exposed to a 2-step in vitro digestion assay to simulate the gastric and the small intestinal phases of digestion. In experiment 1, dicalcium phosphate improved (P ≤ 0.05) phosphorus solubility in the gastric phase, which did not change in the small intestine, except when phytase was supplemented, which reduced (P ≤ 0.05) phosphorus solubility in the small intestine. The small amount of calcium present in corn is highly soluble, but limestone, dicalcium phosphate, or phytase reduced (P ≤ 0.05) calcium solubility in the gastric phase. Solubility was further reduced (P ≤ 0.05) in the small intestine in the presence of limestone.

In experiment 2, phosphorus was more soluble (P ≤ 0.05) in the presence of limestone, dicalcium phosphate, or phytase in the gastric phase and compared with P solubility in the SI phase. Calcium solubility was reduced (P ≤ 0.05) in the small intestine compared with the gastric phase, except when dicalcium phosphate or limestone was supplemented. In conclusion, phosphorus and calcium solubility were influenced by the change in pH between the gastric and SI phases, differences in diet composition, and the Ca:P ratio. Limestone, dicalcium phosphate, and phytase increased phosphorus solubility in the gastric phase and reduced phosphorus solubility in the small intestine. Phytase had more of an effect on phosphorus and calcium solubility in soybean meal than in corn and this is possibly a result of the high amount of phytate in soybean meal.

Boling, S.D. and co-workers. 2001. Efficacy of phytase for increasing protein efficiency ration values of feed ingredients. Poultry Sci. 80(11):1578-1584.