The digestibility of phosphorus (P) and calcium (Ca) must be corrected and standardized for accurate assessment. The importance of expressing the digestibility of phosphorus using the standardized total tract digestibility (STTD) was emphasized over the use of apparent ileal digestibility (AID) or apparent total tract digestibility (ATTD), as P is primarily absorbed in the small intestine, with minimal hindgut absorption or secretion. Therefore, there is no difference between small intestinal and TTD of P, but determining TTD is less costly and less labor intensive than determining small intestinal digestibility. However, correcting ATTD values for the basal endogenous losses is crucial for calculating STTD values. For practical diet formulation, STTD values are more appropriate because STTD values from individual ingredients are additive in mixed diets, unlike ATTD values.
The same principle applies to calcium (Ca). Regardless of the presence of phytase, increasing Ca levels affect the ATTD of Ca. However, standardizing for endogenous losses of Ca to calculate the STTD of Ca ensures that values that are independent of diet Ca concentrations are calculated.
