Effects of primary calcium source on phytase efficiency in weaned piglets

An 18-d experiment tested the hypothesis that using calcium formate (Ca-formate) instead of limestone in piglet diets, lowers the dietary buffering capacity, reduces stomach pH, and improves phytase efficiency. At weaning (29.8 ± 1.10 d), 60 boars (body weight 8.8 ± 0.31 kg) housed in metabolism crates were allotted to a randomized complete block design with 10 experimental diets and 2 weaning rounds. Diets were formulated following a 5 × 2 factorial design with 5 dietary treatments (Trt) and 2 main Ca sources (limestone or Ca-formate). The 5 Trt included a positive control diet [PC; 0.76% Ca, 0.34% standardized total tract digestible (STTD) P] and 4 negative control diets (NC; 0.56% Ca, 0.21% STTD P) supplemented with phytase at 0, 750, 1,500, and 3,000 FYT/kg. At day 10, piglets were individually housed to allow fecal and urine sample collection. On days 0, 7, and 18, blood samples were collected. At day 18, samples of gastrointestinal content and the metacarpus were collected, and apparent jejunal, ileal, and total tract digestibility (AJD, AID, and ATTD) of Ca and P was calculated. Digestibility and blood data were analyzed as a 2-way and 3-way interaction, respectively. Phytase dose-response regressions for ATTD Ca and P concentrations were performed per Ca source. Regardless of Trt, Ca source did not affect stomach pH, but Ca-formate diets had greater AID and ATTD of Ca (+5.7% and 3.6%; P < 0.01) and lower AID and ATTD of P (-3.2% and 2.4%; P < 0.01) than limestone diets. Increasing phytase levels increased the ATTD and AID of Ca and P (P < 0.01). An interaction between Trt and Ca source was observed for AJD of P (P < 0.01). There was no effect of Ca source, except at 750 FYT, where the Ca-formate diet had lower AJD of P than the limestone diet. Based on the dose-response analyses, to reach a dietary ATTD P of 3.84 g/kg, 1,232 and 1,944 FYT are needed in the limestone and Ca-formate diets, respectively. Compared with the NC, increasing levels of phytase and the PC diet increased serum P (mg/L) and P retention (%), phytase inclusion increased plasma inositol (µmol), but only the PC diet increased bone ash (g/kg; Trt main effect; P < 0.001). In conclusion, replacing limestone with Ca-formate does not lower stomach pH, increases Ca but reduces P digestibility, and decreases phytase efficiency, likely due to greater Ca solubility, which induces the formation of insoluble Ca-P-phytate complexes. This highlights the importance of considering a digestible Ca system in piglet diets.