Effects of Primary Calcium Source on Phytase Efficiency in Weaned Piglets

Abstract 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, 1500, and 3000 FYT/kg. At d 10, piglets were individually housed to allow fecal and urine sample collection. On days 0, 7, and 18, blood samples were collected. At d 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, 1232 and 1944 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.