The effects of microbial phytase, citric acid, and their interaction in a corn-soybean meal-based diet for weanling pigs.

Crossbred weanling pigs (an equal number of barrows and gilts) with an average initial weight of 7.4 (Exp. 1) or 9.6 kg (Exp. 2) were used in two 4-wk experiments (Exp. 1, n = 96; Exp. 2, n = 96) to investigate the effects of added phytase or citric acid on performance, rib mineralization, gastric pH, and digestibility measurements. A corn-soybean meal-based diet low in Ca and P was used in both experiments. In Exp. 1, three citric acid levels (0, 1.5, or 3.0%) and four phytase levels (0, 250, 500, or 750 U/kg) were used in a 3 x 4 factorial arrangement of treatments. In Exp. 2, two citric acid levels (0 or 2.0%) and three phytase levels (0, 250, or 500 U/kg) were used in a 2 x 3 factorial arrangement of treatments. Phosphorus was maintained at .33 and .34% in Exp. 1 and 2, respectively. Calcium was maintained at a 2.5:1 ratio with total available P (available P plus the estimated released phytate P by phytase) in Exp. 1 and at a level of .44% in Exp. 2. In both experiments, BW and feed consumption were measured weekly, and pen fecal samples were collected twice daily for 5 d during wk 4. At the end of wk 4, the barrow in each pen was killed following a fast-refeed-fast (22-1-2 h) regimen for collection of 10th ribs and stomach digesta. In Exp. 1 and 2, phytase addition did not affect (P > .05) performance but linearly increased (P < .05) rib shear force, shear energy, dry bone weight, ash weight, ash percentage, and Ca and P digestibilities. Addition of citric acid in both experiments reduced dietary pH and stomach digesta pH (P < .05). The addition of citric acid improved (P < .05) ADG, feed efficiency, and Ca digestibility in Exp. 1, but it had no effect on performance and Ca digestibility in Exp. 2. In summary, the additions of citric acid and phytase to weanling pig diets were each beneficial, but no synergistic effects were observed.