PHYTASE SUPPLEMENTATION OF LOW-PHOSPHORUS GROWING-FINISHING PIG DIETSIMPROVES PERFORMANCE, PHOSPHORUS DIGESTIBILITY, AND BONE MINERALIZATION AND REDUCES PHOSPHORUS EXCRETION

Two experiments using 413 crossbred growing-finishing pigs were conduc ted to assess the use of a commercial microbial phytase (Natuphos(R)) in corn-soybean meal diets to improve phytate P bioavailability and th us reduce inorganic P supplementation and fecal P excretion. In Exp. 1 (n = 189), the following diets were used: 1) .50/.40% total P, respec tively, for grower and finisher phases, and no phytase; 2) .40/.35% P and no phytase; 3) diet 2 plus 250 U phytase/kg; and 4) diet 2 plus 50 0 U phytase/kg. The total Ca level was .58/.48% for diet 1 and .53/.43 % Ca for diets 2, 3, and 4 in the grower and finisher phases, respecti vely. Feeding the low-P diet without supplemental phytase resulted in an overall 18% reduction in ADG (P <.05), 15% reduction in ADFI (P <.0 5), and 3% poorer feed efficiency (P <.08). Adding 250 to 500 U phytas e/kg to the low-P diet restored ADG, ADFI, and feed conversion to leve ls not significantly different from and within 96% of that observed fo r pigs fed the adequate-P diet. The overall apparent digestibility of P was linearly (P <.01) improved with addition of 250 and 500 U phytas e/kg to the low-P diet, but Ca and DM digestibilities were not affecte d by phytase or P level. In Exp. 2 (n = 224) the following diets were used: 1) .38/.33% total P, respectively, for grower and finisher phase s, and no phytase; 2) .42/.37% P and no phytase; 3) .46/.41% P and no phytase; 4) diet 1 plus 167 U/kg phytase; 5) diet 1 plus 333 U/kg phyt ase; and 6) diet 1 plus 500 U/kg phytase. All diets contained .41/.36% Ca for grower and finisher phases, respectively. Pigs fed the low-P c ontrol diet grew slower (P <.01) and less efficiently (P <.10) than pi gs fed diets with added P or phytase. With increasing levels of supple mental phytase or P there was a linear increase (P <.01) in ADG, diges tibility of P, and digested P and a quadratic improvement (P <.05) in feed efficiency. Tenth rib mineralization based on shear force and ash were linearly increased (P <.0.8 to .001) as phytase or P was added t o the low-P diet. There were generally no effects of P or phytase leve l on carcass quality. Using prediction equations derived from the resp onse traits of ADG and P digestibility in Exp. 1 and ADG, P digestibil ity, and bone shear force in Exp. 2 to added phytase or P, we estimate d that 500 U phytase released an amount of phytate P that was approxim ately equivalent to .87 to .96 g of P from dicalcium-monocalcium phosp hate supplements. Fecal P excretion was estimated to be reduced 21.5%.