Effectiveness of Natuphos phytase in improving the bioavailabilities of phosphorus and other nutrients in soybean meal-based semipurified diets for young pigs.

Crossbred pigs (n = 96, BW = 7.5 kg) were used in a 5-wk trial to determine the effectiveness of supplemental Natuphos phytase in improving the bioavailabilities of P and other nutrients in a semipurified diet with soybean meal as the only P source in the basal diet. Two available P (aP) levels (.05 and .16%) and five phytase levels (0, 350, 700, 1,050, and 1,400 units/kg of diet) were used in a 2 x 5 factorial arrangement of treatments. In addition to the 10 diets, two extra diets were formulated to supply the recommended level of aP (.32%) with 0 and 1,400 units (U) of phytase/kg of diet. Graded levels of phytase resulted in linear increases in ADG (P < .02), ADFI (P < .01 at .16% aP only), and gain:feed ratio (P < .03). Effects of adding phytase to the diet with .32% aP were observed only in the first 14 d of the study with increases in ADG (P < .06) and gain:feed (P < .02) for added phytase. Apparent digestibility (or absorption) coefficients (ADC) of DM, P, Ca, and N were estimated using chromic oxide as an indicator during wk 4 and 5. When phytase and P were added to the low P diet, the ADC of P was increased (P < .01), but only small and variable changes in the ADC of DM, Ca, and N were observed. Fecal P excretion (grams per day) decreased as microbial phytase was added (P < .01) and increased with added P (P < .01). In comparison to the results with the .32% aP diet, fecal P excretion decreased 25 to 50% by the addition of phytase. The addition of phytase to the diet with .32% aP further improved (P < .01) the ADC of P (54.5 vs 61.8%) and decreased (P < .01) fecal P excretion (1.62 vs 1.38 g/d). Characteristics of 4th metacarpals and 10th ribs were consistently improved by increasing dietary levels of both phytase and P. On the basis of an assessment of R2 values from secondorder translog equations, ADG, ADFI, P apparent absorption, bone ash percentage, and bone shear force were sensitive indicators to evaluate phytase efficacy of P availability in diets. Phosphorus equivalency of microbial phytase was calculated by using response equations for ADG and apparent P absorption. The average function of the release of P (Y, grams per kilograms) by microbial phytase (X, units per kilogram of diet) was developed with aP levels of .05 and .16%: Y = 1.546-1.504e-.0015X. The replacement of 1 g of inorganic P would require about 676 U of microbial phytase. This represents 77% of released P from phytate.