IMPROVING PHOSPHORUS AVAILABILITY IN SOYBEAN-MEAL FOR BROILERS BY SUPPLEMENTAL PHYTASE

A 21-d experiment was conducted with day-old male broilers (n = 840) t o evaluate the effectiveness of supplemental phytase for improving the availability of phytate P in soybean meal when varying levels of P we re fed. The semi-purified basal diet (.18% phytate P) contained soybea n meal as the only protein source. Seven levels of phytase (0, 200, 40 0, 600, 800, 1,000, and 1,200 U/kg diet) were added to diets formulate d to contain .20,.27, or .34% nonphytate P (nP; or .38,.45, and .52% t otal P, respectively). The desired levels of nP in the three basal P d iets were achieved by adding varying amounts of defluorinated phosphat e. A 2:1 Ca:total P ratio was maintained in all diets. Body weight gai ns and feed intake were improved (P < .001) by phytase at all nP level s, but the magnitude of response was greatest at low nP levels, result ing in an nP by phytase interaction (P < .01). Gain:feed was unaffecte d by phytase addition. A high mortality (35 to 45%) was observed for t he .20 and .27% nP diets without added phytase, but this declined to n ormal levels with the addition of 200 to 400 U phytase/kg diet. Ash pe rcentage of toes and tibia and shear force and stress of tibia increas ed with added phytase. These responses clearly show that the phytate-b ound P in soybean meal was made more available to broilers by microbia l phytase, and the total response was related to the phytase and nP/to tal P levels. Based on the high R(2) values for the second order trans log equations, BW gain, feed intake, and toe ash percentage were the m ost sensitive indicators to assess P availability, followed by tibia f orce and ash percentage. Derived nonlinear and linear equations for BW gain and toe ash percentage at the two lower nP levels were used to c alculate P equivalency values of phytase for inorganic P. Using the av erage function of P released (Y) by microbial phytase (X) derived with nP levels of .20 and .27% for BW gain and toe ash percentage, Y = 1.1 20 - 1.102e(-.0027X) 1 g of P could be released with 821 U of phytase. The amount of P released increased with increasing levels of phytase, but the amount of P released per 100 U of phytase decreased. Released P ranged from 31 to 58% of phytate P for 250 to 1,000 U of phytase/kg of diet.