RESPONSE OF TURKEY POULTS TO TIERED LEVELS OF NATUPHOS(R) PHYTASE ADDED TO SOYBEAN MEAL-BASED SEMI-PURIFIED DIETS CONTAINING 3 LEVELS OF NONPHYTATE PHOSPHORUS

A 3-wk feeding trial using 920 day-old turkey poults was conducted to evaluate the addition of seven levels of phytase (Natuphos(R); 0, 200, 400, 600, 800, 1,000, and 1,200 U/kg of diet) to diets containing thr ee levels of nonphytate P (nP) (.27,.36, and .45%). A positive control diet contained .60% nP. Semi-purified basal diets contained soybean m eal as the only protein source. The increase in BW gain from added phy tase was greatest for the lowest nP diet (nP by phytase interaction, P < .001). At .27% nP, gains improved (P < .001) to 800 U of phytase/kg of diet and then reached a plateau. At .36 and .45% nP, increases in gains were observed only for 200 U of phytase/kg of diet. The highest phytase addition to .36 and .45% nP diets produced gains equal to thos e of the positive control diet. Feed intake increases paralleled those of BW gains. Gain:feed was lowest for the .27% nP diets without phyta se, but improved (P < .001) to 800 U of phytase/kg of diet and then re ached a plateau. The high incidence of leg disorders and high mortalit y (40%) observed for the poults fed the .27% nP diet without added phy tase declined with the addition of 200 to 400 U of phytase/kg of diet. Ash percentage of toes and tibias increased as the levels of nP (P < .001) and phytase (P < .01) increased; the magnitude of the response t o phytase decreased as nP in the diet increased, resulting in an nP by phytase interaction (P < .001). Tibial shear force and stress respond ed in a similar manner to increasing levels of nP and added phytase. R esults show that 652 U of microbial phytase is equivalent to 1 g of P from defluorinated phosphate in turkey starter diets using soybean mea l as the only source of phytate P. The response per 100 U of phytase d ecreased as the total amount of phytase added was increased.