PHOSPHORUS EQUIVALENCE OF MICROBIAL PHYTASE IN TURKEY DIETS AS INFLUENCED BY CALCIUM TO PHOSPHORUS RATIOS AND PHOSPHORUS LEVELS

Male day-old turkey poults (n = 768) were fed 0, 300, 600, or 900 U of phytase/kg of a corn-soybean diet in combination with four Ca:total P (tP) ratios of 1.1, 1.4, 1.7, and 2.0:1, and two levels of nonphytate P (nP) of 0.27 and 0.36% in a 21-d trial. Dietary Ca:tP ratios were o btained by varying defluorinated phosphate and limestone at the expens e of cornstarch. The calculated dietary percentage of phytate P was 0. 266 for all diets. Phytase additions linearly increased (P < 0.05) BW gain, feed intake, gain:feed, toe ash content, and apparent retentions of Ca and P at each Ca:tP ratio and nP level, but the response was in fluenced by dietary Ca:tP ratios and P levels. The detrimental effect (P < 0.02) of widening the Ca:tP ratio was observed for all measuremen ts at each phytase and P level, and was greatest at lower phytase and P levels. Widening the Ca:tP ratio from 1.4 to 2.0 decreased the phyta se efficacy by 7.4 and 4.9%, respectively, for 0.27 and 0.36% nP diets , which was close to the decrease in the phytase activity in vitro by 7.5 and 6.7%, respectively. The largest responses to supplemental phyt ase were achieved when poults were fed diets with 600 and 900 U of phy tase/kg diet, respectively, for 0.36 and 0.27% nP, and for Ca:tP ratio s ranging from 1.1 to 1.4:1. Second-order translog equations were gene rated for the phytase, Ca:tP ratio, and P effect, and nonlinear and li near equations for the phytase and Ca:tP ratio effect. Based on an ass essment for the R(2) and P values of equations, BW gain, feed intake, toe ash content, and P retention were sensitive measurements of the re sponse to phytase addition. Equivalent equations were developed to det ermine the P equivalency of supplemental phytase. About 652 and 963 U of phytase were equivalent to 1 g nP, respectively, for 0.27 and 0.36% nP diets in turkey poults from hatch to 21 d of age.