Impact of response criteria (tibia ash weight vs. percent) on phytase relative non phytate phosphorus equivalance.The current study was conducted to evaluate the impacts of using tibia ash percentage or ash weight as the response criteria on estimated phytase relative equivalence.  Straight run broilers were fed treatment (Trt) diets from 7 to 21 d age (6 birds/pen, 8 pens/Trt).  The corn-soy based Trt were formulated to contain 0.80% Ca and 4 non-phytate phosphorus (nPP) concentrations (0.20, 0.27, 0.34, and 0.40%).  Monocalcium phosphate was the inorganic phosphate source added to achieve 4 different dietary nPP concentrations and against which the nPP relative equivalence of phytase was determined.  A 6-phytase (Danisco Animal Nutrition, DuPont Industrial Biosciences, Marlborough, UK) was added at 500 or 1,000 phytase unit ( FTU: )/kg to the 0.20% nPP diet resulting 6 total Trts.  Tibia ash was determined at 21 d age.  Phytase fed at 500 or 1,000 FTU/kg increased tibia ash weight and ash percentage compared to that of birds fed 0.20% nPP diet without phytase (P<0.05).  Graded nPP were log transformed and regressed against tibia ash (weight and percentage) to calculate phytase nPP relative equivalence.  The R2 obtained from pen value regressions were 0.81 and 0.84, for tibia ash weight and percentage, respectively.  Ash percentage from birds fed 500 and 1,000 FTU phytase/kg fell within the range obtained with the MCP additions.  Ash weight (842 mg/tibia) from birds fed 1,000 FTU phytase/kg exceeded (P<0.05) maximum weight (773 mg/tibia) measured in birds fed the greatest nPP Trt (0.40%), thus the nPP relative equivalence was only calculated in birds fed 500 FTU phytase/kg Trt.  The nPP relative equivalence in birds fed 500 FTU phytase/kg were 0.117 and 0.168% based on ash percentage and weight, respectively (P<0.05).  The nPP relative equivalence in birds fed 1,000 FTU phytase/kg was 0.166% for ash percentage.  Results suggested that ash weight better reflects the amount of bone mineralization as compared to ash percentage and using ash percentage may lead to an underestimation of phytase efficacy.
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