The interplay of dietary nutrient specification and varying calcium to total phosphorus ratio on efficacy of a bacterial phytase: 1. Growth performance and tibia mineralization.

A 14-d experiment was conducted to study the effects of 2 dietary variables on efficacy of a 6-phytase from Citrobacter braakii on broiler growth performance and tibia mineralization. Diets were formulated with or without nutrient matrix values for phytase as negative or positive control (NC or PC, respectively) and with 2 Ca:total P (tP; 2:1 or 2.5:1). The diets were supplemented with 0, 1,000, or 2,000 phytase units (FYT)/kg, thus producing a 2 × 2 × 3 factorial arrangement. Birds and feed were weighed on d 7 and 21, and tibia bones were collected from all the birds on d 21. The main effects of nutrient matrix, Ca:tP, and phytase supplementation were significant (P < 0.05) for all the growth performance responses (except for G:F for which there was no effect of matrix). The Ca:tP × phytase and matrix × phytase interactions were significant (P < 0.05) for weight gain. In the PC diets, phytase increased weight gain (P < 0.05) relative to the control only in diets with 2,000 FYT/kg, whereas in NC diets weight gain increased (P < 0.01) only from 0 to 1,000 FYT/kg levels. Broilers consuming diets with 2.5:1 Ca:tP had lower (P < 0.05) tibia ash, whereas phytase increased (P < 0.01) tibia ash, Ca, P, and Zn but decreased (P < 0.01) tibia K. Phytase supplementation of diets with 2:1 Ca:tP increased (P < 0.05) tibia P in birds receiving 1,000 FYT/kg relative to the control with no further increase at 2,000 FYT/kg, whereas each level of phytase supplementation increased (P < 0.05) tibia P in the diets with 2.5:1 Ca:tP. It was concluded that the best response to lower phytase supplementation (1,000 FYT/kg) was in NC diets with narrow Ca:tP, whereas the best response to higher level of phytase supplementation (2,000 FYT/kg) was achieved in diets in PC diets with wide Ca:tP.