Influence of supplemental phytase on performance of broilers four to six weeks of age.

The influence of dietary phytase on phytate P availability was investigated using Ross x Hubbard male broiler chicks. A randomized complete block design with a factorial arrangement of 2 x 3 was used with eight replicates (n = 2,400; 50 chicks per replicate) per treatment. Diets were formulated to contain two levels of nonphytate P (NPP; 0.225 and 0.325%) and three levels of phytase [0, 300, and 600 phytase units (FTU)/kg] with 0.75% Ca. An additional diet with 0.425% NPP and 0.85% Ca was used as a positive control (n = 400). Prior to assigning treatments, all chicks were fed a commercial starter mash adequate in all nutrients until 3 wk of age. Neither performance nor bone strength was significantly influenced by a reduction of NPP to 0.325% and Ca to 0.75% as compared to the positive control. However, when NPP was reduced to 0.225% and Ca to 0.75%, significant negative impacts on body weight, feed consumption, feed efficiency, and bone strength were observed. Phytase significantly increased BW at the lower NPP level but not at the higher NPP level. A significant NPP by phytase interaction occurred in bone criteria and livability. Phytase (300 FTU/kg) had greater influence on bone mineral content, bone density, bone breaking strength, and livability in broilers fed 0.225% NPP than in broilers fed 0.325% NPP. This study indicates that supplementing phytase in grower diets containing reduced levels of NPP and Ca significantly improved performance and bone strength of broilers. In diets containing marginal to deficient levels of either NPP or Ca or both, the addition of microbial phytase at 300 to 600 FTU/kg feed prevents P deficiency symptoms. Increasing phytase levels from 300 to 600 FTU/kg feed provided no additional benefit.