Influence of dietary phytase supplementation on incidence and severity in broilers divergently selected for tibial dyschondroplasia

A study was conducted to determine the effect of dietary supplementation of phytase on the incidence and severity of tibial dyschondroplasia (TD) in chickens selected for high (HTD) and low (LTD) incidences of TD for 11 generations. By feeding a phosphorus-deficient diet (0.1% nonphytate phosphorous; nPP), HTD and LTD chickens were further identified as high-sensitivity birds (HS) and low-sensitivity birds (LS) to phosphorus deficiency based on mortality. Two hundred forty 1-d-old chicks from HTD and LTD lines (five replications of four birds per treatment) were randomly assigned to a control diet with 0.5% nPP and two treatment diets (0.1% nPP) with and without 600 phytase units (FTU) Natuphos phytase/kg. Feed consumption and growth rate were measured for 3 wk, and both tibiae were scored for TD incidence, average TD score, and total number of TD lesions with the most severe form of the abnormality (lesions that were scored 3). The addition of phytase had no influence on TD incidence and lesion scores of 3 in HTD chicks. However, a nonsignificant reduction in TD incidence (P = 0.07), TD score, and no. 3 lesions (P < or = 0.01) were observed in LTD chicks. Interactions between sensitivity (to P deficiency) and phytase (P < or = 0.01) and sensitivity and nPP (P < or = 0.01) were observed for no. 3 scores in LTD chicks. These results indicate that phytase was effective in reducing TD incidence and severity in LTD chicks but not in HTD chicks.