Avian tibial dyschondroplasia: the interaction of genetic selection and dietary 1,25-dihydroxycholecalciferol.

The effects of dietary 1,25-dihydroxycholecalciferol on the incidence and severity of tibial dyschondroplasia (TD) were assessed in broiler fowl selected for TD (high-TD line) and against TD (low-TD line). Assessment was by examination with a portable Lixiscope to identify lesions in live birds, gross pathology and histopathology. Eighty broilers in four groups of 20 were examined. When fed on standard diets the low-TD line had a 15% incidence whereas the high-TD line had a 63% incidence. The chicks in the high-TD line, when fed on a diet supplemented with 5 mug or with 10 mug 1,25-dihydroxycholecalciferol/kg diet, showed a marked reduction in the incidence of TD, to 21% and 0% respectively. The growth plate in the proximal tibiotarsi of the high-TD line showed accumulations of transitional chondrocytes, supporting the hypothesis that a failure of chondrocyte differentiation is the cause of dyschondroplasia. This hypothesis is further supported by studies which indicate 1,25-dihydroxycholecalciferol has a major role in chondrocyte differentiation.