Effectiveness of dietary 25- and 1-hydroxycholecalciferol in combating tibial dyschondroplasia in broiler chickens.

1. Three experiments were carried out to determine the effects of feeding diets containing different concentrations of cholecalciferol, 1 alpha-hydroxycholecalciferol (1-HCC), 25-hydroxycholecalciferol (25-HCC), 1,25-dihydroxycholecalciferol (1,25-DHCC) and ascorbic acid on the incidences and severities of tibial dyschondroplasia (TD) at 3 weeks of age in male broiler chicks. 2. In experiment 1, replacing 75 micrograms cholecalciferol/kg with the same weight of 25-HCC decreased significantly (P < 0.01) the incidence of TD from 65 to 10%. 3. In experiment 2, the incidence of TD in the control group was lower, but feeding amounts of 25-HCC up to 250 micrograms/kg had a linear effect on the incidence of TD that was significant at P = 0.06. There was no effect or interactions with dietary addition of 250 mg ascorbic acid/kg. Dietary addition of 5 micrograms 1-HCC/kg decreased TD incidence from 21 to 5%, though the effect was not significant (P > 0.1). 4. TD incidence in experiment 3 was too low to determine an effect of25-HCC or 1,25-DHCC on TD incidence, though in this, as in both other experiments, the severities of TD lesions were always lower with diets containing cholecalciferol metabolites. 5. Hypercalcaemia was not observed after feeding up to 250 micrograms 25-HCC/kg in either experiments 2 or 3. 6. It is concluded that 25-HCC may be an effective practical means of improving broiler leg health by alleviating the incidence and severity of TD.