The role of calcium and phosphorus in the etiology of tibial dyschondroplasia in young chicks.

By means of a central composite rotatable design with dietary calcium levels of 0.63, 0.70, 1.10, 1.50 and 1.67% and total phosphorus levels of 0.53, 0.61, 0.81, 1.01 and 1.09%, practical-type rations were fed for 2 weeks to commercial broiler-type chickens. The design involved three replicates for each rotatable point and 15 replicates for the central point with 10 cockerels per replicate or a total of 390 birds. When the experiment was terminated, the chickens were weighted and killed; one tibia was used to determine bone ash and the other tibia for scoring the incidence and severity of tibial dyschondroplasia. The incidence of tibial dyschondroplasia in chickens fed the various diets ranged from 0 to 37%. A high incidence of tibial dyschondroplasia was associated with diets containing high phosphorus and low calcium levels. Tibial dyschondroplasia score and percentage of bone ash were not correlated. No obvious relationships existed between tibial dyschondroplasia incidence and percentage retention of calcium and phosphorus. A second experiment confirmed the finding that increased dietary calcium levels would reduce the incidence of tibial dyschondroplasia when chickens were fed diets high in phosphorus.