Susceptibility of a dwarf strain of chickens to rickets.

A sex-linked dwarf strain of chickens fed a practical chick starter had a high incidence of rickets while other strains, including the Cornell K strain reared under the same conditions, had no evidence of this disease. In studies of the calcium, phosphorus and vitamin D requirements of the dwarf and K strains, no strain differences in requirement for phosphorus or vitamin D for maximal bone mineralization were apparent. The vitamin D requirement for growth was less for dwarfs than for the K strain; the reverse was observed for phosphorus. A marginal level of calcium (0.6%) did not depress growth or bone ash in either strain. Dwarfs had significantly lower bone ash than chicks of the K strain when dietary levels of calcium, available phosphorus and vitamin D were adequate (1.0%, 0.73% and 400 I.U./kg., respectively). When the diet contained a low level of one nutrient (0.6% calcium, 0.33% of phosphorus or 100 I.U./kg. vitamin D), neither strain was affected. Low levels of two nutrients significantly decreased bone ash in dwarfs but not in the K strain. Low levels of all three nutrients depressed bone ash in both strains, but most markedly in the dwarfs. These studies indicate that chicks of the Cornell dwarf strain have a low bone mineral content and are unusually susceptible to rickets.