Lack of response of bone mineralization of chicks fed egg yolks from hens on dietary 1,25 dihydroxycholecalciferol.

White Leghorn hens, age 34 wk, fed 0 to 3 micrograms/kg of 1,25 dihydroxycholecalciferol (1,25(OH)2D3) as the only source of dietary vitamin D3 with 2.5 or 3.5% calcium failed to achieve normal embryonic survival and hatchability of their fertile eggs. Improved egg production was observed over a 24-wk period as the level of 1,25(OH)2D3 was increased. Eggs of hens fed the vitamin D-deficient diet showed significantly lower (P less than .01) egg weight and shell strength with a higher percent of soft shell eggs than those of hens fed 1,25(OH)2D3 or vitamin D3. To study the possibility that 1,25(OH)2D3 cannot be transferred into the egg yolk, yolks from the hens receiving varying levels of 1,25(OH)2D3 and vitamin D3 were fed to day-old chicks. A standard vitamin D3 assay was developed and bone ash was determined on fat-free tibial bone. Significantly lower (P less than .01) vitamin D activity per gram of yolk was observed in yolks from hens fed 0 to 3 micrograms of 1,25(OH)2D3 than in yolks of hens fed vitamin D3. These results demonstrate that insufficient 1,25(OH)2D3 is incorporated in the egg yolk to support normal embryonic survival and hatchability. This may be due to a decrease in available receptor sites in the egg yolk or to the inability of 1,25(OH)2D3 to cross the vitelline membrane.