Safety of 25-hydroxyvitamin D3 as a source of vitamin D3 in layer poultry feed.

A target animal safety study investigated the effects of providing 25-hydroxyvitamin D3 (25-OH-D3) in laying hen feed at levels ranging from 0.5 to 10 times the level commonly used for vitamin D3 supplementation in the poultry industry. Following a 28-day preconditioning period, 5 groups of laying hens were fed commercial diets containing 68.9 micrograms of vitamin D3/kg feed (control) or 41.25 (0.5x), 82.5 (1x), 412.5 (5x), or 825 (10x) micrograms of 25-OH-D3/kg feed. The study compared the effects of the control level of vitamin D3 and the various test levels of 25-OH-D3 on health, performance, hematology, and 25-OH-D3 tissue concentrations in laying hens from 0 to 112 d of treatment and on health, performance, gross pathology and histopathology from 113 to 224 d of treatment. Gross pathologic and histopathologic examination of selected tissues after 224 d revealed no lesions attributable to vitamin D toxicity at any level of test material. Concentrations of 25-OH-D3 in edible tissues at 112 d were similar for birds in the control and 1x groups. On the basis of all variables monitored, including body weight gain and feed conversion, the 10x level of 25-OH-D3 produced clear toxicity (but no mortality), the 5x level caused limited threshold toxicity, and the 1x level induced no toxicity. These results indicate that 25-OH-D3 is safe for use in laying hen feed as a source of vitamin D3 at 82.5 micrograms/kg feed (1x), with a margin of safety of approximately 5x between the proposed 1x level and the 5x level (412.5 micrograms/kg feed) that constitutes threshold toxicity in layers.