Cholecalciferol (VIT-D3) and 25-hydroxycholecalciferol (25-OH-D3) were each used to provide 125, 250, 500, 1,000, 2,000, or 4,000 lU/kg of vitamin D activity in a nutritionally complete corn-soybean meal diet. Each treatment was assigned to four pens of 60 male broilers (Cobb 500) grown in an open-sided house with sidewall curtains. At 21 and 42 d, BW and feed conversion were determined. Samples of birds (8 per pen) were killed at the same ages; the right tibia was subjected to bone ash determination, and incidence and severity of tibial dyschondroplasia (TD) were determined on the left tibia. At 21 and 42 d, the BW of birds fed the 25-OH-D3 were significantly greater than those of birds fed the VIT-D3. At 21 d, vitamin D in excess of 500 IU/kg appeared necessary to maximize BW regardless of source. At 42 d, approximately 1,000 IU/kg was needed to maximize BW of birds fed VIT-D3, whereas no significant difference in BW was noted among birds fed the various levels of 25-OH-D3. Bone ash at 21 and 42 d was significantly greater for birds fed the 25-OH-D3 as compared to those fed the VIT-D3. Approximately 2,000 IU/kg of VIT-D3 was needed for maximum bone ash, whereas there were no significant differences in bone ash content of birds fed from 250 to 4,000 IU/kg from 25-OH-D3. The incidence and severity of TD was significantly lower for birds fed 25-OH-D3 and was reduced by increasing levels of vitamin D regardless of source. Results of the study show that 25-OH-D3 is more metabolically potent. on a per unit basis than VIT-D3 for support of BW, tibia ash, and reduction in incidence and severity of TD. The differences were observed primarily at lower levels of vitamin D; at typical industry levels, few differences were observed between the two sources. Use of the 25-OH-D3 may allow for supplementation with lower levels or may provide a greater margin of safety.
