NRC guidelines for a 1000-lb heifer is 140 IU of vitamin D per pound of dry matter. Assuming a heifer consumes 2.5% of her body weight or 25 lb of dry matter, 3500 IU of vitamin D is recommended. For lactating cows, we suggest 30,000 to 40,000 IU of vitamin D and avoid levels over 100,000 IU/day of added vitamin D. Toxicity has been suggested at 300,000 IU per cow per day. Since your heifers are large animals, the cow guidelines could be used.
Mike Hutjens, Extension Dairy Specialist, University of Illinois - Illini DairyNET
