Vitamin A plays a critical role in young calf health. Calves are born with very limited vitamin A stores because little transfers from the dam during gestation. Newly born calves rely heavily on colostrum for their supply of vitamin A, making the cow’s late-gestation diet critically important. Milk is a poor source, so calves depend on the colostrum for their Vitamin A supply for the first few months of life.
For most herds calving in late winter or spring, cows are fed stored or stockpiled forages during their last trimester.
Source : unl.edu