Haylage rations also may be low in vitamin A if they do not have good green color. Use of pasture or green chop at a minimum of 50 percent of the forage dry matter for several months can replenish liver stores. Vitamin D supplementation at proper levels may improve calcium and phosphorus utilization, metabolism, and reproductive performance. Low intakes of vitamin D may result in rickets and weak bones, as well as weak or silent heats, especially in young stock. Vitamin D should be included in the formulation of the ration to avoid excessive levels.
Excessive intakes occur quite frequently.
An intake of 80,000 units per head daily may depress production. An intake exceeding 100,000 units for an extended period may increase the incidence of milk fever as well as infertility, joint problems, lameness, and heart failure. Vitamin E is most apt to be limiting with a ration high in hay or corn silage and haylage lacking green color. Both vitamin E and selenium are necessary for good resistance to disease. Low intake of vitamin E makes the animal more susceptible to infections and has a pronounced effect on the ability of white blood cells to kill organisms and on the production of antibodies.