Resistance to disease is greatly dependent on antibodies or immunoglobulins and can be either active or passive in origin, says Oklahoma State University Extension specialist Glen Selk, PhD. Young calves depend on passive immunity, which largely depends on the calf receiving antibodies from its dam via colostrum within hours after birth. Research shows successful transfer of passive immunity during the first 24 hours enhances disease resistance and performance through the first two years of life including the feedlot phase. The calf’s ability to absorb immunoglobulin from colostrums decreases linearly from birth, Selk says. The transfer ideally takes place within six hours after birth, and by 24 hours, the calf absorbs little or none of the critical components of colostrum. Selk recommends providing high-risk calves, such as those born to thin first-calf heifers or calves that endured dystocia, at least 2 quarts of fresh or thawed colostrum within the first six hours of life and another 2 quarts within another 12 hours. This is especially important for those baby calves too weak to nurse naturally. Thaw frozen colostrum slowly in a microwave oven or warm water, as overheating can denature the proteins. If at all possible, feed the calf natural colostrums first before feeding commercial colostrums substitutes. If natural colostrum is not available, commercial colostrum replacers containing 100 grams or more of immunoglobulin per dose can be fed within the first six hours.