Newborn calves gain temporary (passive) immunity against disease when they ingest colostrum from the dam, since this “first milk” contains maternal antibodies. After a few weeks or months, this temporary protection begins to wane, however, and calves must build their own immunities. Vaccinating calves at the proper time can help protect them until weaning age. Vaccinating them too soon, however, may not stimulate much immune response. If the calf still has maternal antibodies in its system, these tend to interfere with building its own immunity.