During pregnancy, certain molecules important to immune system defense called antibodies, are not passed from dam to calf. When calves are born, they have inactive immune systems and therefore, need protection against potential diseases until their own immune systems mature. Colostrum provides just that as it contains antibodies called immunoglobins. Immunoglobins have important roles in the immune system recognizing and destroying pathogens. Bovine colostrum contains 3 types of Immunoglobin – A, G, and M. However, IgG is over 85% of the total immunoglobin concentration in colostrum. However, the calf's gut can only absorb IgG in the first 24 hours of life. The earlier that calves receive colostrum the better though as the gut's absorptive ability slowly decreases over those first 24 hours. Really consumption within the first 4 to 6 hours is best. Failure to receive an adequate amount of IgG results in failure of passive transfer of immunity. Calves with failure of passive transfer are more likely to get sick.
Research done at USMARC in Clay Center, NE found click here