Research indicates that passive immunity is often inadequate due to low concentration of colostral immunoglobulins, insufficient methods of feeding colostrum, and low absorption of immunoglobulins in the calf.
Good quality colostrum should contain more than 50g/L of IgG (the primary immunoglobulin in colostrum), which can be confirmed only by testing for antibody content. Quality may be inadequate if dams are young, nutritionally stressed, have a poor immune status, or produce large volumes of lower quality milk. Colostrum-deprived calves are 50 to 75 times more likely to die within the first three weeks of life, most of them in the first week. Commercially available colostrum supplements or replacers can play an important role in calf health, but it is necessary to understand the differences in these products based on their formulations and how to use them for optimum results.
Failure of passive transfer (FPT) is usually determined by a test evaluating plasma IgG concentration within 24-48 hours after birth. The critical level used to indicate FPT in calves is less than 10 g/L. Products are classified by their ability to raise plasma IgG concentrations. Colostrum supplements do not raise the plasma concentration above the species standard of 10 g/L, while replacement products do. Research results indicate that calves fed replacer products perform similarly to those fed maternal colostrum in terms of IgG levels, health, and growth rates.
The USDA Center for Veterinary Biologics regulates colostrum products containing IgG. In general, products that contain less than 100 g IgG/dose are categorized as colostrum supplements, and are designed to be used when feeding low or medium quality colostrum. Replacer products can be used to completely replace colostrum, as they contain greater than 100 g IgG/dose and also supply additional nutrients required by the calf (carbohydrates, protein, fat, vitamins and minerals). However, the quantity of IgG provided by each product does not accurately predict how much is actually available to the calf. The amount of IgG that can be measured in the plasma 24 hours after birth is known as the “apparent efficiency of absorption” (AEA).
Research data suggests that absorption efficiencies typically range from 20-35% in maternal colostrum and many supplements. Most veterinarians recommend that calves receive 100 g of IgG within the first 24 hours of birth; however, efficiency of absorption must be considered when determining type and amount of supplement to be provided. Considering average AEA’s, the actual amount of IgG consumed should be between 103 and 180 g in order to reach the critical plasma level of 10 g/L. Factors that can affect AEA may include source of IgG, method of processing, amount and type of protein, and presence of fat and lactose. Some research has shown that the addition of some colostrum supplements may actually reduce IgG absorption from natural colostrum.