The consumption of colostrum by the neonatal calf has significant effects on the gastrointestinal tract [154] . The intestinal uptake in the immediate period after birth is transient and nonselective in species such as cattle, sheep, goats, swine and others. The intestinal cells become unable to absorb macromolecules within 24-36 h after birth probably as a result of developmental processes occurring in the enterocytes [155] . The process whereby the intestinal cells gradually stop absorbing macromolecules is termed -closure‖. Before closure, the enterocytes will nonselectively absorb large molecular weight proteins and other molecules [155] . Macromolecules so transported are released into the lamina propria and then are absorbed into the lymphatic or portal circulation. Failure of passive transfer of immunity in these species is defined as occurring when a threshold concentration of IgG is not reached before closure occurs, which in the calf corresponds to serum IgG levels less than 10 mg/mL [156] . The maternal IgG in the calf's blood gradually declines over the initial month after birth, and has a half-life of approximately 16 days [157] .