The immunity a calf attains by consuming colostrum is called passive immunity. Passive immunity is received from an outside source. Newborn calves achieve passive immunity by consuming colostrum shortly after birth, thus providing essential proteins called immunoglobulins (Ig).
Attainment of adequate passive immunity is often associated with blood IgG concentrations greater than or equal to 10 g/L at 24 hours of age. The incidence of calf death is increased when blood Ig levels fall below this threshold (Figure 1). Absorption of Ig must occur before the calf’s intestine becomes impermeable to the large Ig proteins. The development of impermeability is called gut closure. Typically, a healthy calf that has access to liquid feed or has consumed colostrum will undergo complete gut closure by about 24 hours after birth (Figure 2).
Figure 1. Effect of passive immune status (blood immunoglobulin concentrations) on newborn survival
Immunoglobulin may be fed to calves using a nipple bottle or an esophageal feeder. Healthy calves should consume three to four quarts of colostrum within twelve hours of birth. The esophageal feeder is a long, narrow, rigid tube that is inserted down the esophagus of the newborn calf. A bottle or bag attached to the other end of the tube contains the fluid that flows into the calf upon release.
Producers may wish to have a stored supply of quality colostrum available. This can be achieved by freezing high-quality colostrum collected from the first milking after calving. The concentration of Ig in colostrum is a common indicator of colostral quality. Colostrum containing greater than 50 g Ig/L is considered to be high quality. Colostrum may be stored frozen for up to a year with no significant loss in Ig. Measuring colostral quality on the farm can be achieved by the use of a colostrometer. The colostrometer is a hydrometer device calibrated to associate specific gravity of colostrum with Ig concentration. When using this instrument, it is critical to control the temperature of the colostrum being tested. Variations in colostrum temperature impact specific gravity. To avoid Ig destruction, it is important not to overheat frozen colostrum during thawing. A good practice to follow is to submerge the sealed, frozen container in a bath of warm (not hot) tap water until it thaws completely. Thawing time will vary depending on container size. Storing two quarts in a one-gallon sized freezer bag laid in the freezer allows for quick thawing at this critical time.