Concentration of IgG in colostrum varies according to many factors, including a cow’s disease history, volume of colostrum produced, season of the year, and breed. Research has shown that IgG levels vary widely from one cow to the next and range from less than 20 to over 100 mg/mL. The difference between 20 and 100 mg/mL of IgG in colostrum can mean the difference between failure and success in passive transfer of immunity in calves. Measurement of IgG concentrations in colostrum can be very useful in managing colostrum quality and monitoring colostrum feeding practices.
Although high quality colostrum is typically very thick and creamy, appearance alone does not reliably predict IgG content. Volume of first milking colostrum also can be misleading and is not a recommended method for estimating colostrum IgG content. In addition, although IgG concentration can be measured very accurately in a laboratory, these tests are time consuming and not typically available to farmers. Colostrum containing 50 mg/mL or more of IgG is considered to be a high quality feed for newborn calves. Hydrometers and refractometers can be used on the farm to estimate colostrum IgG, separate high quality colostrum from low quality colostrum, and improve your ability to provide calves with enough IgG to attain successful passive transfer of immunity. Colostrum containing 50 mg/mL or more of IgG can be fed to newborn calves or stored for future use. Avoid feeding any other colostrum during the first or second feeding; lower quality colostrum can be mixed with transition milk and fed to calves that are at least two days of age.
The colostrometer is a hydrometer that measures specific gravity and, using a color-coded scale calibrated in milligrams per milliliter (mg/mL) of immunoglobulins (Ig), converts specific gravity to Ig concentration. The colostrometer is placed in a cylinder containing colostrum and allowed to float freely. Colostrum that tests “green” contains greater than 50 mg/mL of Ig, “yellow” contains 20 to 50 mg/mL, and “red” contains less than 20 mg/mL of Ig.
Colostrum components other than Ig affect specific gravity, so this method is somewhat variable in its ability to accurately estimate Ig concentration. In research investigating the colostrometer, the relationship between IgG and specific gravity has been reported to have an R2 value of 0.3 to 0.4. The practical implication of the variability in the colostrometer measurements is that the exact values reported on the scale are of less value than the categorization by color. The colostrometer is best used to separate good quality colostrum from poor quality colostrum.