Calculating Apparent Efficiency of Absorption

Apparent Efficiency of Absorption (AEA) describes the proportion of ingested immunoglobulin G (IgG) that appears in the calf’s circulating blood after colostrum feeding.

It is commonly calculated as:

The amount of IgG present in plasma is estimated from the calf’s serum or plasma IgG concentration and estimated plasma volume. A commonly used form of the equation is therefore:

with appropriate unit conversions.

Biological basis

Newborn calves are born with essentially no circulating maternal immunoglobulins because antibodies do not cross the bovine placenta in meaningful quantities. Protection during the first weeks of life therefore depends on absorption of immunoglobulins from colostrum.

During the first hours after birth, IgG molecules can cross the intestinal epithelium and enter the circulation. This ability declines rapidly with age and eventually disappears as intestinal closure occurs.

Not all IgG consumed appears in the bloodstream. Consequently, AEA is an apparent measure of absorption rather than a direct measurement of intestinal transport.

For example, if a calf consumes 200 g of IgG and an estimated 60 g subsequently appears in its circulating plasma:

AEA in the literature

AEA has been reported extensively in colostrum research because it allows passive transfer to be evaluated independently of the amount of IgG fed.

Typical values in well-managed newborn calves are often approximately 20 to 40%, although substantially lower or higher values can occur depending on factors such as:

• Age at first feeding 
• Amount of IgG consumed 
• Colostrum management 
• Maternal heat stress 
• Calf physiology 
• Experimental conditions 

AEA generally decreases as the calf becomes older at feeding. The CMC model also accounts for the observed relationship between AEA and cumulative IgG intake.

How CMC calculates AEA

CMC does not assign every calf a single fixed AEA. Instead, AEA is predicted using the selected CMC equation.

The available equations describe AEA as a function of variables such as:

• Age at feeding 
• Cumulative IgG intake 
• Prepartum heat stress, when using the Full model 
• Pasteurization, when using the Full model 

For calves receiving multiple feedings, CMC predicts AEA separately at each feeding using the calf’s age and cumulative IgG intake through that feeding.

The amount of IgG absorbed from each feeding is then calculated as:

The absorbed IgG from all feedings is summed before serum IgG concentration is calculated.

Thus, AEA provides the biological link between the amount of IgG fed and the amount predicted to reach the calf’s circulation.