As newborn calves have no antibodies, they are vulnerable to infection from pathogens. Timely intake of sufficient high quality colostrum transfers antibodies to the calf, providing what is known as passive immunity.  

Speaking at the Calf Action Conference in April 2026, Ryan Davies from SCCL shared how to achieve higher levels of passive immunity in calves via colostrum management, which sets calves up for success from day one.  

 

What does excellent look like?

Immunoglobulin G (IgG) is the most abundant antibody found in colostrum, and is used as a measure of quality. Ryan explained that calf rearers are aiming to ensure a sufficient mass of antibodies are fed and also absorbed by calves.

Colostrum management can be monitored by testing IgG levels in the blood within a week of birth. Ryan defined 25 g/L IgG as ‘excellent’ and recommended this as the standard for calf rearers to aim to achieve.

 

How much colostrum is needed?

It is the mass of antibodies fed which is critical to achieving passive immunity in calves. The quantity of the colostrum fed and the quality – the concentration of IgG – both matter. The size of the calf also influences mass of antibodies needed.

This can be summarised neatly in an equation:

Mass of antibodies = (Quantity x Quality of colostrum) ÷ blood volume.

A calf can take in 10% of its bodyweight in its first feed, and 5% in its second. Because there is a limit to the quantity of colostrum that can be fed, this is why quality is important too. 

 

Measuring colostrum quality

Understanding colostrum quality can be used to provide information on how much is needed, or if it needs to be supplemented with powder to provide sufficient antibodies.

A Brix refractometer can be used to measure the quality of colostrum, as Brix scores provide an estimate of IgG concentration. As a guide:

  • 22% Brix ≈ 50g/L IgG
  • 25% Brix ≈ 75 g/L IgG
  • 30% Brix ≈ 100 g/L IgG

 

Estimating a calf's antibody requirement and whether it is being supplied

Ryan shared that the mass of IgG required by a calf can be calculated by multiplying its bodyweight (kg) by 7.5.

By combining information on calf bodyweight, the volume of colostrum fed and its quality, calf rearers can estimate whether enough antibodies are being supplied. If the calculated mass of IgG meets or exceeds the calf's requirement, they can be confident that they are feeding a sufficient mass of antibodies via colostrum.

 

How to ensure antibodies are absorbed

The quantity and quality of colostrum fed is not the only factor in ensuring transfer of passive immunity, as sufficient antibodies actually have to be absorbed by the calf.

Timing is critical here, as feeding within two hours of birth ensures the antibodies are received when the calf is most able to absorb them.

Other factors include:

  • Providing a clean environment can result in better absorption
  • Stress can limit absorption, which may be caused by a prolonged labour, handling stress or the calf simply being cold
  • A high IgG also delivers better absorption of antibodies.

 

Assessing colostrum management

Routine blood testing is the best way to check if colostrum management practices are working. This is best done as soon a possible after the gut stops being able to absorb antibodies, between 24 hours and 7 days after birth.

Ryan shared that the target is 6.2 g/dL serum protein, as this indicates an excellent level of passive immunity has been transferred. If serum protein levels are low, he recommends either testing colostrum and saving the highest quality, or adding powder to increase the Brix score. The SCCL Colostrum Calculator App can be used to do this.

Ryan's session on colostrum management was one of several at the Calf Action Conference that focused on achieving consistency in calf rearing through practical, evidence-based approaches. Throughout the day, farmers, calf rearers, nutritionists, vets and researchers shared experiences and explored how technology and training can support the delivery of the GB Calf Strategy.