As calf research continues to progress, we are better able to intervene at key moments to positively alter a calf’s outcome. For instance, we know that testing maternal colostrum with a refractometer gives us the ability to ensure the calf will receive enough immunoglobulins in the first meal to reduce their risk for morbidity and mortality. With new research, we have a better understanding of other factors and management strategies that can be used to keep a calf on target:

1. Fine tuning our consensus of what defines successful passive transfer and failure of passive transfer in calves;
2. Benefits of multiple colostrum feedings in the first days of life;
3. Implications of heat stress during gestation.

It isn’t about passing or failing anymore

By increasing calf serum IgG concentration, the risk for morbidity and mortality decreases, thus, a new study (Lombard, 2020) is suggesting a graded system for evaluating transfer of passive immunity in calves. Calf illness continues to be a significant challenge, and one that has long lasting implications into future performance. Maximizing calf serum IgG content is one way to naturally prevent, or certainly lessen the severity, of common calf diseases (see Table 1). (adapted from Lombard, 2020):

Previously, a serum level of 10 g/L of IgG or more was considered a “pass”, however, a calf with “fair” passive transfer status is still nearly 8% more likely to experience illness than a calf with “excellent” transfer of passive immunity. This represents more interventions needed such as electrolyte therapy, antibiotics, and lost future milk production.

Is it just the first feeding that is important?

The timing of the first meal is critical, but also feeding colostrum beyond the first 24 hours of life. When comparing apparent efficiency of absorption (AEA) when feeding colostrum at birth, versus six to twelve hours after birth, the AEA drops from nearly 52% down to 35% (Fischer, 2018). Likewise, delaying the first meal of colostrum out to six or more hours has a negative impact on the number of beneficial bacteria in a calf’s gut. Moreover, extending colostrum, or transition milk, feeding beyond the first days has positive impacts on the early structural development of a calf’s small intestine (Pyo, 2020). Also note, a calf is born with minimal fat reserves, thus extending the supply of energy and nutrient dense colostrum, or transition milk, will have further benefit to the calf. With nearly 70% of the immune system located in the gut, it is critical to provide colostrum early to promote the proliferation of healthy bacteria and maintain feeding colostrum or transition milk for a few days after birth to stimulate early gut development.

Transfer of passive immunity is hindered by the heat

Under heat stress conditions during late gestation, a number of negative effects are imparted on the calf, but perhaps the earliest and most significant challenge is the reduction in a calf’s ability to absorb immunoglobulins (IgG) from colostrum. It is unclear what the exact mechanism is behind this, but there are two clear examples showing how significant this problem is. Both Tao (2012) and Laporta (2017) showed that calves born to heat stressed cows (no additional cooling measures provided during late gestation) had lower serum IgG concentration at 24 hours, demonstrating their reduced ability to absorb IgG.

AEA is a function of colostral IgG content, the amount fed, the size of calf (blood volume), and the end serum IgG result. As a practical example, if a 40kg calf was born to a cooled cow (AEA 33.6%, Tao 2012) and fed 4L of 22% Brix colostrum, the theoretical serum IgG level would be 24.3 g/L of IgG. However, if we took that same calf, but was born to a heat stressed cow (12.3% AEA, Laporta 2017), the theoretical serum IgG would then be 8.9 g/L of IgG, which would represent failure of passive immune transfer.

What management strategies can be used to overcome this challenge?

1. Provide additional cooling measures (extra fans, sprinklers) in the dry cow pen, as this will have benefits beyond
   improving effectiveness of colostral IgG absorption by the calf;
2. Testing all maternal colostrum, and increasing the target Brix value to compensate for reduced absorption efficiency,
   especially for calves born at night;
3. Freezing high value colostrum (e.g. 28%+ Brix) for use during summer months;
4. Supplementing maternal colostrum with a colostrum replacer to increase and standardize IgG content for all
   calves. As a quick rule of thumb, Grober Nutrition’s Calf’s Choice Total colostrum replacer provides a 1% Brix
   increase for every 15g of powder used per litre.
5. Above all, always remember that equipment used to collect and
   feed colostrum must be clean, as microbial contamination will
   further hinder the calf’s ability to absorb IgG.

For more information on the use of an extender with your whole milk please reach out to one of our Young Animal Specialists.