Farmers urged to act on heat stress as autumn-born calves are at risk

Dr Jon Mayer, ruminant veterinary advisor at MSD Animal Health, warns that heat stress during late gestation can compromise calf development in utero, with lasting knock-on effects after birth. 

He said: “From my experience both in clinical practice and on the family dairy farm, I’ve seen first-hand the negative impact of heatstress on colostrum quality and calf health. 

“Cows experiencing heat stress in the dry period often have shorter gestations, leading to smaller calves with reduced immunity. The dam’s lower dry matter intake also limits nutrients available for calf growth, which increases the risk of reduced birth weights, poorer immune transfer, and compromised performance.” 

The expert explained that knock-on effects from in utero heat stress can include: 

• Reduced birth weights 
• Lower survival rates 
• Impaired passive transfer of immunity 
• Slower growth rates in the rearing phase 
• Reduced lifetime milk yields and fertility in heifer calves. 

“This year, farmers should assume calves could be more vulnerable than normal, especially where cows were dry during the July and August heatwaves,” Dr Mayer added.

Colostrum management  

Colostrum management will be key to offsetting some of these risks. Dr Mayer recommends testing every batch with a Brix refractometer, feeding only colostrum that scores 22% Brix or higher, or boosting lower-quality colostrum with a high-quality replacer to reach the target Brix level.  

“Relying on poor-quality colostrum will simply add to the challenge,” he said. 

Vaccination to boost immunity 

Another key tool is vaccination of the dam ahead of calving, ensuring high levels of antibodies are present in colostrum to protect the calf from scour pathogens. 

“Calf scour remains the single most common cause of disease and mortality in pre-weaned calves. The main culprits are rotavirus, coronavirus, E. coli K99, and Cryptosporidium,” Dr Mayer added. 

Two vaccines are available from MSD Animal Health to help mitigate the risk: 

• Bovilis Rotavec Corona – a single-shot annual vaccine given three to 12 weeks before calving, protecting calves against rotavirus, coronavirus and E. coli F5 (K99) and F41 via colostrum antibodies. 
• Bovilis Cryptium – a two-dose primary course in the third trimester, followed by an annual booster, which protects against cryptosporidiosis and can be used alongside Bovilis Rotavec Corona. 

“Vaccinating cows before calving can enhance colostrum quality, potentially mitigating heat stress effects. This ensures calves should receive more antibodies against major scour pathogens,” Dr Mayer said. 

He also advises paying close attention to dry cow rations to support calf development in heat-stressed cows. 

“It’s vital the diet is properly balanced, with the right levels of energy, protein, fats and minerals. Using higher quality forages, adding supplements such as yeasts, and including rumen bypass proteins can be beneficial,” he explained. 

Water is equally critical. For every 1°C rise in temperature, cows can drink an extra litre of water. Farmers should check there is enough trough space, that flow rates are fast (at least 20 L/min), and that water is clean and potable. 

Wider herd implications 

Heat stress in dry cows is an increasing concern for UK herds as summers become warmer and more prolonged.  

But Dr Mayer stresses that it is not just about high temperatures: “Heat stress can strike at any time of year when humidity is high, so looking beyond the thermometer and considering the Temperature Humidity Index (THI) is often more appropriate.” 

Alongside reduced calf viability, heat stress can also affect cow fertility, transition health, and milk yield in the following lactation. 

Monitoring tools, such as SenseHub Premium, can highlight group heat stress, allowing mitigating actions to be taken before irreversible damage occurs.  

Dr Mayer also added that heat stress does occur in younger animals, so using SenseHub Youngstock tags offers vital 24-hour monitoring during such critical periods. 

“Calves born this autumn may carry the hidden cost of summer heat stress for months—or even years—to come,” he warned. “Simple steps around colostrum testing, hygiene, and vaccination will be more important than ever to protect calf health and long-term performance.” 

Key actions for farmers 

• Test colostrum quality with a Brix refractometer (aim for 22% Brix or higher)
• Store, test, label and freeze high-quality colostrum for use when needed 
• Use a trusted colostrum replacer with stated IgG content if quality is low 
• Consider feeding colostrum or transition milk for a longer period to support calf development, gut health and future performance. 

Read more livestock news.