Heat stress and dairy cows: A look into the internal and external consequences

As the days last longer, heat becomes an increasing concern for producers. Inside the barn, cow density increases heat and humidity. The high temperature doesn’t just affect the cows’ welfare; it disrupts the balance of their rumen microbiota, impacting digestive comfort and milk production. As a result, cows will change their behavior to cope with this environmental stress.

Heat stress occurs when the temperature-humidity index (THI) is above 68, and it has a profound effect on the rumen microbiota of dairy cows. Heat stress can affect milk yield but also milk components. Milk composition shifts, mirroring the internal turmoil caused by the rising temperatures.

Changes in behavior occur in pastures: Look for shade and fly-load sharing

Once heat stress occurs, it’s well know that cows reduce their feed intake (Bouraoui et al., 2002). A recent study coming from UK studied bunching behavior. This behavior can occur due to social interactions, stress, or shared interest in resources. It is more frequent at higher temperatures, possibly due to fly-load sharing or seeking shade.

A study tracked a UK dairy herd’s spatial position and activity, finding increased bunching behavior at temperatures above 20°C. This behavior correlated with localized temperature variations but decreased at higher barn temperatures. Bunching may increase localized temperatures, causing heat stress. Observing cows with this bunching pattern could help identify heat stress early and prompt corrective measures.

Changes in the herd’s spatial positioning during certain times of the day can disrupt normal behaviors like feeding, especially if stocking rates at the feed face increase (Hill et al., 2009). These changes can disproportionately affect cows with health issues, such as lame cows. These cows may adjust their feeding times to avoid competition (Blackie et al., 2011; Barker et al., 2018).

Internal changes: The rumen microbiota

The rumen is a complex ecosystem where various microorganisms play essential roles in digestion and overall health. High temperatures disrupt the balance of these microorganisms, leading to several physiological changes.

According to the study “Differential Dynamics of the Ruminal Microbiome of Jersey Cows in a Heat Stress Environment” (Kim et al., 2020), heat stress alters the microbial community within the rumen, which is essential for digestion and nutrient absorption. Under heat stress conditions, heat alters the diversity and abundance of microbial communities in the rumen. One of the first changes is a decline in beneficial bacteria crucial for fiber digestion and energy metabolism. This leads to less nutrient absorption and lower digestive efficiency.

This study also shows that heat stress increases lactate-producing bacteria and decreases acetate-producing bacteria in the rumen.

Lactate-producing bacteria, such as Streptococcus and certain Enterobacteriaceae, thrive under heat stress conditions. This leads to higher levels of lactate in the rumen. High lactate levels can lower ruminal pH. This creates an acidic environment that harms the growth of beneficial microbes.

On the other hand, acetate-producing bacteria, such as Acetobacter, decrease under heat stress. Acetate is a crucial volatile fatty acid for milk fat synthesis, and its reduction can negatively impact milk production. The imbalance between lactate and acetate production can lead to metabolic disorders, reduced milk yield, and compromised milk quality.

External changes: milk composition

Heat stress doesn’t just affect the internal workings of dairy cows; it also has noticeable impacts on the composition of their milk, negatively affecting its nutritional quality and  properties for cheesemaking. In 2024, Albenzio et al., concluded in their study, that “milk quality of Italian Mediterranean buffalo was affected by different classes of THI, which occurred during late spring and summer.”

Heat stress negatively affects the case in patterns and coagulation traits in buffalo milk. Specifically, milk from buffalo cows exposed to a high THI showed poorer milk clot characteristics, such as a lower rate of clot formation and reduced curd firmness.

Heat stress impairs selenium status

Exposure to high THI also results in lower selenium content in buffalo milk. Selenium is essential for antioxidant defense and immune function. The reduction in selenium levels under heat stress conditions is likely because of increased metabolic demands and oxidative stress, which deplete selenium reserves. This can impair the overall health and productivity of the buffalo cows.

Mitigating heat stress

To combat the effects of heat stress, farmers can adopt various strategies. Providing shade, ventilation, and cooling systems in barns can help reduce heat exposure. Also, adjusting feeding practices to include easier-to-digest feeds and supplements can help with rumen comfort. Using a rumen-specific live yeast, such as LEVUCELL SC (Saccharomyces cerevisiae CNCM I-1077), can help balance the microbiota at this specific challenging period where stressful conditions alter microbial communities. Using LEVUCELL SC has been shown to improve feeding behavior and increase rumination time, in addition demonstrating to higher milk yield (+ 2 kg energy corrected milk – ECM ) and fat milk content (+0.12%) than cows under heat stress not supplemented with the live yeast (De Santos et al. 2020).

Using an antioxidative solution, such as a selenium enriched yeast like ALKOSEL, can also support selenium levels in cows under heat stress conditions by providing a highly bioavailable selenium source to the cow through the diet.

Taken together, these tactics can mitigate the negative effects of heat stress, inside and out.

At Lallemand Animal Nutrition, we offer integrated solutions to help dairy farmers manage heat stress and support feed efficiency and the selenium status of animals.