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Let’s Talk About Heat Stress in Dairy Cows

As we head into summer, it’s time to talk about heat stress in dairy cows. Why is heat stress a serious issue for modern dairy cows you may ask? Well, heat stress results in over $900 million in economic losses to the U.S. dairy industry due to reduction in milk production, increased culling, and compromised reproduction (St-Pierre et al., 2003). The continued genetic selection for improved feed intake and milk production in the absence of consideration for heat tolerance, results in cows that are less heat tolerant. Unfortunately, with ongoing climate change, heat stress will become worse for dairies in the foreseeable future.

The thermoneutral zone for lactating dairy cows, which is the optimal temperature range for production within which no additional energy above maintenance is needed to regulate body temperature, is between 31 to 68F, and research suggested that the upper critical air temperature for Holsteins is 78F (Berman et al., 1985). In the U.S., large numbers of dairies will experience at least some degrees of heat stress especially when coupled with high humidity throughout the summer season. It’s no surprise that lactating cows are more sensitive to heat stress than non-lactating cows due to the elevated feed intake and metabolism from milk production. Similarly, high-producing cows are more vulnerable to heat stress than lower yielding animals (Spiers et al., 2004).

In addition to high temperature and humidity, if the cow has over 80 breaths/min, it’s a sign that the animal is too hot. An immediate coping mechanism for the animal when heat stressed is to reduce feed intake. This will cause a decrease in available nutrient for milk synthesis. In addition, the activation of thermoregulatory system will result in an increase in basal metabolism, which can increase the metabolic maintenance requirement by up to 25%, thus further exacerbating the metabolic stress and reduction in milk yield (Polsky et al., 2017). We know that adding shades and a cooling system such as water sprinklers are very helpful. For the animal itself, panting has been the main way for the cows to dissipate body heat when experiencing heat stress, which allows the cow to blow off hot, moist air from the lung, thus decreasing body temperature. However, the cows will not be able to chew their cuds and thus the normal buffering (sodium carbonate) to the rumen from saliva decreases dramatically. As a result, adding sodium bicarbonate and potassium carbonate to the diet is especially important during heat stress. Recent research also shows that adding potassium carbonate to the diet will improve the milk fat yield, possibly due to the decrease in intermediate metabolites (trans 10 CLA) in the rumen that suppress milk fat synthesis.

Fei Sun, PhD, PAS
Dairy Product Technical Manager, Origination LLC.

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