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Cope with physiological challenges during heat stress

July 15, 2019

By Kai Yuan Ph.D, Senior Research and Technical Advisor

Heat stress costs US agriculture over 4 billion per year. Modern high producing cows begin to experience heat stress at a temperature-humidity index (THI) of 65-68. During heat stress, cows have decreased milk and milk components, reduced body condition, increased rumen acidosis, impaired gut health, and a significant drop in pregnancy rate.

Key physiological changes during heat stress:

  1. Cows have increased respiration rate. The increased expired CO2leads to a drop in blood HCO3. To compensate, the kidney dumps HCO3, consequently, there is less buffer available to the rumen. Rumen acidosis becomes more of a problem.
  2. Cows have increased sorting against forages. This is because that fermentation of forages generates more heat increment compared with the fermentation of concentrates. Oxidation of acetate (a byproduct of fiber fermentation) generates more heat than other types of nutrients. Therefore, cows have more sorting against forages, which further drop rumen pH. Depression of rumen pH is directly associated with low milk fat.
  3. Cows utilize much more energy for a non-production purpose. Research showed heat-stressed cows secrete 400 g less lactose than pair-fed thermal neutral controls. Those cows actually utilized much more energy (over 400 g/d glucose) in non-mammary tissues. Independent of the reduction in feed intake, heat-stressed cows require extra energy, especially glucose.
  4. Heat stress impairs gut health. During heat stress, a massive diversion of blood flow to skin and extremities to dissipate heat, causing coordinated vasoconstriction in intestinal tissues. This reduces nutrient and oxygen delivery to enterocytes and leads to oxidative stress. As a consequence, the tight junctions of enterocytes open and permeability of intestinal cells increases. The so-called leaky gut allows toxins to enter gut barrier and trigger inflammation. Cows have reduced nutrient uptake from the gut and impaired health and production.
  5. Heat stress impairs the production of both dams and offspring. Recent research from the University of Florida showed that dry cows under heat stress produced over 10 lb/d less milk throughout the lactation compared with cool cows. More importantly, after the calves were born, the calves had reduced growth rate and they also produced 10 lb/d less milk throughout the lactation. Therefore, the issue of heat stress can pass on to several generations. Strategies to alleviate heat stress should generate good returns, especially for transition cows.


Strategies to alleviate heat stress

  1. Heat stress abatement is the key. Improve management factors: ventilate and cool; reduce time in holding pen; exit lane cooling; don’t lock up cows during mid-day; feed early in the morning and late in the night; feed more frequently; push up often; provide adequate shade; clean water tanks daily.
  2. Feed high quality forages. Since cows have more sorting against forage intake during heat stress, high-quality forages will promote cows to eat more forage. Because rumen function is impaired during heat stress, high-quality forages that are free of toxins and molds are important to maintain rumen health.
  3. Feed molasses-based QLF products. Maintaining forage and fiber intake during heat stress is essential to long-term rumen efficiency, milk, and component production. Adding a liquid feed at 3-6 lb/cow/day helps reduce TMR sorting. Also, sugars in molasses-based liquid feeds improve NDF digestion, helping cows receive more nutrients and energy from forages in the TMR.  Feeding sugar to partially replace and complement search maintains a more optimal rumen pH, to help enhance rumen efficiency, forage digestion, and microbial protein production. Furthermore, heat-stressed cows require extra energy especially glucose, and sugars can efficiently provide energy to the cow through improved forage digestion and rumen efficiency.  Molasses-based liquid feeds enhance TMR consistency and palatability, encouraging cows to eat during warm summer weather.  Also, given the increased gut permeability and susceptibility to toxins, QLF is an excellent carrier for additives such as TMR preservatives and mycotoxin adsorbents.
  4. Feed additives that help cows cope with heat stress. Some of this type of additives include rumen buffer sodium bicarbonate; supplemental dietary potassium to compensate for sweat loss; beneficial yeast supplements that can improve rumen fermentation; TMR preservatives and mycotoxin binders to improve feed quality; supplemental Zn for gut health etc.


Cows experience significant physiological and metabolic challenges during heat stress. Decreased production and cow health contribute to economic losses. Strategies that help cows alleviate heat stress should be implemented to minimize long-term losses.

Monteiro et al. 2016. Journal of Dairy Science 99:8443-8450.

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