As I write this, it is snowing – for the 917th day in a row. OK, that may be an exaggeration, but it is cold, and heat stress is the last thing on my mind right now. I am more worried if the milk truck can make it up the driveway, if the waterers in the heifer barn are going to freeze (again) and if my kiddos will get another snow day (hopefully not). A few hot summer days sound amazing right now; then I remember that my currently comfortable cows may disagree. Right now, my girls are living their best lives – protected from the elements inside the barn, with a balanced ration, comfortable beds and low stress.

Bryan keith
Technical Services Manager – Ruminant DFM and Silage Inoculants / Chr. Hansen Animal Health & Nutrition
Morrill kimberley
Technical Service Manager / Chr. Hansen Animal Health & Nutrition
Marketing Manager / CHR Hansen

The imminent summer season that I am looking forward to brings a new set of challenges, and heat stress tops the list. Heat stress is more than just a buzzword. It can negatively impact every animal on your farm – regardless of age – along with you and your team.

Heat stress not only affects the productivity and health of lactating dairy cattle – which is easily observed – but also has a negative impact on dry cows, the developing fetus and calves born from heat-stressed dams. The estimated economic losses (around $1.5 billion per year) are driven by production and reproductive losses, along with the increase in morbidity and mortality of lactating cows. It does not include the impacts on dry cows or replacement heifers.

Using over 10 years of production data, a researcher from the University of Florida evaluated the impact of hyperthermia (heat stress) on the dam during late gestation and its impact on offspring performance across multiple generations and lactation. First-generation heat-stressed offspring had lower milk production in their first (4.85 pounds per day), second (5.07 pounds per day) and third lactation (14.33 pounds per day), as compared to offspring from dams that were cooled during their dry period.

The researchers also observed that late-gestation heat stress exerts a carry-over effect on at least two generations. The granddaughters of the heat-stressed dams produced less milk in their first lactation (2.86 pounds per day), as compared to the granddaughters of the dams that were provided heat abatement. Suppose we extrapolate the first-generation milk production numbers for 305-day lactations. In that case, that is greater than 7,000 pounds of milk lost per animal over three lactations, due to an event that occurred prior to birth. If milk price is $17 per hundredweight (cwt), that equates to $1,190 per animal. With this information, fans and sprinklers begin to look more affordable, and the economic losses tied to heat stress continue to increase.


The above results help highlight the economic advantage of investing in heat abatement infrastructure – such as shade, fans and soakers – for both lactating and dry cows and heifers. In conjunction with infrastructure, managers can also implement feeding strategies to improve the animal’s ability to manage heat stress. Understanding what is happening inside the cow during heat stress is just as important as any mitigation practice.

What’s happening inside the cow during heat stress?

Cows respond to hyperthermia with physiologic and behavioral attempts to minimize heat production and increase heat dissipation. Decreased feed intake is a key component of reducing metabolic heat production and is generally proportionate to the intensity of the heat load. Other behavioral changes to mitigate heat stress include increased time standing and altered meal patterns. Water intake also increases, largely to replenish evaporative water loss. In addition to these behavioral changes, blood flow redirects to the periphery to maximize radiant heat loss.

This means that less blood flow is going to the internal organs. Reduced blood flow and oxygen lead to reductions in digestive efficiencies and breakdowns in barrier function. Additionally, electrolyte and acid-base disturbances occur as consequences of sweating and increased respiration rate. Respiratory alkalosis stimulates the renal secretion of bicarbonate (HCO3_) to maintain a stable blood pH. The HCO3_ loss results in compensatory metabolic acidosis. Reduced salivary HCO3_, buffering capacity, reduced volatile fatty acid absorption, decreased dry matter intake, decreased rumination and increased respiratory rate can all contribute to reducing the rumen pH in lactating dairy cattle.

On top of the physiological changes occurring within the animal, heat-stressed cows have altered meal patterns and eat fewer, larger meals than those in thermal neutral conditions, which can also contribute to a decrease in rumen pH.

In addition to physiological and behavioral challenges, we continue to increase subacute ruminal acidosis (SARA) due to management practices (high pen populations, overcrowding and inadequate water access), improperly balanced rations (elevated fermentable carbohydrates, inadequate dietary fiber and inadequate electrolyte supplementation) and poor feeding management.

What strategies can I implement to mitigate heat stress?

  1. Water availability. Ease of access to clean drinking water cannot be overlooked. Dehydration limits the animal’s ability to thermoregulate and can increase the risk of lethal heat stress. Facilities should be designed or retrofitted to meet the water intake needs on the most challenging days.

  2. Diet composition. A nutritionist is truly an artist. Their goal during periods of heat stress is to balance the fermentable carbohydrates and digestible fiber to provide all the nutrients needed for production on lower dry matter intake (DMI) and without increasing the risk of acidosis. The use of effective probiotics can help support the normal integrity and function of the rumen and lower gastrointestinal tract. The main benefits of live yeast within the rumen are to remove oxygen and, by doing so, encourage an increase in cellulolytic bacteria and lactic acid utilizers while reducing lactic acid producers, which helps to stabilize pH. Probiotics improve the production of important volatile fatty acids, like propionate, that can be absorbed through the rumen wall and transported to the liver, where it is converted to glucose. In the lower gastrointestinal tract, probiotics can compete with enteric pathogens, increase the digestibility of the feed through effective enzyme production, enhance mucosal immunity and improve tight junction protein function – which helps alleviate leaky gut syndrome.

  3. Feed management. Do your cows have access to stable, clean feed when they want to eat? While it might not be convenient for the person in charge of feeding, the timing of feed delivery may need to be changed during periods of heat stress.

  4. Being observant and proactive. Do you need to make adjustments to other management practices? Are you acknowledging bottlenecks – such as overcrowding – and creating temporary solutions (e.g., more water access) while developing a long-term plan?

There is still a lot to learn about the physiology of heat stress and its long-term implications, but we need to focus on being proactive versus reactive. What are we doing today to prepare for this summer and reduce the risk of heat stress on our animals? While you might not be adding new waterers or fans in the middle of winter, you can develop a plan for where they need to be placed once the weather is decent. You can have conversations with your nutritionist and team about potential bottlenecks and changes that should occur prior to warmer weather. Reducing the risks of heat stress through multiple mitigation practices can help minimize health and production losses down the line.