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“HEAT BUSTERS. Who You Gonna Call?”

My mailbox and inbox have recently been overflowing with downloads and brochures about HEAT STRESS.  These written pieces are emphasized by capital letters and exclamation marks as many on-line suppliers and consultants provide information, strategies and, of course, their particular product that will combat this costly annual challenge. But great information is no good if it winds up in the garbage.  At the Bullvine we like to remind each other to ask the second question.  “What can I do with this?” If you have the herd contact person, ask the simple question, “What do you think we could do better to handle heat stress in our herd?”  

HEAT STRESS: Early Warning Signs and Symptoms

  • Increased breathing
  • Open mouth breathing (panting)
  • Slobbering
  • Trembling and loss of coordination
  • If they go down, recovery is unlikely

Take action when the first signs of heat stress are observed.  Survival depends upon effective intervention. Be particularly observant during the evening when cattle are trying to dissipate the heat built up during the day. Record observations and measurements.

HEAT STRESS AWARENESS TOOL: The Temperature Humidity Index 

Cows are large and their daily living processes means that they themselves are producing heat, in addition to the heat of the environment that they are living in. The effects of heat stress on dairy cattle are caused by a combination of high environmental temperature and relative humidity. These combined effects are measured by the THI Temperature Humidity Index. And used to assess the risk of heat stress and prevent harmful effects. Studies of THI have concluded that heat stress in cattle is avoided as long as temperatures are below 64 degrees Fahrenheit and when humidity is under 15%.  The optimal temperature for dairy cattle lies between 23 degrees F and 64 degrees F. At a temperature of 68 degrees F and humidity of 80%, a cow is already suffering from heat stress. It is clear that these conditions are repeatedly exceeded for extended periods of time during warmest months of dairy operation.  We can be sure that even though we humans may be comfortable; our cows are already experiencing heat stress. THI adds important analysis information. (for more information Excellent examples of how THI is formulated can be found online)

NEXT: Get Ready to Refine Results Beyond the THI Index

THI started being studied in the 1950s and has been available since the 1980s. There are apps available for doing the calculations.  One application doesn’t fit all situations.  It is necessary to know the predominant conditions in the area you are in as well as the relative humidity. Results are different in areas of dry heat (semiarid climates) or moist heat.  Present-day dairy operations need to plan ahead for the microclimatic changes caused by global warming and pollution. The actual Index also needs continual modification to more precisely interpret 24-hour results over extended time periods. Moving ahead, combining THI, body temperature and other indices (i.e. activity) will make it possible to individualize and effectively forecast heat stress.  

YOUR DAIRY HERD:  Who Else is Hot?  

Calves:  Two recent studies conducted at the University of Florida reported a lower pre-weaning average daily gain of calves from heat-stressed cows than those from cooled cows. As well, calves that experience in utero heat stress during the dry period maintain a lower body weight at least until 1-year-old compared to in utero-cooled calves. Multiple studies report that calves born to dry period heat-stressed cows had reduced efficiency to absorb immunoglobulin G (IgG) from colostrum, resulting in lower serum IgG concentrations during the first month of life.

Dry Cows: An article by Mark Pearce (Dairy Australia May 2016) stated that heat stress on dry cows has a dramatic effect on the development of mammary tissue in the udder and leads to decreased milk production in the following lactation.

KEEP COOL CHECKLIST: Take Immediate Action  

  • Check ventilation capacity and reduce any barriers to airflow
  • Increase ventilation rate when necessary (mechanical ventilation)
  • Make adjustments to achieve effective natural ventilation
  • Make sure all water troughs are clean at all times
  • Increase access to clean fresh water.
  • Keep all feed rations fresh and palatable
  • During hot periods, only have the cows on pasture during the night or during the cool moments (evening, early morning) of the day

COOLING OFF:  Dairy Stress Nutrition Strategies  

There are many sources who can provide advice and support when your herd is facing heat stress.  Don’t overlook the effect that targeted nutrition strategies can provide. Don’t consider the cost input without also calculating the dollars lost to dropping production or rising health problems.  Feed special rations (supplemented with additional minerals and vitamins) at least two times a day. In an experiment conducted at the University of Illinois (Pate et al, 2020 Journal of Dairy Science) the following was reported: “Protein in milk declines seasonally, just like butterfat, and the lowest point is reached in summer. “Heat-stress also reduces milk protein and milk fat depression during summer.” Protect against milk protein depression in summer with amino acid balancing and rumen-protected methionine supplement with a high bioavailability. 


Addressing heat stress from a genetic perspective presents a longer term solution. Relatively new on the breeding scene is breeding for the Slick gene in Holsteins.  It produces a shorter and smoother coat.  This is a gene with dominant heritability (like the polled gene) so that it makes it easier to introduce it into a population.  Sires are now available for carriers of the Slick gene. Slick animals in the tropics have been found to have 30% more sweat gland areas and 1.6 degrees Fahrenheit lower surface temperature.  University of Florida research shows Slick gene cows, 60 to 90 days in milk, produce 10 lbs. more milk per day in hot environments.  As well, calving interval for Slick gene cows was 30 days shorter than for normal Holsteins.

HOT STUFF: The Multiplying Costs of Heat Stress.  

In May of 2013 Hoards Dairyman published an article “How Much will heat stress cost you this summer?” It provided very interesting numbers to support the expensive side of dairy cattle heat stress. “It is estimated that heat stress costs the dairy industry anywhere from $900 million dollars to $5 billion each year depending on the calculation used.  The level of stress experience by an animal and resulting financial losses fluctuate as temperature and humidity go up and down.” “Knowing that heat stress does not typically happen for one day only, consider if a cow suffered heat stress for a period of 45 days; the losses for a 500 cow herd grows to $36,000 to $126,000. If the herd is milking 1,000 cows the losses become even more significant ranging from $72,000 to $252,000. These numbers don’t take into account reproduction losses and extended days open.”  These may not be your numbers but they may inspire you to take a realistic look at the financial impact of dairy heat stress on your operation.


As we move through human learning regarding responses to the Covid-19 pandemic, we are learning about the effects of crowding and physical distancing.  While heat stress isn’t contagious, crowded conditions are certainly another way that temperature impacts our herds. Cows that have spacious pens or pastures may still come into heated conditions while moving through holding areas. Barn fans are a mechanical solution to the moving air that is needed for groups of animals.  Assessments should be made to determine whether the moving air is actually on the animals or if it is largely blowing down alleys over people movement areas. Sometimes the fans are in the right place but the machinery we use for feeding and cleaning may block effective air flow onto the cows.

HEAT STRESS: Exercise Can Help Cows Adapt to Heat 

Studies have reported that cattle that exercise regularly spend less time in an elevated temperature, so they are less susceptible to hot days.  This can provide the added benefit of more milk components. Tim Rozell, an animal scientist with Kansas State University says, “We see increased protein in milk from exercised cattle. Last year, for example, we exercised pregnant heifers up to three weeks before they underwent parturition, and even 15 weeks or so into milk production, we saw increased protein in their milk, elevated lactose and other improvements in milk production.” Abi Wilson, A K-State master’s student in biology reports, “At the beginning and end of each trial, we take muscle biopsies. We are looking at specific enzymes, hormones and any changes in the skeletal muscle that may enhance their tolerance to heat, pregnancy rates and milk production.”

CLIMATE CHANGE: Will it Make a Dairy Difference?  

According to a recent study, the average number of days that feel hotter than 100 degrees in the U.S. will more than double by 2050. Scorching weather and lack of rain damages the quality of crops and the grass used to feed farm animals. This is even more concerning if weather conditions include the other extreme of too much rain and subsequent flooding. Some scenarios predict that climate change could lead to a 5 to 11% reduction in dairy production per year between 2020 and 2029 after controlling for other factors (see Journal of Dairy Science, Issue 12, December 2015, Pages 8664-8677). Research and extension efforts are needed to promote suitable dairy adaptation strategies.  You might ask, “Do animals beat the heat better by being inside or by being out outside?”  There are arguments to be made that pastured animals may be more vulnerable to the effects of climate change than cows that are housed. This is because housing provides shelter and technological options to mitigate the extremes of weather. There are no absolute answers.

HEAT STRESS: Simply Surviving a Few Hot Periods is NOT a Success Strategy 

Making it through to cooler temperatures may seem like a heat stress win which we might attribute to survival of the fittest.  Unfortunately, that attitude means accepting the long-term damage to current and successive generation of the dairy herd. It isn’t something that may happen.  It will damage your herd.

If the gene pool is too slow or too expensive, you might consider a more economical solution such as misting or water evaporation. For many, the solution of water misting seems obvious but, here again, it will depend on how well you manage the resulting humidity.  The plan is that the solution won’t make the problem worse instead of better. 


Multiple forces act on dairy cattle to send their body temperatures beyond normal levels. The goal of dairy management is to make it possible for each cow to meet her full potential for milk yield and fertility, without damaging heat stress. More research is needed to identify improved comprehensive cow-side measurements that can indicate real-time responses to elevated ambient temperatures. With this knowledge, effective heat abatement management decisions can be acted upon in the right way, right now!  It’s your call.




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