Summer has arrived. As we do every year, we hope to survive it without too many hot spells that make the cows (and us) miserable. There’s seldom a summer in which we don’t get at least one prolonged period of oppressive heat and humidity that knocks the milk production down by a gallon or more per cow per day. Heat stress also will cause a decrease in fertility that results in the lengthening of lactations. It’s not a matter of “if” it gets hot, but “when” it will do so. Dealing effectively with heat stress in dairy herds is essentially a management issue.

Heat stress results from a combination of environmental factors that exceed a cow’s comfort zone and ability to keep cool. Managing heat stress in cows begins with understanding what environmental conditions are optimum for cows and how they will react to temperatures and humidity above these norms. There are four environmental factors that influence the severity of heat stress:

  • Air temperature
  • Relative humidity
  • Air movement
  • Solar radiation

The optimal temperature range (thermoneutrality) for cow comfort is between 25 and 70 degrees Fahrenheit (-4 to 21 degrees Celsius). It’s easy to see that, for many regions of the country, the cows are most comfortable during the three months of the year that don’t include summer. Once temperatures climb over 75 F, with moderate humidity, cows will experience mild levels of heat stress. Under mild heat stress cows begin to cool themselves by sweating and breathing rapidly. However, cattle sweat only about 1 percent as much as humans, so increased breathing rates, all the way to open-mouthed rapid panting, becomes the main response for cows trying to keep cool.

Stifling effects of humidity

High levels of humidity exacerbate heat stress. Relative humidity in the atmosphere is a measure of the amount of water vapor in the air. Air containing higher levels of water vapor reduces the potential for evaporative cooling in a cow’s environment. If body heat is not allowed to dissipate by way of evaporation of sweat, the cow retains that heat and becomes more uncomfortable. A cow’s next response is to start panting to get cool air into her body. Even if humans are not feeling discomfort in a warm barn, a barn full of panting cows indicates that the cows are already experiencing heat stress.

Increased air flow is the first alternative to reducing heat stress as the movement of air slightly lowers pressure and allows for more evaporative effect. However, when the air is already highly saturated with water vapor, any additional evaporation from air movement will be minimal. The temperature/humidity chart illustrates the range of temperature and humidity combinations that correspond to different levels of heat stress. A temperature humidity index of 72 is considered to be the beginning of mild heat stress on cows. The chart illustrates that temperatures in the mid-80s with humidity levels over 50 percent will result in moderate heat stress. A temperature humidity index in the mid- to upper-80s is cause for concern for cow health and on-farm cooling strategies should be implemented.

Keeping cows out of the direct sun and providing them with shade when outdoors will reduce the potential for heat stress. Facilities and barn yards should be designed to minimize overcrowding while incorporating ways to improve air flow and evaporative cooling. Ample supply and unlimited access to water is absolutely critical to reducing heat stress.

Cows suffering from heat stress quickly stop eating largely in an effort to diminish the heat coming from rumen fermentation. The reduction in feed intakes will reduce milk production and put the cow at risk for various health issues. Because higher producing cows have higher feed intakes, they produce more metabolic heat and will be at greater risk. During periods of heat stress and reduced nutrient intake, cows can quickly fall into a negative energy balance. Less feed in the rumen means less fermentation and volatile fatty acid production as well as a reduction in rumen microbes and metabolizable protein. This results in fewer nutrients being available for milk production.

The temperature/humidity chart illustrates the range of temperature and humidity combinations that correspond to different levels of heat stress. Keeping cows out of the direct sun and providing them with shade when outdoors will reduce the potential for heat stress.

Nutritional needs during heat stress

Higher quality forages should be fed during periods of heat stress since they are more digestible, contain more energy and produce less heat during fermentation compared with poorer quality forages. Fiber fermentation, while necessary for healthy rumen function, can be as much of a hindrance as a help during heat stress. Forages lower in neutral detergent fiber should be made available when cows are under heat stress. However, there’s a fine line between supplying enough or not enough effective fiber in a ration – especially for high-producing cows in early lactation. Feeding forages that are too low in fiber will quickly result in acidotic rumens. It’s highly recommended that an additional buffer, such as sodium bicarbonate, is added to a diet when fiber levels are reduced.

An effective way to maintain energy density in a diet is by adding a rumen inert fat. Rumen bypass fats are not subject to rumen fermentation. These fats provide energy by being absorbed in the small intestine. Sugars can be added to the diet since they produce less heat than grains and other commodity byproducts.

Digestible protein levels in heat stressed cows will also decline as feed intakes decline. While it’s highly recommended that milk cow diets are balanced for appropriate metabolizable amino acids and not over-feeding crude protein, it’s advisable to increase rumen degradable protein (RDP) sources from feedstuffs (which increases total crude protein) so long as the energy status of the cow is maintained. Excessive RDP in a diet in which metabolizable energy balance is in decline will cause the cow to use more energy to dispose of the excess nitrogen in the protein and further exacerbate the negative energy balance.

While avoiding heat stress on dairy farms can be impossible in some years, decreases in feed intakes and the occurrence of other metabolic problems can be mitigated through advanced planning. Heat stress in dairy herds, while not possible to completely eliminate, can be managed satisfactorily with minimal negative impact on cow health and milk production.

3 Tips to Managing Heat Stress in Cows

Looking for ways to prevent heat stress? Here are tips and tricks to keep your dairy herds safe during the hot summer months. By Yelena Tischenko

1. Have Clean Water at All Times

According to the University of Minnesota Extension, water helps with dehydration and helps cool the calf or heifer internally. About 25 gallons of water per cow per day is required. The Washington State University Veterinary Medicine Extension recommends providing water right after milking because cows drink 50 to 60 percent of their total daily water intake immediately after milking.

Along with having enough water available, fans and sprinklers help with cooling cows. The combination of fans and sprinklers encourages greater feed intake and milk production, according to the University of Georgia Cooperative Extension.

2. Provide Shade Outside 

According to our Dairy Nutrition columnist John Hibma, cows are most comfortable in a temperature range of 40 degrees and 50 degrees Fahrenheit and will remain comfortable even in 80 degrees Fahrenheit as long as the humidity remains low.

Barns that provide adequate shade are popular. The University of Georgia Cooperative Extension recommends a feeding/cooling barn with or without free stalls. They have high roofs to minimize transfer of heat energy from the metal roof to the cows.

Avoiding overcrowding in pens is also key in keeping cows comfortable. If overcrowding is happening, that reduces the amount of airflow and makes it harder for the cow to reduce the heat.

3. Perform Activities in the Morning

Activities such as vaccinating, moving and dehorning should be performed in the morning when the temperature is lower and the sun isn’t out yet. According to the University of Minnesota Extension, the care and management of calves and heifers should be a high priority during the summer months.

While it’s impossible to change the temperature and humidity completely, it is possible to manage it and make sure your cows stay as comfortable as possible during the hot summer months.

Read more: Warm Weather Can Trigger Hypomagnesemia: Are You Prepared?


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