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OSU Extension

College of Food, Agricultural, and Environmental Sciences

May 25, 2021 - 8:00am --

With warm weather in the forecast, it is time to start thinking about heat stress and its impacts on your dairy. Just as we have our comfortable temperatures in which we keep our homes in the summer or winter, dairy cows are the same. They have upper and lower critical temperatures, or temperature thresholds at which cows must expend extra energy to cool or warm themselves. Dairy cows have a pretty large threshold – between 25- and 65-degrees Fahrenheit – meaning that below 25 or above 65 degrees their energy requirements will change. However, humidity also plays a large role in heat stress. Because of daily variability in humidity and temperature, the temperature-humidity index (THI) has been created to neatly portray the weighted effects across a wide range of values for each factor. According to researchers at the University of Arizona, the THI at which heat stress begins is 72. So, for example, on a sunny July afternoon in Ohio the temperature may be 84˚ with 5% humidity. The THI in that case is 70, below the threshold. Now, let’s say it’s the same July day except a pop-up shower just rolled through. Now, the humidity is 15%. That bumps the THI to 72 and your cows may start to feel the effects of the heat.

But what are these effects that we’re concerned about when temperature and humidity are on the rise? The negative impact of heat stress on milk production is likely the greatest concern when we approach these warm months. The decrease in production has a 1–2-day lag time from the time that the heat was impactful, however some research has shown the THI needs to be 74 or greater for at least 4 days before lactation is affected. Milk yield and milk fat yield is cyclical to begin with and bottoms out in the summer but adding extreme heat to the mix can likely exacerbate the issue. This decrease in milk yield is probably attributed to the decreased dry matter intake during the heat spell. Cows tend to decrease their appetite when they are hot to minimize the endogenous heat production caused by ruminal fermentation.

            Additionally, rebreeding cows may also be delayed in the summer for several factors related to heat stress. Estrous behavior is noted to be reduced in both time and intensity, decreasing the chance for visual observation. If estrus detection aids are used, such as a breeding indicator patch, cows may still be missed for rebreeding as mounting behavior is also decreased in summer.

            Other changes that may be observed in heat stressed dairy cows are increased thirst and respiration rate (panting). Dairy cows become severely dehydrated when water loss equals 12% of their body weight, and with exhalation being a main form of water loss, panting and water loss go hand in hand. On extremely hot days, providing enough water is essential to prevent hyperthermia and its effects such as inability to quench thirst and muscle fatigue.

            To prevent your cows from hitting the summer slump too hard, consider the following heat abatement strategies:

  • Industrial fans – The THI can be decreased by 1 unit with every 1 mph increase in wind speed. High-volume, low-speed fans or box fans are both options depending on space availability, but make sure to keep up on maintenance of both for optimum air and electricity efficiency.
  • Misters – Spraying a fine mist into fans can cool the air in the barn, thus decreasing the temperature of the air cows breathe in.
  • Sprinklers – Setting up overhead sprinklers to soak the cows’ hide works in the same way that sweating does, except the cow doesn’t have to spend the energy to perspire.
  • Adding shade structures – If cows are housed on pasture, a tree, shade cloth, or simple roofed structure can provide some relief from direct sunlight.
  • Barn architecture – If a new or remodeled facility is in your future, think about its position for both sunrise/sunset and prevailing winds. University of Kentucky Extension recommends an east/west direction to prevent as much direct sunlight as possible and likely matches natural wind direction. A 4:12 roof pitch with ridge vent and cap are suitable for moving hot air up and out of the barn. Large enough sidewall gaps are also essential for maximum airflow in the summer.

Regardless of the tactic you use, be sure to remain vigilant in monitoring your cows for signs of heat stress or dehydration and remember to stay cool yourself!


Haley Zynda is an OSU Extension Agriculture and Natural Resources Educator and may be reached at 330-264-8722.

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