Dealing with the summer heat and humidity and the effects on cow health and milk production is one of the most significant management challenges faced by dairy farmers each year. Summertime heat can be a tough gig for dairy cows. High heat coupled with high humidity quickly takes its toll on milk production, reducing a cow’s milk production by 50 percent when conditions are bad enough. Prolonged exposure to high heat and high humidity can prove lethal.

Dairy cows produce a lot of internal heat and naturally tolerate cooler outdoor temperatures much better than high ones. Cows are most comfortable in a temperature range of 40-50 degrees Fahrenheit and, as long as the humidity remains low, will remain comfortable even into the 80 degrees Fahrenheit range when not in direct sunlight. However, many U.S. locations see daytime temperatures exceed 100 degrees Fahrenheit for extended periods during the summer, and many more will experience both high humidity levels from 60 percent to 100 percent along with temperatures above 80 degrees Fahrenheit. These extreme conditions cause significant discomfort and will quickly reduce feed intakes along with milk production and health.

When cows are hot and uncomfortable they will crowd around a water trough on hot, humid days. If they’re outside they might be standing or lying in a mud hole, perhaps under a tree or next to a shed. Along with simply not being able to cool down, cows that are miserable in the summer heat are not inclined to eat. Dairy diets need to be reformulated to increase nutrient density to compensate for lower feed intakes.

First, during the summer, cows must have an unlimited supply of cool, clean water available night and day. When it’s hot and humid this single item will go further than anything else in reducing heat stress and milk production losses. From an environmental management perspective, focus on evaporative cooling with body sprinkling and good air movement with the generous use of fans, if possible. Avoid overcrowding in wash pens and feeding areas. If cows spend a lot of time out of the barn during the summer, make sure they still have access to plenty of water and provide shade to get them out of the direct sunlight for part of the day.

Consider these other important facts regarding environmental heat stress:

  • Cows must be cooled down starting when the temperature in their environment is over 80 degrees Fahrenheit and humidity is over 30 percent.
  • Cows will begin panting when the temperature is over 80 degrees Fahrenheit and humidity is over 30 percent.
  • Cattle sweat only 10 percent as much as humans.
  • Panting can increase the cow’s maintenance requirement by 20 percent.
  • When the temperature climbs over 80 degrees Fahrenheit in high humidity, a cow’s feed intake will decrease by 8 percent to 12 percent or more.
  • When the temperature is 90 degrees Fahrenheit or higher, milk production has decreased by 3 percent to 20 percent or more and conception rates can be nonexistent.
  • Five to six pounds of feed dry matter can translate into a gallon or more of milk loss.

Cows pant in an effort to keep themselves cool. Under optimum conditions the respiration rate of cows is around 65 breaths per minute. As cows approach mild heat stress their breathing rate increases above 70 breaths per minute. Respiratory rates above 80 breaths per minute are indicative of a serious heat load in the cows. All of this discomfort will prevent a cow from eating the way she should.

As cows reduce their feed intakes, metabolizable energy and protein take a steep dive. The main feeding challenge during hot weather is to maintain energy intake as much as possible, while maintaining dietary fiber levels and proper rumen health. Diets should be reformulated to reflect a higher energy density to adjust for the reduction in total pounds of feed consumed by the cow.

Maintaining a balanced ration can be difficult because of the following:

  • Feed intake drops.
  • Cows prefer grain to roughage in hot weather, which can lead to rumen acidosis.
  • If forage quality is variable, sorting may occur, again disrupting the absorption of nutrients.
  • Cows that freshen in extreme heat and humidity are much more susceptible to additional metabolic problems such as a displaced abomasum, ketosis and laminitis as a result of poor feed intakes.

Dairy nutrition models help predict and evaluate dry matter and milk production losses when temperatures and humidity become excessive. As an example, a ration that’s formulated to support 85 pounds of milk in the cooler spring weather will indicate a cow reducing her dry matter intake by 5 pounds and a corresponding decrease of 2 gallons of milk when the temperature soars to 90 degrees Fahrenheit and humidity is at 80 percent. The energy density in that ration needs to be increased significantly. If a feed ration is not adjusted to account for the reduction in dry matter intake, nutrient intakes will be depressed and body conditions will also suffer.

Low fiber levels can lead to rumen upset, which often will further reduce feed intake. Trying to compensate for reduced energy intake by adding more grain to the ration may only compound the problem. When a cow pants excessively she’ll drool large amounts of saliva, losing potassium and sodium that are necessary for rumen buffering and other metabolic functions.

Here are additional nutritional guidelines to help manage nutrition during heat stress:

  • Feed high-quality forages but keep the physically effective neutral detergent fiber (peNDF) over 20 percent or rumen health will be affected.
  • Keep the rumen buffered with sodium bicarbonate (or related products) especially if the effective fiber levels decrease through increased grain consumption.
  • Potassium is critical in maintaining proper acid-base balance during periods of extreme heat. Maintain potassium at least at 2 percent of the total ration dry matter. When potassium levels are increased, magnesium must also be kept near 0.4 percent of the ration, as well. Research has also shown that increasing potassium levels can also increase dry matter intakes.
  • Provide most of the ration during the night or cooler periods of the day.
  • Add extra water to the total mixed rations, silage or haylage if dry matter intakes drop seriously. This can encourage cows to keep eating.
  • If possible, feed ensiled feedstuffs more frequently to compensate for shorter bunk life during hot weather to prevent heating and spoiling. Keep the feed in the manger or bunk fresh.
  • Adjust energy and protein densities when dry matter intakes decrease.
  • Feed a rumen protected fat as opposed to increasing starch or tallow to increase the energy density of the ration.
  • Feeding a yeast product has been shown to improve fiber fermentation and dry matter intakes.

Read more: How to Manage Heat Stress in Dairy Cows