Current Progressive Dairy digital edition

1008 PD: Summer heat stress and nutrition

John Hibma Published on 30 June 2008

One of the most significant environmental challenges dairy farmers face each year is dealing with the summer heat and humidity and its affect on cows and milk production.

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˚ and 50˚F and, as long as the humidity remains low, will remain comfortable even into the 80˚F range when not in direct sunlight. However, many U.S. locations see daytime temperatures exceed 100˚F 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˚F. These extreme conditions will quickly reduce feed intakes along with milk production and health.



We’ve all seen our cows crowded around a water trough on those hot, humid days. Or, 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 in order to compensate for lower feed intakes.

Don’t forget the water and keeping cows cool
Before you begin changing the ration, you need to make sure your cows have an unlimited supply of cool, clean water all summer long. When it’s hot and humid, this single item will go farther than anything else in reducing heat stress and milk production losses. After that, 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 your cows spend a lot of time out of the barn during the summer, make sure they have access to 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:

• If the temperature in your cows’ environment is over 80°F and humidity is over 30 percent, you need to start cooling things down.

• When the temperature climbs over 80°F in high humidity, a cow’s feed intake has decreased by 8 to 12 percent or more.

• When the temperature is more than 90°F, milk production has decreased by 3 to 20 percent or more and conception rates can be as low as 0 percent.


• Cattle sweat only 10 percent as much as humans.

• Panting can increase the cow’s maintenance requirement by 20 percent.

Nutritional management
There are a number of things we can do to manage nutrition during the hot summer weather. The main feeding challenge during hot weather is to maximize energy intake, while maintaining ration fiber levels and proper rumen health. Maintaining a balanced ration can be difficult because:

• 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.


• Cows that freshen in extreme heat and humidity are much more susceptible to metabolic problems such as displaced abomasums and ketosis as a result of poor feed intakes.

The Cornell-Penn-Miner (CPM) dairy nutrition model is an excellent tool to evaluate expected dry matter and milk production losses when temperatures and humidity become excessive. As an example, the CPM model showing 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 two gallons of milk when the temperature soars to 90˚F and humidity is at 80 percent. 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 condition 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. Along with that, 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 some nutritional guidelines to help manage nutrition during heat stress:

• Feed high-quality forages but keep the physically effective non-detergent fiber (peNDF) over 20 percent or rumen health will be affected.

• Keep the rumen buffered with sodium bicarbonate, 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. When potassium levels are increased, magnesium must also be kept near .4 percent of the ration, as well. Research has also shown that increasing potassium levels can increase dry matter intakes.

• Provide most of the ration during the night or cooler periods of the day.

• Add extra water to the TMR, 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 fresh in the manger or bunk.

• Adjust both energy and protein densities when dry matter intakes decrease.

• Feed a rumen-protected fat as opposed to increasing starch or tallow to increase the caloric density of the ration.

• Feeding a yeast product has been shown to improve fiber fermentation and dry matter intakes.

References omitted but are available upon request at

John Hibma for Progressive Dairyman