It’s no secret that warm temperatures cause stress for dairy cows. While some effects are noticed immediately, like a drop in milk production, others rear their heads weeks or months later, like reduced reproductive performance and even lameness.

Coffeen peggy
Coffeen is a former editor and podcast host with Progressive Dairy. 

So what is really going on with the heat-stressed cow? John Bernard from the University of Georgia explained the series of events and their impact on the cow.

When the temperature-humidity index creeps up to 68 or higher, the cow begins panting as a way to relieve heat (Figure 1). As she pants and her respiration rate increases, she expels more carbon dioxide than a normal exchange, consequently reducing the critical balance of carbonic acid and bicarbonate in her bloodstream.

The dairy cow's response to heat stress

This can lead to a state of respiratory alkalosis if carbon dioxide levels are low and pH levels rise. The cow responds by “dumping” bicarbonate into the urine to keep things in balance.

This reduces the amount of bicarbonate that is recycled through saliva into the rumen to help buffer the acids produced during fermentation of feed. If she is unable to compensate for the amount of lost bicarbonate, the result then is a lower rumen pH. This change disrupts normal ruminal fermentation and rumination contractions.


As conditions in the rumen become increasingly acidic, receptors detect the change and slow down rumination, which makes a bad situation worse. As the cow chews less often, she produces less saliva, which means her natural buffer is in short supply.

“The body actually fights against itself,” Bernard added.

The problem actually compounds at night when temperatures cool down. The cow pants less, so carbonic acid rises, creating inconsistent levels throughout the day. This is when you may see metabolic acidosis.

“All this sets up a cow for increased incidence of subacute rumen acidosis (SARA),” he explained. Unlike acute rumen acidosis, commonly associated with slug feeding, SARA has been shown to be a direct contributor to laminitis. However, those symptoms usually take a month or two to reveal themselves, sometimes popping up long after the heat is gone.

Heat stress affects the cow more than just metabolically. She releases stress hormones like cortisol and epinephrine. At this time, her behavior also changes. Her natural instinct is to stand more so she can expose as much of her body as possible to air flow to try to cool off.

Simultaneously, the cow’s blood flow is being redirected. In an attempt to move heat outward, blood increases to extremities, such as skin and ears, and the regular pattern to the udder and feet is interrupted.

Battle heat stress at the bunk

With so many factors affecting the cow during times of heat stress, it’s no wonder we see her challenged to keep up with her normal performance. A good heat abatement plan certainly improves comfort, but don’t neglect the importance of balanced nutrition.

“The cow is always teetering back and forth during the summertime to maintain that balance,” Bernard said. “They do a fairly good job, but this is where we have to make some changes nutritionally to try to help her.”

One way to do this is by maintaining high-quality forages and high-fiber/high-digestibility feeds in the ration.

“During heat stress, one of the things we need to do is resist the urge to drop the forage or fiber level and increase the grain,” he noted. “We want to maintain fiber levels in those diets, preferably from forage.”

When cows aren’t eating as much, they won’t make as much milk, but giving them too much grain sets them up to be acidotic. It may be necessary, however, to increase energy density by adding fat to the diet which does not add to the acid load or require additional buffering and leaves room for forages and high-fiber feeds.

Bernard further suggested adjusting the ration for minerals and buffers. Help cows battle heat stress by increasing sodium and potassium, both of which are lost through sweating. Also, be sure to adjust for trace minerals, vitamins and other additives that promote healthy rumen function.

It’s not just what you feed that makes a difference; how you feed during heat-stress days impacts cows’ eating behavior, too. Aim to put fresh feed in front of cows at the times when they want to eat. Cows prefer to eat during cooler times of the day, so if possible, shift your feeding patterns.

Cleaning the bunk daily will also promote intakes. Keep in mind that during warm weather, some feeds are subject to secondary fermentation and spoilage, which can occur just a few hours after delivery.  PD

This information was presented at the 2016 Hoof Health Conference in Atlanta, Georgia, in February.

Peggy Coffeen