Heat stress costs the dairy industry millions, if not billions, of dollars. In fact, one calculation by Normand St-Pierre, dairy scientist at Ohio StateUniversity, placed the figure at between $900 million and $1.5 billion a year based on a number of assumptions, including milk at $13 per hundredweight, death loss at $1,800 per animal and a reproductive cost of $2.50 for each day open. It was a careful analysis that tracked all of the ramifications of heat stress in an average or typical year.   

Yet, the triple-digit temperatures that struck California over a two-week period last summer — and the resulting losses to the state’s dairy industry — were anything but normal. That bout

alone cost an estimated $1 billion. When the losses outside of California were factored in as well, the cost of heat stress went much higher.    

Going into this year, many people are focused on cow-cooling equipment, such as misters and fans. Often, that requires a large capital investment, and there isn’t always the need for the equipment once it’s installed. A more flexible approach involves nutrition. 

Based on new research from the University of Arizona, we have assembled the following questions and answers regarding a nutritional strategy for heat stress.

Q:  Under what circumstances does heat stress occur?

The Temperature-Humidity Index is often used to gauge the effects of climate on dairy cows. Heat stress occurs whenever the index reaches or exceeds 72. This can occur at temperatures as low as 75 degrees F, provided the relative humidity is 65 percent or higher. A copy of the index appears on page 32.

Q:  How does heat stress affect the cow physiologically?

Not only do cows tend to eat less during heat stress, but their bodies partition the nutrients differently. 

Much of it has to do with the way the cow utilizes glucose, a precursor for milk production. As the illustration on page 28 shows, glucose that would normally be used for milk production often gets diverted for other uses during times of heat stress.

Burning glucose generates less metabolic heat than burning fat, so a heat-stressed cow prefers to utilize more glucose in her muscle and organs, says Lance Baumgard, assistant professor of nutritional physiology at the University of Arizona. “The heat-stressed cow goes into survival mode. And by switching over to glucose, it produces less heat and, as a consequence, the animal can hopefully survive the bout of heat stress,” he says.

But this comes with a price. With less glucose reaching the mammary gland, the body is deprived of an important building block for milk production. 

Q: Does new research shed light on the problem?

Last year, researchers at the University of Arizona conducted an experiment to further explain the drop in milk production seen among heat-stressed cows. 

For 14 days of the 21-day trial, heat-stressed cows were kept in warm conditions ranging from 85 degrees F to 102 degrees F. They were compared to an equal number of “underfed” cows whose access to feed was restricted (to coincide with the drop in feed intake among the heat-stressed cows) and whose environment was kept at a constant 68 degrees F.

With feed intake the same, ambient temperature became the key variable. 

Milk yield in the heat-stressed cows dropped by 45 percent, compared to 19 percent in the “underfed” cows. The difference, says Baumgard, had to do with the physiology of the cows. The “underfed” cows (with no heat stress) metabolized fat for muscles and organs, but the heat-stressed cows metabolized more glucose because it burned cooler in their bodies.

Q: What specific nutritional strategies can come out of this?

Based on his research, Baumgard concludes that about half of the reduction in milk yield is related to reduced feed intake and the other half to physiology and the way heat-stressed cows partition nutrients.

There may be opportunities to affect the latter through nutritional intervention and modification.   

“I think there are some opportunities for the industry to recoup some of that milk,” Baumgard says, including:  

  • Feeding an ionophore like monensin. Research has shown that monensin increases production of a volatile fatty acid known as propionate. The idea is to increase the pool of available propionate that is later re-converted to glucose and makes its way to the mammary gland for milk production.  (See graphic on the opening page.)
  • Using bovine somatotropin to alter insulin sensitivity. By decreasing insulin effectiveness, less glucose is diverted to the muscles and more is used by the mammary gland to make milk.

And, don’t forget these traditional strategies during times of heat stress:

  • Provide plenty of water.
  • Feed during the evening or early morning when
    it’s relatively cool outside.
  • Cow cooling in general. “It’s probably the most important thing you can do because it lowers the stress on the cow and keeps her dry matter intake up,” says Joe West, dairy scientist at the University of Georgia’s Tifton campus. Accomplishing those things will make nutrition a whole lot easier.
  • Feed highly palatable, highly digestible forages. That way, you help keep dry matter intake up and also gain more energy from the forage portion of the diet. Often, in the summer, people are tempted to feed more grain or fat to keep energy levels up. But the heat-stressed cow is already prone to acidosis (see below), so feeding more grain can be a problem. If a larger portion of the energy can be supplied by the forage — relative to concentrates — you hopefully can avoid some of the conditions that lead to acidosis, West says. And, the more digestible the forage is, the less time it will stay in the rumen, which has its advantages during times of heat stress. “Digestion of fiber creates more heat in relation to concentrates or fat,” West says. But if the fiber passes through the system quickly, it produces less heat of fermentation. 
  • Feed sodium bicarbonate or potassium carbonate. A cow that’s totally heat-stressed and panting will blow off a lot of carbon dioxide, and that’s a problem because carbon dioxide helps create bicarbonate (and buffering capacity) in the cow’s system. With less buffering capacity, the cow is more prone to acidosis. When you give a cow sodium bicarbonate or potassium carbonate in the diet, you help supplement her natural buffering capacity. “The advantage of potassium carbonate is that it also supplies potassium, and the dairy cow has been shown to need more potassium in hot weather,” West says.
  • Consider adding yeast culture to the diet. A diet containing supplemental yeast culture may help the rumen utilize nutrients more efficiently.