By Brad Haire
University of Georgia
Cows can be stressed when average daily temperatures rise above 77 degrees Fahrenheit. In Georgia, this happens up to six months every year, said Joe West, a dairy nutritionist with the UGA College of Agricultural and Environmental Sciences.
West and CAES graduate student Lani Pacetti are working on a dairy feed study at the Coastal Plain Experiment Station in Tifton, Ga.
Cows are less able than humans to get rid of excess heat, West said, because they have less surface area compared to internal body mass.
Dairy cows are most comfortable when outside temperatures are in the 40s and 50s. Most Southeastern dairies have sprinklers and fans to help cows stay cool. But it often isn't enough during Georgia's summers, when the heat index can reach into the 80s at night.
A hot cow will eat less and give less milk. Milk production can drop in the summer by as much as 20 percent in Southeastern states. Heat stress can hinder reproduction, too, and lead to health problems.
Part of the problem is that eating can heat up a cow, too. A meal high in fibrous carbohydrates, West said, takes more energy to digest, generating more heat inside the cow. A diet high in fat, though, takes less energy to digest and generates less heat.
West and Pacetti hope to mix common feed components like cottonseed, alfalfa hay, soybean hulls and corn silage into a ration that will keep cows from producing excessive heat during digestion while still getting the nutrients they need.
The theory is that by simply adjusting feed rations, farmers could help keep their cows cooler in summer. Cool cows produce more milk.
Pacetti said the eight-week study, which began June 13, will use three groups of 10 cows. She and West will monitor and track each cow's body temperature, weight variation, respiration and blood components, as well as the milk volume and fat content. They will also record the environmental conditions surrounding the cows.
Each cow in the study wears a magnetic key around her neck that lets that cow eat only from a certain trough, or gate, where she gets her experimental diet.
Group 1 will be fed a diet high in fiber and carbohydrates and low in fat. This should generate the most heat during digestion.
"It would be like us eating a big bowl of bran flakes," Pacetti said. This group can eat as much as it wants during each feeding. But cows should feel full with this diet and eat less than they normally do in cooler weather.
Group 2 will get a diet high in fat and starch. This should generate less heat during digestion. "This would be like us eating ice cream," Pacetti said.
This group can eat as much as it wants, too, which will probably be more than Group 1, she said.
The cows in Group 3 will be fed Group 2's high-fat-and-starch diet but won't be able to eat as much as they want. They'll get the same amount of calories that Group 1 decides to eat.
"The best outcome would be the cow producing less total body heat," West said. "This would make the cow comfortable and more likely to consume more feed and use the nutrients more efficiently, which would mean more milk."
A dairy cow can eat 100 pounds of feed each day. A good cow gives 11 gallons of milk daily. Georgia dairy cows produced more than 16 million gallons of milk last year, worth about $200 million.
(Brad Haire is the former news editor with the University of Georgia College of Agricultural and Environmental Sciences.)