Cows, like dogs and people, like a good scratch. Outside, they'll rub their bodies against fence posts or trees to remove parasites or just stay clean. Some do it so much, they can break radio transmission towers if you don't fence it off.

But many dairy cows in the United States never go to pasture. And even when they do, cows may spend winters tied up in a barn. So if a cow has an itch to scratch — what's a cow to do?

In a lot of places, nothing.

But in some places, there's the mechanical brush.

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This bristly, swiveling, motorized apparatus spins when a cow touches it, allowing the animal to reach places it couldn't on its own. On average, cows will spend seven minutes a day rubbing their heads, necks and backs on these bulky body buffers. And some researchers think these mechanical brushes aren't just some spa amenity for dairy cows — they're important to the animal's well-being.

"We have no idea how these cows think," said Marina von Keyserlingk, who studies animal welfare at the University of British Columbia in Canada. "But what we do know is that she's highly motivated to brush. And what happens if she can't?"

Testing the animals' willingness to work for access to fresh feed, mechanical brushes and empty space, Dr. von Keyserlingk and her team trained pregnant, healthy, indoor dairy cows to open a weighted gate. By looking at how much weight they were willing to push before giving up, the researchers got an idea of the relative importance of each resource to the cows.

The researchers suspected the brush would come in second for hungry cows. But the cows worked just as hard for the brush as fresh food. Their results, published Wednesday in Biology Letters, suggest that a cow may need mechanical brushes for grooming indoors and that dairy farmers should consider having these in their barns.

The brushes may benefit farmers by keeping cows from destroying surfaces inside barns and pleasing consumers who increasingly want to know that the animals are healthy and, more important, happy.

For the cows, using mechanical brushes may ward off parasitic outbreaks, scratch itches and remove dead skin. But grooming also helps many species — perhaps including cows — cope with stress.

"The way I see a cow move under that brush goes way beyond just relieving that itch," said Temple Grandin, a researcher at Colorado State University, known for her work on farm animal behavior and welfare and who was not involved in the study.

Animals have emotions, perhaps not as complex, but similar to humans, she said (although the nature of these emotions is a hot topic among animal behaviorists). They have the same basic nervous system and the same neurotransmitters in emotional parts of the brain.

"What they don't have is a gigantic cortex that can do things like fly to the moon or build that gigantic computer you're using right now," said Dr. Grandin. "I'm going to say the dairy cow enjoys it. It's like going to the spa."

But Dr. Grandin and Dr. von Keyserlingk think because the cows are so determined to get to the brush, grooming with it may be more like the need to clip your nails.

Mechanical brushes are required in Denmark, but not in the United States or Canada. And it may be some time before we see more of them, said Dr. von Keyserlingk, because states and provinces govern animal care in the United States and Canada. They are also expensive and high maintenance.

Instead, industry and consumers may drive the demand for cattle brushes.

"If you can get industry to take charge and they can adapt these best practices, they're much more nimble than legislation," Dr. von Keyserlingk said.

In the end, she said, "science can only tell us what the options are. It can't tell us what we ought to do."