Around the summer of 2006, she says, she started seeing a new kind of problem among the cattle, especially when the weather got really hot. "You had animals that were stiff and sore-footed, animals that were reluctant to move," she recalls. "They act like the floor is red-hot. They don't want to put their feet down. And I had never seen these kinds of symptoms before, ever!"

The problems, she says, affect as many as 1 out of every 5 animals. She's become increasingly convinced that the problems result from the drugs called beta agonists.

Beta agonists are similar, chemically, to the adrenaline that our bodies produce. In humans, this class of drugs is used to treat asthma. When fed to cattle or pigs, though, the animals grow more muscle.

"These beta agonist drugs have a dramatic and profound and beneficial impact on production," says Guy Loneragan, a professor of food safety and public health at Texas Tech University in Lubbock. "They can add somewhere in the range of 15 to 30 pounds of beef to a carcass in the last three to four weeks of the fattening period." That makes a significant difference to a beef producer's bottom line, and as a result, these drugs are widely used.

Yet Loneragan, too, has come to believe that these profitable drugs may be causing problems.

Two weeks ago, at a meeting held by the National Cattlemen's Beef Association, he presented data showing that feedlot cattle consuming beta agonists died more often. At that same meeting, Grandin gave a talk about her observations of cattle that seemed reluctant to move. And an animal welfare expert from a big meat packer, JBS, showed a video of cattle that appeared lethargic, unable to walk properly.

Grandin says that video made people angry. "You've got a lot of people in the cattle industry who care about cattle, and they don't like to see cattle that are lame," she says.

As that conference was ending, another big meat packer, Tyson Foods, announced that it would stop buying cattle that had been fed one particular beta agonist: Zilmax. Zilmax, or zilpaterol, is considered the most powerful beta agonist.

Loneragan later had a chance to go through information that the company had collected on the effects of beta agonists. "As they pieced together their information — and as I had the opportunity to review their information — they felt that they had to act, and I felt that they made the right decision," he says.

Merck Animal Health, which makes Zilmax, initially defended its product, but decided last week to suspend sales of the drug. The company says that it remains convinced that Zilmax is safe for animals, and it expects to answer all the questions that people now have. Meanwhile, feedlot operators still can use another beta agonist, which is sold by the company Elanco.

The Food and Drug Administration, which approved these drugs, remains a bystander for now. In an email, an FDA representative said only that the agency "has received a very small number of reports" of problems with these drugs and that the "FDA will review any new information to determine if there is a safety issue."

Grandin, for her part, is not pushing for a ban on beta agonists. She thinks it may be possible to use these drugs in ways that avoid causing harm.

"Maybe you just don't do it in the summer, when it's hot," she says. "The doses are probably going to have to be cut back. But these problems have got to stop. I've laid awake at night about it. I've worked all my career to improve how animals are handled, and these animals are just suffering. It has to stop!"

Copyright 2013 NPR.

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