Cattle eat a ration of feed on a farm August 3, 2012 near Cuba, Ill.
Cattle eat a ration of feed on a farm August 3, 2012 near Cuba, Ill. - 
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What’s the difference between a steer with Zilmax and a steer without? Just about 30 pounds. 

“Cattle are similar to humans,” says Len Steiner, co-author of the Daily Livestock Report, “you keep eating and eventually you stop growing and you put on fat.”

Zilmax changes that. It’s like a get-buff drug for cattle. “Instead of putting on fat, they put on some additional muscle,” says Steiner. And we’re talking a whole lot of muscle on a whole lot of cattle.

Merck, which makes Zilmax, estimates that 70 percent of U.S. cattle sold to slaughterhouses are fed either Zilmax or a similar but less potent drug, Optaflexx.

Now, without Zilmax, feedlots are looking to switch fast.

Turns out there just aren’t that many untapped ways to beef up beef. “We already, in American agriculture and the cattle feeding industry, use all the available nutritional information that we have to maximize growth rate of cattle in the feed lot,” says Dan Schaffer, an animal sciences professor at the University of Wisconsin, Madison, “so there is nothing that is held back.” 

A cattle industry without Zilmax will be a smaller-cattle industry. “We’re looking at a total of about a 1percent drop in nationwide beef production from this issue by itself,” says Rich Nelson from Allendale Inc.

Less beef equals higher prices. At least, slightly higher prices. “On a per pound basis we’re probably looking a maybe an increase of about one to three cents per pound,” Nelson estimates.

He says one thing to watch is just how much consumers care that these drugs go in to making their steak.

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Follow Adriene Hill at @adrienehill