Merck & Co., Inc. (NYSE:MRK) said Friday it was temporarily suspending sales of Zilmax, the leading animal-feed additive used to quickly fatten up cattle for slaughter, after two meat packing companies said it caused heifers to have trouble walking.

Tyson Foods Inc. (NYSE:TSN), the largest U.S. meat packer, said last week that it would stop using meat from Zilmax-fed cattle in its products.

Just last week, Merck told International Business Times in an email that it was “confident” that Zilmax was not responsible for the symptoms Tyson described in a letter to cattle feedlots from which it buys livestock. The drug, a beta-agonist, speeds up weight gain in animals.

Then, earlier this week, Reuters reported that a second of the four major meat processors, privately-held JBS USA Holdings Inc., screened at a cattle industry conference on Tuesday a video showing cattle unable to walk properly. The U.S. Food and Drug Administration told Reuters on Friday that it was investigating the drug with Merck and the U.S. Department of Agriculture.

It’s unclear whether JBS suspended purchases of Zilmax-fed cattle. The company did not respond to calls requesting comment.

Cargill Inc., another leading meat packer, said last week that it had “no plans” to stop buying cattle fed the growth enhancer.

“About 75 percent of beef cattle in the U.S. is fed these types of feed supplements,” Mike Martin, a Cargill spokesman, told IBTimes at the time. “Cargill has no plan to change current cattle procurement practices.”

Leucadia National Corp. (NYSE:LUK), the other top-selling meat processor usually known as National Beef, could not be reached for immediate comment on Friday.

Shares of Merck were down 0.31 percent on Friday afternoon to $47.82.