SPRINGDALE, Ark. – Tyson Foods Inc. will no longer purchase cattle that are fed a growth promotant called Zilmax, a feed additive. The company expressed concern about cases of cattle with difficulty walking, however it did not know the cause of the problem. Tyson will suspend purchases starting Sept. 6.

According to a Bloomberg report, Tyson sent a letter to cattle suppliers stating: “Some animal-health experts have suggested that the use of the feed supplement Zilmax, also known as zilpaterol, is one possible cause” for the animals being unable to walk. The company iterated the issue is a question of animal welfare, not food safety.

News of the company's decision sparked a rally in Chicago Mercantile Exchange cattle futures Aug. 7. Removing Zilmax from feed would cause the cattle to lose weight, resulting in tighter supplies of beef and higher prices for beef.

Merck Animal Health, a division of Merck & Co. Inc., manufactures Zilmax. The company said the drug was deemed safe for cattle after extensive reviews by regulatory authorities.

"We are surprised by Tyson’s letter," the company said in a statement. "We are confident that, based on all of the available data on Zilmax, the experience reported by Tyson is not attributable to Zilmax. Indeed, Tyson itself points to the fact that there are other possible causes and that it does not know the specific cause of the issues it recently experienced. We will continue to work with Tyson to help it identify those other causes. Again, we are confident that the totality of our data does not support Zilmax as being the cause of these experiences, and we remain confident in the safety of the product."