Merck (NYSE:MRK) announced today that it would suspend sales in the U.S. and Canada of Zilmax, a dietary supplement that promotes growth in beef cattle. The move follows an announcement from Tyson Foods (NYSE:TSN) on August 8 that it would stop accepting beef cattle fed with Zilmax beginning on September 6.

According to a report in Beef Magazine, Tyson sent a letter to its cattle suppliers entitled "animal well being," in which the company laid out its concerns about potentially deleterious effects in cattle from Zilmax supplementation. The letter elaborated:

There have been recent instances of cattle delivered for processing that have difficulty walking or are unable to move. We do not know the specific cause of these problems, but some animal health experts have suggested that the use of the feed supplement Zilmax... is one possible cause.

In its initial response on August 8 to Tyson's decision, Merck asserted its commitment to animal well-being, and defended the safety of Zilmax:

Worldwide regulatory agencies have reviewed extensive data on Zilmax and have concluded that use of Zilmax according to the label is safe in cattle. It is important to understand these data included rigorous animal health safety and well-being studies-conducted by University experts-that found the behavior and movement of cattle fed Zilmax is normal.

In its announcement today of the sales suspension, Merck characterized its decision as illustrative of its commitment to responsible beef. The company intends to conduct a scientific audit of Zilmax in conjunction with independent experts, following the full lifecycle of beef cattle to evaluate possible explanations for compromised mobility.

Zilmax sales in Canada and the U.S. were $159 million in 2012.