Merck plans feedlot testof Zilmax

NASHVILLE, Tenn. — Merck Animal Health is looking for 240,000 feedlot animals on which to test the effects of Zilmax.

The company voluntarily stopped sales of its growth enhancer last August after reports about lameness in finished cattle. 

Tyson Meats had earlier an-nounced it would no longer accept cattle that were fed the product because of problems detected in animals coming to its plants. 

Ron Bryant, head of Merck’s ruminant products group, outlined the company’s plan to investigate the product during the National Cattlemen’s Beef Association convention held in Nashville Feb. 3-7.

It will be tested in different weather conditions and locations to assess whether the product causes lameness or other foot problems. Animals will be assessed at feeding, transportation, offloading and staging at the processing facility. 

The data will be collected and audited by a third party. 


“Industry will let us know when the questions have been answered and when they are comfortable with the technology,” Bryant said. 

Some feedlots said they are willing to join the study, but packers must accept the cattle when they are ready to go to market. 

Taiwan and South Korea have rejected shipments when they tested positive for zilpaterol, the main ingredient in Zilmax.

Merck has also asked academics, veterinarians, nutritionists, feedlot owners and stocker-backgrounders to form an advisory board that will meet every few weeks. 

As well, the company is starting an annual certification program to ensure that producers who feed Zilmax have been trained to use the product properly. 


Producers who take the course must pass a test before certification is achieved. They then receive a certification number, which is required when obtaining Zilmax.

“There was some concern that maybe we had lost some control over who used Zilmax,” Bryant said.

“That is not an unfair statement.”

There have been cases where Zilmax was fed indiscriminately without Merck’s knowledge, he added. 

The company’s website reported that Zilmax sales in the United States and Canada were worth $159 million in 2012. 


Company and independent studies have found that zilpaterol hydrochloride fed for 20 to 40 days at the end of the finishing period enhanced growth performance and carcass muscle deposition for steers and heifers by about 15 pounds.