Beta agonists add on the pounds

Optaflexx, Zilmax tested | Heavier carcass weights achieved, including those of Holstein steers

Beta agonists in livestock feed have come under fire, but the products have proven results in increased weights, carcass leanness and improved dressing percentage. 

They were first developed for asthma patients to alleviate breathing problems, but later research discovered a positive growth response, first in mice and later livestock, said Lee-Anne Walter, a PhD candidate at West Texas A & M University.

“There is a huge pool of results on beta agonists,” she said at a veterinary conference in Calgary June 19-20. 

Producers have to make their own decisions about their use, but research trials from universities and manufacturers show a considerable improvement in carcass weight gain. 

Elanco manufactures Optaflexx, which contains ractopamine hydrochloride, and Merck produces Zilmax, which is made from zilpaterol hydrochloride. Another product, Actogain from Zoetis, is awaiting approval. It also contains ractopamine.

Many countries have banned meat containing ractopamine.

Last year, Zilmax was voluntarily pulled from the North American market pending further tests because the product was suspected of causing lameness. The manufacturer did not respond to requests for further information on the Zilmax trials. 

The products have been tested on thousands of feedlot animals with consistent results.

Elanco tested 26,500 animals that were fed varying levels of ractopamine.


“The more Optaflexx they are fed, there will be an increase in the amount of body weight gain,” Walter said.

Feed efficiency and average daily gain improved from 2.97 pounds per day among the control animals to 3.8 lb. when they received 300 milligrams per head. 

Hot carcass weight gain also im-proved.

Cattle that received 100 mg of the product had a seven lb. carcass weight gain, while those fed 300 mg saw carcass weight improve by 30 lb. over the control group. However, marbling scores went down. 

Fifty-nine percent of cattle in the control group graded Choice compared to 54 percent of those receiving the additive. Choice is the equivalent of Canada AAA. 

“You start to see some shifts in those quality grades … but it is not a major shift,” Walter said.

“It is definitely something that can be managed.”

A test using 26,600 steers fed Zilmax resulted in a 19 lb. body weight gain and 33 lb. of hot carcass weight. Yield grades also improved, but there was a seven percent drop in Choice or AAA and an increase in AA grade.


“You need to be able to manage that as a producer if that is an important part of your value proposition with the packers or whatever Choice-Select spread you are locked into,” Walter said. 

“Quality grades can be managed with longer days on feed with both Zilmax and Optaflexx.”

Other studies that focused on heifers showed Zilmax had positive results. 

A more impressive result was found when feeding Holstein steers. These cattle are longer, taller and light muscled, and the carcasses do not fit well into plant grids because the rib eyes are narrow and the meat is lean. 

This additive helped, but the cattle need to be on feed for much longer. 

Elanco studies showed ractopamine resulted in an 11 lb. advantage in terms of hot carcass weight, and Merck showed Zilmax added 18 lb. 

These products could change the economics of feeding Holstein steers because they gained better and increased their feed efficiency. Marbling scores were also generally better, but it took 305 days on feed to achieve those results. 

These products improve carcass weight rather than producing a heavier body weight.