Use of Beta-Agonists in Cattle Feed

By Dr. John Comerford

Photo by pippalou/

A recent furor in the beef industry over the use of the product Zilmax in feedlot rations has again raised questions about the use of feed additives in general. Cattle feeders use feed additives like Zilmax to increase the rate of weight gain without additional feed intake, making feed efficiency greater. On August 7, 2013, a major player in the beef processing industry, Tyson Foods, announced they would no longer purchase cattle that had been fed Zilmax, due to animal welfare concerns.

The reason Tyson gave was that there were incidences of cattle suffering from stress and lameness after transportation to slaughter plants. Further, it was noted that the issue was not one of food safety, but of animal welfare. With an eye on their customers and their perceptions of food quality, whether confirmed or not, Tyson took a huge step in alleviating those concerns.

On August 16, 2013, Merck & Co., the makers of Zilmax, suspended the sale of the product in the U.S. and Canada.

Merck recruited animal welfare specialist Dr. Temple Grandin to help consult with a review of the product. In an interview with Reuters on August 22, Grandin indicated that there had been incidents of stiffness, soreness and heat stress in packing plants since the use of beta-agonists began. In cases where problems were seen, Grandin said all of the cattle would be fine at the feedlot, but half of them would show some level of stress or symptoms of lameness after arrival at the packing plant. While these conditions were not shown in the safety approval process for Zilmax, such welfare incidences and actions by Tyson could make the product unavailable for a considerable period of time as the evaluation process continues.

Beta-agonists are a class of nonhormonal compounds fed to cattle. Their mode of action is to bind to receptors on fat cells in the animal's body and redirect and reduce the metabolism of fat. Consequently, less fat is produced and less fat is stored in the carcass. At the same time, the compounds bind to receptors on muscle cells and redirect and increase the size of muscle fibers. Muscle fiber size replaces some of the weight normally found from fat, and the total carcass contains a higher percentage of lean muscle. These actions reduce the energy supplied by the feed to produce weight gain. With more weight produced by the same level of feed intake, feed efficiency is increased.

The products go through an extensive approval process by the Food and Drug Administration before they can be used in livestock feeds. There are two compounds available to cattle feeders: Zilmax (zilpaterol) and Optaflexx (ractopamine), made by Eli Lilly and Co. Both products are fed at low levels (200 milligrams per head per day for Optaflexx and 60 to 90 milligrams per head per day for Zilmax) for a short duration just before slaughter (25 to 35 days).

A recent report suggested that 60 to 80 percent of feedlot cattle in the U.S. are fed a beta-agonist (Micik, 2013.) The Encyclopedia of Meat Sciences (edited by Dikeman and Devine, 2004) reported that Optaflexx increased average daily gain by 15 to 25 percent with no additional feed intake. Slightly higher results are shown for Zilmax. Neither product has shown any effect on meat quality at accepted feeding rates, and lean yield of the carcass was increased.

Smaller cattle feedlots seldom use beta-agonists. When pens of cattle are being sorted weekly or biweekly for sale, it becomes impossible to effectively use these products because of the defined feeding period prior to slaughter. In some cases, however, they may be used in pens of smaller-framed heifers or steers to increase carcass size from the additional weight gain and/or decrease fat and improve yield grade. This serves to avoid marketing discounts for small carcasses (under 600 pounds) or Yield Grade 4 or 5 carcasses. Feeding the additive and marketing must still be done on a pen basis.

Since such a large number of cattle are fed beta-agonists, the loss of one of the products can have a significant impact on beef markets. The report from Micik (2013) quotes DTN Livestock Analyst John Harrington as saying a reduction of 10 to 15 pounds off of all beef carcasses due to the reduction in use of beta-agonists is 0.5 billion pounds of beef, or about 1 to 1.5 percent of production. Typically, this result would raise the price of live cattle by $3 to $4, according to Harrington. Similarly, other features of the beef market and economic profiles for cattle feeding could be impacted. One example is more corn needed to finish cattle, which raises the competition for other livestock uses.

Cattle feeders will have the option of shifting to Optaflexx in their rations, since it is still available. However, there may still be a small reduction in weight for each carcass. Corn prices that trend lower encourage cattle feeders to feed cattle longer and increase carcass weights, and this result could offset some of the impact of not using beta-agonists.

Technology will drive the improvement of beef production and help keep food prices low now and in the future. Setbacks in the use of feed additives, implants, drugs and other technology will slow that improvement. However, customers require a positive perception of the food they buy, and it is necessary to effectively answer their concerns.

Dr. John Comerford is an associate professor and extension beef specialist at Pennsylvania State University.