“Zilmax Update”
by Chris Reinhardt, feedlot specialist
You may have heard a great deal about Zilmax® (zilpaterol hydrochloride) lately in either the popular or beef industry press. Zilmax is a growth promotant feed additive in the class called beta agonists, which is used during the final days of the finishing phase to increase carcass weight and lean muscle mass in beef cattle.
Beta agonists have been used in the U.S. cattle finishing industry since 2004 and in the swine industry since 2000, at which times ractopamine hydrochloride was made available for use in cattle (Optaflexx®) and swine (Paylean®). Zilmax was approved for use in cattle in 2006 and became widely available in 2007.
Since their respective approval dates, implementation of both beta agonists increased steadily through 2012 when approximately 70-80% of the finished cattle in the U.S. received a beta agonist. Extreme heat stress conditions occurred in various cattle feeding areas during the summers of 2011, 2012, and 2013, which coincided with greater than anticipated late-term mortality. Because growth in beta agonist use, and Zilmax use in particular increased during that same time frame, questions surrounded the coincidence. However, because Zilmax increases the lean muscle mass and reduces fat and marbling content of the carcass, cattle are typically fed for an additional number of days and to a greater finished weight, which mitigates the marbling reduction. This increase in number of days fed and the weight of cattle when finished confounds the investigation into changes in heat-related mortality.
In August 2013, a number of packing plants reported that a small percentage of cattle which had been fed Zilmax were “reluctant to move” after arrival at the packing plant, and announced that they would suspend acceptance of cattle fed Zilmax. Merck Animal Health, the manufacturer of Zilmax, suspended sale of Zilmax on August 16, 2013.
In relation to these decisions by the manufacturer and the packing firms, it is important to note that no food safety issues were involved. The decisions were instead made to provide the industry with the opportunity to investigate the issue of certain animals’ reluctance to move after arrival at the packing facility. Also, Optaflexx has not been similarly implicated in this issue.
Merck, Elanco (manufacturer of Optaflexx), the major packing companies, and the beef industry are pursuing investigations into potential causative factors contributing to the impaired mobility issue. Unfortunately, with the cessation of summer heat conditions, and with the withdrawal of Zilmax from the marketplace, thus changing feeding and marketing decisions, it is difficult to duplicate the conditions which may have combined to cause the impaired mobility issue. However, research models are being developed which may provide answers and management practices which will prevent the issue in the future.
