Vet Column 5-17-10 | TheFencePost.com

Vet Column 5-17-10

David L. Morris, DVM, Ph.D.
Fort Collins, Colo.

Regardless of business endeavor, improving efficiencies in creating or delivering a desirable product is often a common goal. Beef production is no different. Recognizing that feed represents a significant portion of beef production costs, if one could produce leaner beef on less feed, it would have great potential for a win-win situation.

Investigators from the University of Illinois recently published data concerning the use of two phenethanolamines, sometimes known as beta-adrenergic agonists, for improving beef feedlot performance. These are organic molecules similar in structure to naturally occurring epinephrine. These substances act by binding to cellular membranes to increase protein synthesis or decrease protein degradation, or both, as well as decrease fat generation and increase fat destruction. In other words, they increase meat production and decrease fat accumulation, both desirable attributes to beef production.

Ractopamine hydrochloride (Optaflexx, Elanco Animal Health) is a category 1 ß-adrenergic agonist that improves growth performance and affects carcass characteristics in several livestock species through increased protein synthesis. Zilpaterol hydrochloride (Zilmax, Intervet) is a category 2 ß-adrenergic agonist which functions through increased protein synthesis and decreasing protein degradation. Optaflexx and Zilmax were approved for use by the U.S. Food and Drug Administration in 2003 and 2006, respectively.

Both of these products have been demonstrated to show improvement in feedlot performance and carcass composition although the effects of both lead to a concern of decreased tenderness. The study recently reported compares the effects of both in the same trial when fed to steers in the last 33 days of the finishing period.

Three hundred crossbred beef steers were grouped by body weight, body condition score, and breed type into three groups. One group was fed Optaflexx at the rate of 200 mg per head per day for 33 days. A second group was fed Zilmax at the rate of 75 mg per head per day for 30 days and removed for three days due to the required withdrawal period. A third group served as a non-treated control. Carcass assessments included 34 different measurement points.

Both Optaflexx and Zilmax increased final body weight, average daily gain, feed efficiency, and hot carcass weight compared with controls. Compared with Optaflexx, Zilmax steers had less average daily gain, ate less, and were lighter at harvest, but they had increased hot carcass weight and better dressing percentages.

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In this experiment, Optaflexx did not affect carcass yield over controls. Zilmax, however, decreased the fat thickness, had larger ribeyes, improved the yield grade, and increased cut-out yields when compared to the controls. Marbling, lean maturity, and skeletal maturity were not different between treatments.

Steaks from Optaflexx steers were tougher (less tender) than steaks from control steers at three and seven days of aging. At 14 days of aging, Optaflexx steer steaks were not different in tenderness from control steers. Steaks from Zilmax fed steers, though, were tougher throughout the 21-day postmortem aging period than both controls and Optaflexx fed steers.

In this study, feeding either Optaflexx or Zilmax to steers in the last 33 days of the finishing period was effective at improving beef feedlot performance. In regard to tenderness effects, it appeared that Optaflexx fed steers were equally as tender as controls with 14 days of aging, whereas Zilmax fed steers were less tender than controls after 21 days of aging.

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