Beta-agonists in the Spotlight

March 1, 2016 05:20 AM

By: Heidi Carroll, SDSU Extension 

The use of beta-agonists in beef cattle production has been highly scrutinized following concerns of animal welfare, which has led to the removal of Zilmax (zilpaterol hydrochloride) from the market by the manufacturer. Improving animal well-being through careful use of these feed additives and managing the animal’s environment to minimize stress has remained a focus of much research. During the February 3rd Animal Care Wednesday Webinar, Dr. Kristin Hales, a Research Animal Scientist at the U. S. Meat Animal Research Center discussed some research findings about the effects of feeding Zilmax and using shade in feedyard pens on beef cattle performance, heat stress, and other important measures.

The research

The purpose of the study was to evaluate the effects of supplementing Zilmax to finishing steers during the last 21 days on feed on performance, carcass characteristics, heat stress, mobility, and body temperature. The study had two different diets, one with Zilmax during the last 21 days and one without it; and the cattle were also in two different housing types, open dirt lots and shaded dirt lots. The study focused on several measurements in addition to growth and carcass characteristics:

  • Body temperature
  • Panting score
  • Respiration rate
  • Mobility score (Tyson mobility scoring system)
    0 = no lameness (normal)
    1 = slightly stiff gait
    2 = fails to keep up with the group
    3 = severely lame and reluctant to move
    4 = non ambulatory (down or refuses to move)


Mobility scores were impacted by the time on feed. Scores observed at the packing plant were worse than scores observed at the beginning of the study, regardless of feed treatment or housing type.

When comparing housing type (open vs. shade pens), there were no differences on growth performance, carcass characteristics, or other observed measures. Though cattle in open pens tended to have slightly greater gains than cattle in shaded pens. Interestingly enough, housing type did not have an impact on the respiration rate or panting scores.

Let’s look at the effects of feeding Zilmax during the last 21 days on feed. There were no differences in growth performance, but carcass characteristics differed when cattle were fed Zilmax. Hot carcass weight increased 14kg (~6.3 lb), dressing percent increased 1.7%, the loin eye area increased 6.4 cm2, and the USDA yield grade decreased 8.57%. Cattle fed Zilmax had higher respiration rates and tended to have slightly higher panting scores; though one should note that the manufacturer’s label states zilpaterol hydrochloride will increase respiration rates. Cattle fed Zilmax had lower average and lower maximum body temperatures, regardless of housing type.

The take home message from this research is that Zilmax improved carcass characteristics while minimally impacting cattle heat stress or mobility. This suggests cattle well-being was not negatively affected by feeding Zilmax during the last 21 days of the feeding period.

Dr. Hales also provided a brief update on other beef cattle research conducted at the U.S. Meat Animal Research Center. Listen to the webinar recording to learn more. Dr. Kristin Hales may be emailed if you have questions about these research results.

Animal Care Wednesday Webinars

For more information on upcoming Animal Care Wednesday Webinars, please contact Heidi Carroll. To view this and past webinars, please visit the animal care resource website.

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