It was just one year ago when concerns surfaced over possible adverse effects of the beta agonist Zilmax on feedyard cattle. Following a special session preceding the 2013 NCBA summer conference in Denver, where experts discussed concerns with lameness and other animal health and welfare issues, Tyson Foods announced a moratorium on purchasing Zilmax-fed cattle. Shortly after that, Merck Animal Health temporarily suspended sales of the product in the United States, a policy that remains in place today.

At the same time, Merck announced a Five-Step Plan to Ensuring Responsible Beef, which includes forming an independent advisory board, conducting new scientific studies and re-certifying feedyards for proper use of the product. Last week, during the Cattle Feeders Business Summit in Denver, Merck Animal Health technical services specialist John Hutcheson, PhD, updated cattle feeders on progress and results of studies carried out over the past year.

At the time of the suspension last August and September, Merck immediately began a large-scale audit of cattle in feedyards and upon arrival at packing plants. Trained observers collected mobility scores on over 60,000 head of cattle either at feedyards shortly before shipping or upon arrival at packing plants, or in some cases, at both stages. The vast majority of cattle in this audit – about 98 percent – were graded with a mobility score of “1,” meaning normal mobility and no sign of lameness. A small percentage had a “2” mobility score indicating slight discomfort and just a few graded “3,” indicating moderate lameness. Mobility scores of 2 or 3 were somewhat more common at the packing plants than at the feedyard, including among those sets of cattle monitored at both locations, suggesting a association between transport and loss of mobility. They found, however, no differences in mobility scores between cattle that received Zilmax and those receiving no beta agonist.

Next, the company helped support a variety of controlled, scientific trials in cooperation with the University of Georgia, New Mexico State University, Oklahoma State University, Texas Tech, USDA and other research institutions. These trials involved a variety of cattle, including various beef breeds, culled dairy cows and calf-fed Holstein steers, fed in a variety of environments and seasons including during the heat of summer.

The trials evaluated behavior and locomotion at feedyards, during loading and unloading at the packing plant and through processing. One trial used pedometers to track the number of steps cattle took each day. In one trial, researchers fed Holstein steers for as long as 12 months, to finished weights as heavy as 1,900 pounds, to provide comparisons among extremely heavy cattle.

None of these trials found any significant differences in lameness or other negative outcomes between cattle treated with Zilmax, untreated cattle or those treated with Optaflexx, the other FDA-approved beta agonist used in finishing cattle.

Several large trials included metabolic studies involving evaluation of blood metabolites, stress hormones, blood gasses, pH and lactate levels. Researchers also used infrared thermography to evaluate heat stress, monitored respiration rates and body temperatures and studied histology on tissues and compared organ weights at slaughter. Researchers found slightly higher respiration rates among Zilmax cattle compared to controls, which Hutcheson says were in an expected range associated with animals with increased muscle mass. They found no significant differences in levels of stress hormones or body temperatures and blood chemistry measures were in a normal range among treated and control cattle. Hutcheson says in these trials, Zilmax did not detrimentally alter the ability of cattle to respond to stress.

At Texas Tech University, researchers actually conducted magnetic resonance imaging (MRI) tests on the feet of control and treated cattle at slaughter weight. These examinations evaluated the lamellae, digital phalanx bones, deep digital flexor tendons and other components in the lateral and medial claws. Almost all of the feet evaluated with MRI technology were normal, and there were no significant differences between treated and control cattle.

Results of these trials and others were reported at the recent American Society of Animal Science (ASAS) annual meeting, and those reports are available online.

Several cattle feeders in the audience expressed frustration that the product, which can significantly improve the value of finished cattle, remains off the market. Merck representatives emphasized they are doing all they can to compile scientific data supporting its safety and to present those data to packers, retailers and other stakeholders. They maintain there is a path forward for restoring the product to the market, and that the company will, at that time, carefully control who uses the product and how they use it, through their feedyard certification process.