Zilmax trial design defended by lead UNL scientist

By Jane Byrne contact

- Last updated on GMT

Zilmax trial design defended by lead UNL scientist
US researchers, who concluded that controversial cattle feed additive, Zilmax, doesn’t have a detrimental effect on cattle health or well-being, said their study is robust despite criticism this week that it was too narrow in terms of scale.

“I am very confident in the design of this study and would disagree that it was, in any way, limited,”​ said Ty Schmidt, an animal scientist based at the University of Nebraska-Lincoln (UNL).

The assistant professor of meat science and muscle biology collaborated with other UNL colleagues along with USDA scientists in research evaluating zilpaterol (Zilmax) in 20 heifers over a 26 day period.

Concerns emerged in some quarters in August 2013 that the Merck manufactured Zilmax might cause lameness in cattle.  


Both Cargill and Tyson Foods have refused to take cattle fed the additive back into their supply chain since then, pending further trials into the beta-agonist.

‘One of the most extensive trials on beta-agonists ever conducted’

Harvey Dietrich, co-founder of a US consumer driven campaign group, Beef Additive Alert, questioned the size of the cohort involved in the UNL trial: “amere 20 heifers in an industry that processes 600,000 cattle per week.”

But Schmidt said it is incorrect to classify it as a small study: 

This is one of the most extensive trials ever conducted on the supplementation of a beta agonist to beef cattle.

It was controlled and in-depth, and we were able to evaluate, in-detail, the animal response when fed Zilmax, contrary to many of the field studies, where the only data collected is live weight, carcass weight, feedlot performance, and carcass merit​.”

And the UNL animal scientist said in-depth, detailed trials such as this one cannot be conducted on large scale, as such an approach would be “physically impossible.”

He said the researchers invested over 5,000 man-hours just to conduct the live animal portion of the trial.

“That does not include the thousands of hours spend analyzing the samples since the trial. Our research group has already examined over 45 different metabolites and hormones, and we still have a large number to look at,” ​added Schmidt.

Research dollars

Beef Additive Alert, this week, questioned whether Merck had supported the UNL research financially.

Schmidt told us that the drug maker “did not fund” ​the trial, which was backed by research dollars from UNL, and was initiated by the university’s animal science department in November 2012, nearly one year prior to Merck halted sales of the beta-agonist in the US.

However, Merck did contact the team and asked them to examine several metabolites and hormones that were outside of the initial trial perimeters. “We agreed to analyze these compounds but at Merck’s expense,”​ said Schmidt.

He said the pharmaceutical giant was not involved in the research trial or data collection or data analysis.

The study

During the 26-day study, the team collected blood, body temperature and video images from 20 heifers.

The heifers were fitted with indwelling jugular catheters and body temperature monitoring devices that recorded body temperature in one minute intervals.

The cattle were divided into two groups, with half receiving Zilmax at the recommended dose and half not receiving it.

On the last day of the trial, four days after Zilmax supplementation was discontinued, heifers were exposed to a simulated stress event to mimic the stress response that would be anticipated in cattle being shipped from the feedlot to packing plant.

At the conclusion of the live part of the study, heifers were harvested at UNL and their hearts, liver, lungs, kidneys and adrenal glands were studied.

The outcomes

The researchers found that heifers supplemented with Zilmax had a decreased production of the stress hormone cortisol, and decreased body temperature during the simulated stress event.

Histopathology of the heart, lungs, liver, kidneys, and adrenal glands revealed some differences between the heifers supplemented with Zilmax and those not fed the additive.

And the livers and right adrenal gland of the Zilmax heifers were slightly smaller than heifers that were not fed Zilmax, but there was no difference in lungs, kidneys, or heart, said the team.

None of these changes suggest that the supplementation of Zilmax detrimentally alters the ability of cattle to effectively respond to stressful stimuli, said the team.

Understanding the role of stress in Zilmax-fed cattle

Schmidt said while variations in body temperature, endocrine and metabolic parameters and histopathology of major organs were seen in the Zilmax fed heifers, these differences are “minor”​ and give no indication that cattle given the beta-agonist were health challenged or their well-being compromised.

This was the first step in understanding the role of stress on cattle supplemented a beta agonist. Once this trial is complete, we are hoping to start a heat stress trial in the autumn,”​ he added.

The scientists have released a technical abstract of the trial, which can be read here​. The study will be presented at the animal science national meetings next week in Kansas City.

“We are in the process of completing two manuscripts for the initial part of this trial, and these will be submitted within the next few months,” ​said Schmidt.

‘Insufficient scope to detect rare events’

Guy Loneragan, professor of food safety at Texas Tech University, who published research in March indicating a link between the administration of Zilmax and thousands of cattle deaths in nine feedlots in the US, told us the size of the UNL trial cohort – 20 heifers – would be sufficient to allow that team detect a consistent and relatively moderate change in parameters.

“But obviously it would be of insufficient scope to detect rare events such as increased mortality as we reported.

I was interested that they observed biochemical, histological, and gross changes but these were interpreted as normal or expected with increased muscle mass.

I am not sure I would conclude that smaller livers or increased creatine kinase ought to be considered inconsequential from the outset.  But I have not seen the paper so an informed critique is not possible​,” added Loneragan.

Merck field trial reported on hold

Merck had intended to conduct a large-scale evaluation of Zilmax this summer as part of its five-step approach “toensuring responsible beef”​ but the Wall Street Journal​ reported this week that industry reluctance to participate had put such a trial on hold.

Kelly Goss, communications spokesperson for the drug maker, would not confirm whether the trial had stalled, simply saying: “We continue to focus on implementation of the various elements of the five-step plan. This process has been more time intensive than anticipated.”

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