Beta Agonist Anxiety

07:47PM Oct 07, 2014
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Researchers take a closer look at the effects beta agonists have on cattle heart rates and cardiac physiology 

The beef industry is more than a year removed from the August 2013 turmoil reporting that severely lame cattle, in a few cases cattle losing hooves, were showing up at packing plants. Consumers and producers had many questions. What was the cause? Who was to blame? How would this impact the industry?

Tyson Foods was the first company to no longer accept cattle fed Zilmax (zilpaterol hydrochloride), believed to be the cause of the lameness. The other major packers followed in short order. Merck Animal Health opted to take Zilmax temporarily
off the market until more research could be done.

Stress magnifies risk. Dan Thomson, veterinarian and director of the Beef Cattle Institute at Kansas State University, has been on the ground floor of animal welfare research involving zilpaterol.

(Read A Feeder's Perspective On Beta Agonists)

Thomson and a group of other researchers went to the packer to conduct necropsies, gather tissue samples and watch video of the cattle’s movement to help determine what was happening. While the hoof sloughing issue captured the headlines, he says the real concern was the muscle tremors and stiffness witnessed in 10% of a pen of 200 cattle taken to slaughter.


“There is a difference in these animals when they are going through this metabolic meltdown or fatigued cattle syndrome than what we see with animals that have an acute injury,” Thomson says.     

The difference was found in the stress levels for the cattle that were euthanized. Normal range for creatine phosphokinase (CPK) is 159 to 332 international units per liter (IU/L), but the levels of the affected cattle were between 5,000 to 8,000 IU/L.

After finding these results, Thomson dug deeper into the cattle stress and conducted a study at Kansas State with a group of steers fed without beta agonists, one fed zilpaterol and the other fed Optaflexx (ractopamine).

“We wanted to look at the effect of beta agonists on heart rates and cardiac physiology because of the reports at feedyards,” Thomson says.  

Blood was pulled from the steers throughout the study to analyze various stress levels. The cattle were also equipped with ECG monitors for 24 hours to measure cardiac rates and arrhythmias.
“These animals were under no stress,” Thomson says. “Their creatine caenise levels were the same regardless of the group. Their lactate levels were in a normal range.”

Similarly, blood was taken from the cattle at the time of slaughter to see if there was an effect from shipping, but no significant differences were observed.

Lessons learned from the hog industry. In the early 2000s the pork industry went through a comparable situation with ractopamine, but it did not come under the public scrutiny Zilmax has experienced.

A study at the University of Illinois showed pigs fed ractopamine that did not suffer from fatigued pig syndrome had similar stress levels in their blood as pigs not fed ractopamine at slaughter.
To see how stress impacted hogs, researchers stressed a group of ractopamine-fed pigs by running them up an alley with an electric prod prior to slaughter. A control group of ractopamine-fed hogs were handled gently.

“They found in the aggressively handled animals, the lactate levels at the time of slaughter was 20.2 mmol/L and in gentle handled animals, 4.0 mmol/L,” Thomson says. “The only animals that went down with fatigued pig syndrome at the plant were in the aggressively handled treatment. None of the gently handled animals went down.”

Of the aggressively handled animals, those with fatigued pig syndrome had CPK levels double the not affected hogs.

“The animals at risk were the heavier muscled animals, the heavier out-weights and those that came from farms with poor hog handling,” Thomson adds. Pigs and cattle going through these fatigued symptoms all resembled tying-up in horses.

“I think it makes a natural, common story to what is going on as we increase muscle mass and the out-weights of cattle,” he says.

Lower stress for cattle. Thomson’s research and lessons from the pork industry isn’t new however. The impact of stress on cattle performance is well documented.

A 1973 study looked at stress in double-muscled cattle compared to normal-muscled animals. Three types of stresses were applied in the study: 1) a nutritional stress where feed was withheld from animals the two days prior to shipment; 2) a simulated psychological stress where cattle received two doses of epinephrine; and 3) an exercise-induced stress where cattle were chased up to five miles.

“At the end of the study, double-muscled cattle responded more negatively to stress than normally muscled animals regardless of the type of stress they were exposed to,” Thomson says. The double-muscled cattle had increases in lactic acid, CPK and death loss.

What we can learn from these past examples is how to move forward. “Let’s start fixing the problem and not continue to keep pointing fingers and having fights,” Thomson says. “There are long-term stresses that we’ll have to look at with cattle coming into feedyards. By long-term I mean the last 30 days on feed.”

Areas to look at include sorting pens, pushing cattle too hard to gain weight and heat stress. “The big one, and the one I would start with immediately, is what we do on the day of shipping,” Thomson says. “Animal handling at this point in time is probably as important as it ever is in a cattle finishing facility.”