New study shows cattle deaths could be caused by b3 agonists
March 31, 2014
A recent study raises serious questions about the feeding of b-agonists, particularly Merck's Zilmax to cattle as part of a finishing ration before slaughter.
Merck's Zilmax (zilpaterol) and ELy Lilly's Optaflexx (ractopamine) belong to a broad family of b-andrenergic agonists, generally classed as b3 agonists, that work by binding to b3 receptors, causing increased feed efficiency and lean muscle mass. Ractopamine is sold under the trade name Optaflexx for cattle and Paylean for swine, and is also fed to turkeys. Merck's Zilmax was approved for use in a cattle finishing ration in 2007, with a three-day withdrawal period before slaughter.
Sales of Zilmax were suspended in September 2013 after meatpackers Tyson and Cargill announced they would no longer accept shipments of slaughter cattle fed the supplement.
This was in response to serious mobility issues in cattle at the point of slaughter.
The new report — "Increased Mortality in Groups of Cattle Fed B-Andrenergic Agonists," published by the scientific journal PLOS ONE and conducted by researchers at Kansas State University and Texas Tech — attempted to "quantify the association between ßAA administration and mortality in feedlot cattle…"
To do this, they studied cattle fed Zilmax in 2011-2012 and used data from earlier studies of cattle fed Optaflexx, in three datasets. The first was a ractopamine dataset that used 79,171 cattle owned by four companies in at least seven feedlots over five different studies in a randomized block pattern having a study group and a control group in each block.
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The second dataset was an unbalanced multi-feedlot group of 722,704 animals in 3110 groups — 637,339 of these animals were fed zilpaterol and 85,365 were not.
The third dataset was comprised of 149,636 animals in a single feedlot — 56.1 percent of these were fed zilpaterol and 43.9 percent were not.
In all of these sets, feeders attempted to follow FDA guidelines for the administration of these products.
After discounting individuals that were at risk at the beginning of the study, the authors found that cattle fed ractopamine in the first study were 91 percent more likely to die than control animals during the at risk period. While company A had no deaths in either group during the study, the average across the four companies was 1.12 deaths per 10,000 animal days for the cattle fed ractopamine and .59 deaths per 10,000 animal days for the control group.
In the second study, there were 1.77 deaths per 10,000 animal days for those cattle fed zilpaterol and 1.01 deaths per 10,000 animal days for the control group.
In the third study, animals fed zilpaterol had an 85 percent increased risk of death during the at risk period than the control group.
The last set of cattle provided interesting further data as well.
Animals fed zilpaterol were 23 percent more likely to require treatment for any condition during the at risk period. While they were less likely to need treatment for diseases of the digestive system, they were 2.3 times more likely to receive treatment for diseases of the respiratory system and 2.3 times more likely to receive a second treatment, often being held back from their peers due to withdrawal at slaughter time.
In addition, 1.59 percent more carcasses of cattle fed zilpaterol were classified as dark, firm and dry than the control carcasses. DFD carcasses receive steep discounts because consumers associate them with low-quality meat.
The authors of the study — while stating that the "data presented herein provide compelling evidence that administration of FDA-approved ßAA to cattle increased both the cumulative incidence (risk) and incidence rate of death" — also noted that a number of factors may have indirectly influenced their conclusions.
The control groups in the zilpaterol studies, for example, were not truly random, but were comprised of individuals not being fed zilpaterol for other reasons, such as a specific marketing program or cattle that were too big to benefit from it.
A second hypothesis is that the increased risk of death came not from the b-agonist supplement, but from management changes relating to its use, such as a delay in feeding time for those cattle fed b-agonists, leading to hungry cattle overconsuming the product.
In addition, the RH studies in particular were not based on mortality, so the authors concede that it is difficult to establish a causal relationship between b-agonist administration and increased risk of death.
The only "consistent modifier" in the studies was the month in which the cattle were slaughtered.
The study suggests that there is less risk of death due to feeding of b-agonists during cooler weather.
Officials with Merck and Co. — having voluntarily suspended sales of Zilmax in September due to mobility problems in slaughter cattle — said they are confident in the "safety and performance" of Zilmax, and criticized the methods used in the study.
The findings were "based on observational information and we disagree with them," the company said in a statement.
But the sterile, scholarly wording of this new report underscored the following: "If a relationship between ßAA administration and increased risk of mortality exists, it ought to stimulate discussion of the pros and cons of the use of drugs approved purely to improve the efficiencies of production yet offering no offsetting health benefits to the animals."
To view the report, visit http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0091177.