Merck Stops Selling Zilmax While Investigating Problems

Last Friday Merck & Company suspended sales of Zilmax a class of beta-agonist cattle feed additive to perform a new study of the drug’s effect on cattle health after Tyson Foods, Inc. announced that it would no longer purchase cattle fed Zilmax earlier this month, due to concern for animal health. Tyson Food is the largest meat processor in the United States by sales.

Beta-agonists were approved by the Food and Drug Administration, FDA, for use in swine in 1999 and in cattle in 2004. These compounds are "repartitioning agents," shifting nutrients away from fat deposition towards lean muscle growth and are used to finish cattle and swine adding an additional 2% in muscle weight shortly before slaughter. That 2% weight gain translates into 24-34 pounds of meat per cow, an average near 30 pounds. There are 24 million head of cattle each weighing around 1,500 pounds harvested each year in the United States, so that is potentially a lot of beef. (It is to be noted that grass fed, organic cattle weigh in around 950 pounds.) Until Merck stopped sales of Zilmax, two beta agonists were available to cattle feeders: ractopamine hydrochloride (Optaflexx; Elanco Animal Health) and zilpaterol hydrochloride (Zilmax; Intervet).

Optaflexx was approved by the FDA for feeding during the last 28 to 42 days of the finishing period. When fed at the recommended dose (200 mg/day), data have shown that body weight and carcass weight are increased, with no increase in feed intake; cows get heavier without the need for additional food. There is no withdrawal period for Optaflexx and the animals are slaughtered at the end of the dosing.

Zilmax, Merck’s product, was approved by the FDA for feeding to feedlot cattle during the last 20 to 40 days on feed. Zilmax demonstrated benefits include increased live weight gains, increased speed of weight gain, increased carcass weight and improved feed efficiency. However, dosing with Zilmax must be stopped three days before slaughter to reduce the presence of the drug in the meat.

Some countries, such as the European Union and China, have restrictions on beta-agonists as a class in response to illegal use of beta-agonists such as clenbuterol, which has caused human illness because of high residues in muscle meats. The World Health Organization and the Food and Agriculture Organization however, voted in 2012 to create a minimum risk level, MRL, for residual in meat of ractopamine hydrochloride (Optaflexx; Elanco Animal Health), the beta-agonists sold by E.I. Lily.

from BCRC video

The Beta-agonists are just the most recent addition to the arsenal of feed additives and growth promoting strategies that have been used to increase what the cattle industry calls feed efficiency, increased carcass weight per pound of feed. Over the past 30 years, weight to feed has improved by 30%. These improvements were due to changes in diet and management of the diet, grain processing, and drugs and hormones used to promote growth.

Cattle grow fatter on grains. It was discovered that the digestibility of grains like corn, barley and oats can be improved by processing. Cracking the outer shell of the grain, allows the rumen microbes to better utilize grain starch and minerals. Processing also allows grain to be mixed with supplements, and can increase palatability to cattle that do not naturally eat these grains.

The other growth promoting drugs and hormones are ionophores, and growth implants. Ionophores are antimicrobials mixed in with cattle feed that improve nutrient availability by killing off certain rumen microbes. Most rumen microbes convert the complex fiber and starch in forage and grain into simple molecules that can be absorbed into the bloodstream to provide energy and protein to the animal, but rumen bacteria known as methanogens convert the dietary fiber and starch into methane gas. Methane contains energy, but it cannot be absorbed by the animal, so it is belched out and wasted. Ionophores improve feed efficiency and weight gain by killing only the methanogenic bacteria, and allowing the other rumen bacteria to make more feed energy available to the animal.

Growth implants are pellets injected under the skin in the animal’s ear. These implants are reproductive hormones that occur naturally in the animal. In steers, implants replace some of the hormones that were lost when the animal was castrated to as part of the feedlot operations. The hormones encourage protein deposition and discourage fat deposition despite the high grain diet. This improves feed efficiency because muscle tissue contains around 70% water, while fat contains less than 25% water. Fat requires more feed than muscle.

This strategy of utilizing growth promoters is what has reduced the relative price of beef over the last 30 years. The FDA, the US Department of Agriculture, the World Health Organization and the Food and Agriculture Organization of the United Nations have all reviewed these growth promoting strategies and have concluded that hormones and growth promoters can be used safely in beef production. These organizations found the residual levels of these growth promoters in food products, such as beef, were too low to be of risk to human health.

We have only recently developed the ability to test for trace exposures and have discovered ubiquitous exposures to many substances. The detection of a chemical or hormone in food or a person’s blood or urine does not mean that it will cause health effects or disease. The guiding principal of toxicology is that there is a relationship between a toxic reaction (the response) and the amount of poison received (the dose). An important assumption in this relationship has been that there is always a dose below which no response occurs. So if the concentration of the contaminant was low enough there would be no toxic reaction, but that principal is being tested with endocrine disruption and advances in analysis. Meanwhile, I buy only organic and organic, grass-fed meat.