For over 40 years the U.S. federal government has approved various natural and synthetic estrogen and androgen hormones in beef cattle to improve the growth rate, feed efficiency (amount of feed necessary for a unit of gain) and carcass leanness.

Five hormone compounds are currently approved to be delivered by subcutaneous implants, including estradiol, testosterone, progesterone, zeranol and trenbolone acetate (TBA). Estradiol, testosterone and progesterone are naturally occurring hormones in animals (and some plants) whereas TBA and zeranol are synthetic hormones.

The natural hormones are thought to increase the circulating levels of growth hormone and other factors involved in regulation of growth resulting in increased protein deposition. Zeranol mimics estrogen, acting similar to the synthetics. TBA acts directly on receptors in muscle to reduce the amount of protein degradation, thereby resulting in a net increase in protein deposition.

With the exception of zeranol, all are steroids or have steroid-like properties. A steroid is a class of compounds that increases protein and bone growth.

The approved hormonal implants are formulated to deliver single hormones or combinations of two hormones. The hormones are compressed into various carrying matrices as pellets for implanting, including lactose, cholesterol or polyethylene glycol. A silicone rubber matrix is utilized by one implant delivering a single estrogen compound.

The rate and duration of release of hormones from implants vary depending on the type of carrier matrix, the type of hormone and the presence of multiple hormones. In general, the duration of useful hormone delivery from commercial implants ranges from 60 to more than 350 days.

In steers, implants serve to replace the hormones naturally produced by the testes, which were removed at castration. Across the spectrum of available implants, the rate of gain for implanted steers is approximately the same as for intact bulls. Implants decrease the amount of fat and increase lean tissue in the carcass of both steers and heifers.

The result is that commercially available implants improve lean composition of carcasses by 5 percent to 8 percent, growth rate by 10 percent to 30 percent, and feed efficiency by 5 percent to 15 percent. The two classes of growth promotants increase lean tissue either by increasing protein building (natural growth promotants) or decreasing protein degradation (synthetic growth promotants).

Implanted hormones achieve these effects through enhanced secretion of natural growth hormone in the treated animal and through stimulation of other cellular mechanisms in muscle and other tissues.

Size-neutral technology

Growth-promoting implants are a size-neutral technology, meaning they do not exclusively benefit large-scale cattle feeding operations and they do not place smaller farms at a disadvantage. Implants are approved for every stage of production in beef cattle production, from cow-calf farms and ranches raising nursing calves to backgrounding and stocker operations and feedlots of all sizes.

The biological effects of implants on growth rates, feed efficiency and carcass leanness occur in individual animals and are not dependent on the size of the herd or group. Although economies of scale exist that can decrease the cost per animal of implanting in operations with larger numbers of animals, implanting is relatively inexpensive and a conservative estimate for the rate of economic return is $5 gained for every dollar spent on implants, independent of the operation’s size. In the cow-calf phase, an average cost for an implant is $1, which can increase to $3 for an implant approved for the final phases of finishing cattle in feedlots.

Read more: Hormones, Antibiotics and the Safety of the Beef Supply

Benefits to producer, consumer

The improvements in growth rate and feed efficiency due to implant use in the beef cattle industry have decreased the cost of gain for producers.

Because feed costs consistently represent the largest annual expense for all phases of beef production, the economic benefits are significant for individual producers and collectively for the industry as a whole.

Consumers benefit from the use of implants because they make cattle leaner, thereby reducing the amount of unwanted fat. Additionally, implants decrease the cost of production, which is passed on to purchasers of processed beef products. Implants have been a major part of the range of technological advances, which have increased meat quality and consumer acceptance as well as decreased retail beef prices more than 25 percent during the past 50 years.

Benefits to the environment

Improved efficiencies attributable to implants have enabled the production of more beef products on less land and have reduced the amount of grain necessary for a given unit of animal growth. Specifically, 21 fewer bushels of corn are now required to finish a steer or heifer, resulting in 3 million fewer acres of corn needed to produce the beef supply for the U.S.

Implants accomplish this without harming the animals treated or adversely affecting the human food supply.

Impact on meat quality

Many factors affect the eating quality of beef. Cattle age, carcass fat, carcass aging and other post-harvest procedures all affect consumer satisfaction. Studies on the palatability of beef from implanted cattle vary depending on fatness of the carcass at the time of harvest. Implants increase the deposition of lean tissue and decrease the deposition of fat. Fatness is positively correlated with palatability. Therefore, implanted cattle must be fed longer so that comparisons can be made at the same level of carcass fatness. When this comparison is made, differences in tenderness and overall acceptability of the beef from nonimplanted and implanted cattle are minimal.

Food safety

FDA oversight. The Federal Food, Drug and Cosmetic Act governs FDA approval of implants. New animal drugs must be demonstrated to be safe and effective for their intended use. Safety must be demonstrated in food from treated animals being consumed by humans, in the health of the treated animal, in the environment at the time of manufacture and use in the animal, and in the human user of the product in animals. Human food safety considerations include drug residues that might occur in meat, milk and eggs. In the case of growth-promoting implants, this only concerns meat since they are not approved in dairy cattle or poultry. Since the mid-1950s several generations of Americans have consumed beef from implanted animals with no adverse health effects.

Placement of implants. Implants are placed under the skin of the posterior aspect of the middle third of the ear of cattle. They remain in this site during the period that they secrete growth-stimulating hormones in extremely small amounts. Any remaining implanted materials, including the carrier matrix and the hormone compounds, are removed at slaughter and processing when the skin and ears are removed. This prevents human consumption of implant carrier matrix materials or hormone residues. The FDA places no slaughter withdrawal time restriction on the use of implants in cattle, meaning that they may be treated and then immediately harvested because the meat will not have violative residues.

Hormone intake. For an implant to be approved in a food-producing animal in the U.S., the no-hormonal-effect level must be determined in the most sensitive experimental animal under well-controlled scientific studies.

A safety factor of 100 is divided into the no-hormonal-effect level to establish an “acceptable daily intake” (ADI) per unit of body weight for humans. The potential daily intake (PDI) is determined by multiplying the human average daily intake of animal tissues by any residue found in them after implanting. To be approved, the PDI may not exceed the ADI.

Wide safety margins between ADI and PDI exist for all approved implants. For example, using the highest residue levels reported, a person consuming 1.1 pounds of beef from implanted cattle would consume an extra 30 to 50 nanograms of estradiol per day. The PDI for a 132-pound person is 100 times this amount (3,000 ng/day). Viewed another way, for a person to reach the PDI, he/she would have to consume 50 to 66 pounds of implanted beef per day containing the highest residue level reported.

Tissue residue allowance. Safe tissue levels have been determined for natural and synthetic hormones used in growth promotants. For implants using naturally occurring hormones, the maximum allowable levels are 6 to 15 times greater than the highest residues found in treated cattle. As an added safety measure, the FDA maximum safe tissue residue allowed for the synthetic hormones is 77 and 150 times greater (trenbolone and zeranol, respectively) than the levels detected in treated cattle.

Endogenous hormone level. Because human tissues and organ systems produce and require hormones as part of their physiological life processes, numerous hormones are normally present in the human body in varying amounts. These include steroid hormones of the same type as those used in implants. The natural production of these hormones in the human body substantially exceeds the PDI from beef treated with implants.

Cancer risk. Multiple assays designed to assess damage to DNA, which can lead to mutations, have been used to evaluate growth promotants. All of these assays have been negative. At extremely high doses, some studies with rodents have reported increased tumor development; however, no studies have reported the same results with higher order mammals. Humans metabolize hormones differently than rodents, which may explain why the carcinogenic link found in rats has not been duplicated in humans.

European response

The European Union does not permit the import of U.S. beef because of the approved use of implanting technologies in the U.S. Nonetheless, the European Agriculture Commission convened a scientific conference on growth promotion in meat production in 1995, which concluded that on the basis of the “data available it seems most unlikely or even impossible that the residues following the use of these compounds according to good agriculture practice will ever exceed the set tolerance levels.”

In addition to the U.S., approximately 30 countries have approved growth-promoting implants for use in cattle. Paradoxically, in those countries where implants are banned, there is evidence that potentially harmful growth-promoting compounds are being used in an illegal fashion to a large and unregulated extent in cattle.

Impact on animal welfare

Minor behavioral changes in treated cattle have been reported. However, these events have been low, related to improper implanting procedures and have been difficult to repeat in controlled studies.

Steroid use in athletes

Some athletes have used anabolic and androgenic steroids to increase performance. The negative effects of their abuse are well known to the general public. The dosage taken by athletes, called supraphysiologic, is 10 to 100 times the rate used in hormone replacement therapy, and is most often injected, which increases the hormone’s potency. Athletes illegally taking steroids will inject 600 to 1,000 mg/wk of testosterone, compared with the highest dosage in treated cattle of 200 mg, which is released over a much longer period of time (100 to 120 days). The amount of hormone injected by athletes results in a dose seven to 10 times that produced by the testes of normal men. The misuse of steroids by humans is not to be confused with proper use in food-producing animals.

Author’s note: This article was co-authored with Jim Floyd, DVM, who is retired from North Carolina State University.


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