Categories of Growth Promotants

Hormones

Hormone implants have been approved for use in cattle since the 1950s, with the most recent (implants containing trenbolone acetate) approved in the mid-1980s. Implants partially replace the hormones that steers lack as a result of castration. Bulls are castrated to eliminate the reproductive hormones that contribute to aggression, reduce animal welfare concerns, avoid unplanned mating, and minimize meat quality problems. However, these hormones also allow bulls to grow more rapidly, efficiently, and develop more lean muscle than steers or heifers. Hormone implants, delivered through a pellet under the skin of a steer or heifer’s ear, replace some of these natural reproductive hormones in steers (or augment them in heifers). This improves the growth rate, feed efficiency and carcass composition of steers and heifers, while mitigating the behavioral and meat quality problems we want to avoid in bulls.

Calf implants can significantly increase weaning weights, particularly in heifers. For example³, heifers implanted with zeranol (Ralgro) at 60 days of age weighed 6% (over 30 pounds) more at weaning than unimplanted heifers.

Use in feeder cattle: The use of growth implant hormones allows a 13% increase in ADG in grass cattle, and a 16% increase in ADG in finisher feedlot cattle².

Hormones used in beef cattle contain one or a combination of three naturally occurring reproductive hormones (or synthetic versions that have similar structure and function):

^*This is not an implant; this is a diet additive, fed at 0.4mg per head per day to control estrus

The implant gradually releases the hormone over time, so the animal is only exposed to a minute amount of hormone each day. For example, the 200mg of TBA and 40mg of estradiol in a Revalor-XS implant are slowly released over a 200-day period.

Caution is warranted when implanting potential breeding stock. Bull calves intended for breeding should never be implanted. Do not implant heifers intended for breeding more than once. Heifers³ implanted with Ralgro at 60d of age had heavier weaning weights compared to unimplanted heifers, with no negative impact on age at puberty, pregnancy rate, calving percentage, calving difficulty or calf birth weight. However, significantly delayed puberty, failure to conceive, and abortions have been observed⁴ when heifers were implanted more often.

One hormone, melegestrol acetate (MGA), is sometimes used as a feed additive to suppress estrous cycles in feedlot heifers. MGA can improve growth rate and feed efficiency because heifers that aren’t cycling expend less feed energy pursuing or avoiding riding behavior. MGA must be managed very carefully to reduce the risk of dark cutting.

Ionophores

Ionophores were introduced in the mid-1970s and are fed to cattle to prevent diseases like coccidiosis, acidosis and bloat, in addition to improved weight gain and feed efficiency. Ionophores improve feed efficiency by acting on rumen microbes. Rumen microbes convert the fiber and starch in forage and grain into volatile fatty acids (primarily acetate, propionate and butyrate) and ammonia that can be absorbed into the bloodstream to provide energy and protein to the animal. Ionophores act on the rumen bacteria to increase the proportion of propionate produced, which is more energetically favorable than acetate. However, some rumen bacteria produce methane instead of acetate, propionate or butyrate. Methane contains energy but cannot be absorbed by the animal, so it is belched out and wasted. Ionophores contribute to reduced methane production, because the proportion of methane produced decreases when the proportion of propionate increases.

Ionophores are antimicrobials categorized as 'Low Importance' in human medicine, meaning they are not used to treat bacterial infections in humans. Ionophores have a very different mode of action than medically important antibiotics, and antimicrobial resistance to ionophores does not result in co- or cross-resistance to medically important antibiotics. Ionophores are often erroneously included in discussions about antimicrobial use in livestock and potential links to antimicrobial resistance in humans.

Beta-agonists

Beta adrenergic agonists are the newest class of growth promotants, commercially available in Canada since 2004. These feed additives are neither antimicrobials nor hormones. Human asthma medications are also beta-agonists. 

‘Beta adrenergic agonist’ is a complicated name that describes what these products do. ‘Adrenergic’ means ‘resembling adrenaline.’ ‘Agonist’ (the opposite of antagonist) means that ‘it works in a similar manner.’ The ‘beta’ refers to the specific receptor that it binds to on the muscle cell surface. So, a beta-adrenergic agonist is a substance that binds to a beta receptor on the muscle, and acts sort of like adrenaline. Adrenaline diverts blood flow from the digestive organs towards the muscle during the ‘fight or flight’ response. Similarly, beta-agonists re-direct nutrients so that more growth occurs in muscle tissue than in internal organs.

The beta-agonists approved for use in beef cattle (ractopamine and zilpaterol) increase protein deposition (muscle growth), growth rate, feed efficiency, and carcass leanness. Some beta-agonists also reduce protein turnover (reduce muscle breakdown), resulting in increased dressing percentage. Beta-agonists are fed at the end of the feeding period, when muscle growth is slowing, fat deposition is speeding up, and feed efficiency is dropping off.

As with aggressive implants, beta-agonists must be managed appropriately, on the right class of cattle at the right time in order to avoid negative consequences on carcass quality (especially reduced marbling levels). The benefit of feeding beta-agonists can be lost if the product is fed for too long, or if the delay between product withdrawal and slaughter is too long.

Some have expressed concerns about the use of beta-agonists in livestock. A 2014 observational study⁵ compared health records from nearly a million US feedlot cattle, some which had received beta-agonists and others which hadn’t. The risk of death was slightly higher in cattle that had been fed a beta-agonist, although no unique cause of death was noted. The main causes of death in the cattle fed a beta-agonist - (digestive and respiratory illness) are also the most frequent cause of death in feedlot cattle that aren’t fed beta-agonists. Other concerns attributed to beta-agonists, including a perceived increased risk of lameness, fatigue and respiratory distress have since been attributed⁶ to improper handling of large cattle in hot weather.

In spite of rigorous pre-approval safety testing that is conducted before pharmaceutical products are approved for use in Canada, some importing nations have a zero-tolerance policy for certain kinds of beta-agonists. This can become an issue in trade negotiations. These concerns are likely related to clenbuterol⁷, a beta-agonist that is not approved for use in cattle, is metabolized differently, can leave residues in meat, and which has been linked to health concerns in humans. Clenbuterol has never been approved for use in cattle in North America but has been used illegally in some countries that do not allow the use of growth promotants. In fact, a person would have to eat more than 13.5 kg (30 pounds) of beef per day, or over 2.25 kg (5 pounds) of liver per day, from cattle administered an approved beta-agonist in order to get the effect of one “puff” of asthma medication.

Regulation of Growth Promotants

Approval

In Canada, veterinary drugs are regulated by the Food and Drugs Act and Regulations. All veterinary drugs go through a Health Canada approval process before they are licensed for use. Health Canada is one of 187 government members of the Codex Alimentarius Commission. The Codex was established by the Food and Agriculture Organization (FAO) of the United Nations and the World Health Organization (WHO) to develop harmonized international food standards, guidelines, and codes of practice to protect the health of consumers and ensure fair food trade practices.

The Health Canada Veterinary Drug Directorate (VDD) evaluates and monitors the safety, quality and effectiveness, and sets standards for the use of veterinary drugs to ensure that, when used according to label directions, they are safe for both animals and humans.

Label and veterinary directions indicate proper administration doses and routes, as well as pre-slaughter withdrawal times that ensure that the product has been metabolized by the animal before it is slaughtered.

For a more detailed explanation of the veterinary drug approval process in Canada, visit

http://www.beefresearch.ca/files/pdf/canadas-veterinary-drug-approval-process.pdf

Withdrawal times

Label and veterinary directions indicate proper administration doses and routes for veterinary products, as well as pre-slaughter withdrawal times. These ensure that the product has been metabolized by the animal before the meat is harvested. Most drugs are completely metabolized during the prescribed minimum number of days between the last administration of the drug and slaughter, and therefore leave no residue.

Inspection and Residue Testing

All animals and carcasses are subjected to pre- and post-slaughter inspections to look for signs of ill health. Random samples of carcass tissues and organs are tested for residues from antimicrobials, growth promotants, and other contaminants.

Frequently Asked Questions Regarding Growth Promotants

How do Growth Promotants Improve Growth Rate and Feed Efficiency?

Implants encourage protein deposition while leaving fat deposition unaffected. This improves weight gain and feed conversion efficiency. Fat deposition requires more than twice as much feed energy as protein deposition does. In addition to this, muscle tissue contains around 70% water, while fat contains around 10% water. This means that for every ten pounds of muscle gained, about three pounds comes from dry feed and seven pounds comes from water. This ratio is reversed for fat growth (roughly nine pounds from dry feed and one pound from water). The net result is that increasing muscle growth more than fat growth improves growth rates and reduces the amount of feed required per pound of body weight gained.

Meat Quality

Negative impacts on carcass quality (maturity, marbling score, tenderness, and possibly color), can occur when aggressive growth promotant regimes are used at the wrong time or on the wrong types of cattle. Cattle feeders are very careful to match the growth technologies used to the type of cattle they are used on, simply because carcass pricing is heavily influenced by marbling score (quality grade). Any reduction in quality grade could negatively affect feeding margins significantly.

Similarly, implants and beta-agonists may negatively impact tenderness, juiciness and flavor of beef when used at the wrong time or in the wrong class of cattle. However, differences reported between beef from cattle raised with or without growth promotants are typically small, are more commonly detected by trained taste panelists than by regular consumers and can be effectively mitigated when beef is aged for 21 days⁸ (the approximate average time beef is aged in Canada it reaches the retail shelf⁹).

Video presentation on implants and carcass quality by Dr. Sandi Parr, feedlot nutritional and production consultant: 

What are maximum residue levels (MRLs)?

There is a zero tolerance for residues of many veterinary drugs in beef allowed for human consumption. Some veterinary products have standardized maximum residue levels (MRLs) established by Health Canada. MRLs are set far below the amount that could pose a human health concern when ingested daily over a lifetime.

Learn how Canadian MRLs are determined in point #6 here: http://www.beefresearch.ca/files/pdf/canadas-veterinary-drug-approval-process.pdf

Can consumers be confident Canadian beef is safe from drug residues?

The CFIA randomly samples the food supply for potential contamination. It is extremely rare that unacceptable residue levels are detected. Foods exceeding the limit are further analyzed by CFIA/Health Canada to determine if a product seizure or recall is necessary. Test results that raise concerns are a very rare occurrence, but regular testing ensures the safety of Canadian beef. The CFIA’s 2015-16 National Chemical Residue Monitoring Program found no residues in Canadian or imported beef that exceeded allowable limits.

The following webinar titled "Hormone use in beef cattle: facts, fears, fantasies" discusses hormone use in beef cattle. 

What would happen if a licensed veterinary product had an adverse effect (Pharmacovigilance)?

Cattle producers, veterinarians, pharmaceutical companies and Health Canada’s VDD work together to identify, report and investigate potential cases of adverse drug reactions.

To learn more about how the VDD monitors livestock drugs on an ongoing basis even after they have been registered, approved and licensed in Canada, visit http://www.beefresearch.ca/files/pdf/pharmacovigilance.pdf

Human health – oral bioavailability

Growth promotant safety has been reviewed by many experts and agencies, including Health Canada, the World Health Organization and the Food and Agriculture Organization of the United Nations. All have concluded that hormones can be used safely in beef production. The levels found in food products, such as beef, are too low to be of risk to human health.

To put these levels into perspective, consider the levels of estrogens that occur naturally in all plants and animals, including humans. This table shows that a person would have to eat 225 tonnes of hamburger every day from cattle administered growth hormones before he or she would be exposed to as much estrogen as the average women produces daily.

Testosterone-containing implants are similar; there is a safety factor of several thousand-fold based on the assumption that people consume the equivalent of 6 to 7 servings of beef per day, which greatly exceeds the average Canadian’s beef consumption (less than one 75g serving of beef daily).