The major growth promoters and metabolic modifiers that are fed to beef cattle include ionophores, antimicrobials administered to decrease the prevalence of liver abscesses, beta agonists, and progestins.
Ionophores (eg, monensin, lasalocid, and laidlomycin propionate) are selective antimicrobials that shift ruminal fermentation to promote energy availability, with the consequence that they improve feed efficiency. Ionophores such as monensin and lasalocid also decrease the incidence of bloat and are effective coccidiostats. Some ionophores (eg, monensin) also decrease meal size, thus having the added benefit of decreasing the risk of overeating and therefore acidosis.
It is important to recognize that ionophores are not used in human medicine.
Tylosin and virginiamycin are often fed in conventional finishing systems to decrease the prevalence and severity of liver abscesses. These antimicrobials require a veterinary feed directive (VFD) for use in the US.
Beta agonists, such as ractopamine, are metabolic modifiers that improve average daily weight gain, feed efficiency, and dressed carcass yield when fed during the final 28–42 days on feed. Beta agonists elicit these responses by stimulating a net increase in muscle protein synthesis.
Progestins, such as melengestrol acetate, are often fed to finishing heifers to suppress estrus throughout the finishing period.
All of the previously described growth promoters and metabolic modifiers are considered by the FDA to be approved for use in medicated feed. Therefore, these products must be used in accordance with their approval and the information provided on the label. Without exception, veterinarians may not prescribe the extra-label or off-label use of a medicated feed in the US. Drugs used in combination to medicate feeds must be approved for use in such combinations. Although not fed to animals, growth-promoting hormone implants are growth promoters that increase lean tissue accretion by providing small amounts of one or more hormones or hormone analogues over an extended period of time.
Growth promoters and metabolic modifiers play a considerable and positive role in the sustainability of beef cattle production, by improving production efficiency, yielding positive returns on investment, and minimizing environmental footprint. In the US, all drugs used in beef cattle, including the growth promoters and metabolic modifiers described here, have been approved for use and deemed safe and effective when used in accordance with their approvals and the information provided on the product label. For more information on growth promoters and metabolic modifiers, see Growth Promoters and Production Enhancers Growth Promotants and Production Enhancers .