ABSTRACT
Metabolic modifiers are compounds that are either fed to animals or implanted to improve rate of gain, improve feed efficiency, dressing per cent, carcass meat yield, visual meat quality, shelf-life or meat palatability. Most metabolic modifiers have been developed and researched to improve growth performance and carcass composition, with fewer of them developed or researched specifically to improve meat quality. Several in-depth review papers have been written on specific categories or types of metabolic modifiers. This chapter presents a general review of the effects of metabolic modifiers on carcass composition, fresh meat quality and cooked meat palatability. Emphasis will be on those metabolic modifiers that are approved for use in cattle in the United States and other developed countries. The following are the main types of modifier discussed in this chapter:
• Anabolic steroid implants • Beta-adrenergic agonists • Vitamins D3, E and A • Somatotropin
2.1 Introduction to anabolic steroid implants Anabolic implants in beef production systems have been studied extensively since their approval for use by the Federal Drug Administration in the 1950s (FDA, 2014). Several products are currently available for use in the United States and several developed countries, each consisting of various compounds and concentrations for use at different stages of animal growth. Most steroids are administered as subcutaneous implants placed in the middle third of the ear, usually as compressed pellets, but some as silastic rods. Release time for most implants range from 50 to 200 days. Approximately 97% of cattle in the United States entering the feedlot have received at least one anabolic implant during their production lives (NAHMS, 2000). Anabolic steroid implants are characterized as oestrogenic, androgenic or a combination of both oestrogenic and androgenic compounds (combination implants). Additionally, these compounds may also be categorized as natural hormones and xenobiotics, or combinations of these. Products available for use in beef cattle production, including doses and sex-specific usage, have been summarized by others (Montgomery et al., 2001; Duckett and Pratt, 2014); see Table 1, which includes the abbreviations used in this chapter. If trade names are not declared, steroid and concentrations used will be reported.
