Mechanisms by Which Metabolic Modifiers Alter Growth Rate and Carcass Composition of Meat Animals

The livestock industries utilize numerous means to alter the performance of meat animals and composition of their resulting carcasses. Inclusion of metabolic modifiers in production systems reflects a continuation of processes that began decades ago with genetic selection and progress in understanding of nutrition, health, and management. Any drug or management practice that affects efficiency of food animal production exerts its influence through at least one of three general mechanisms: • increased energy consumption above maintenance, • altered composition through reduced energy content of the empty body, or • improved efficiency of the processes that result in growth. This paper will focus on the effects of metabolic modifiers currently being used in the livestock industry, the mechanisms by which they exert their effects, and discussion of some practical considerations for their use. The primary example will be steroid growth implants in beef cattle. Other modifiers will be discussed as their effects differ from steroids.