ance programs within individual industries. The structure of the industries also affects the ease of identifying the source of a problem (whether it is a pathogenic microorganism, a drug residue, or an antibiotic-resistant bacteria) and the ease with which consumer preferences flow back through the system to stimulate changes in the genetics and breeding of stock to produce the desired product.

In all of the animal industries, antibiotic drugs are used for three primary reasons: (1) therapeutically, for treating existing disease conditions; (2) prophylactically, at subtherapeutic concentrations¹; and (3) subtherapeutically for production enhancement (increased growth rate and efficiency of feed use). Therapeutic use generally occurs after diagnosis of a disease condition, and treatment is governed by the drug’s label instructions or in accordance with extra-label instructions provided by a veterinarian in the context of a valid and current veterinarian–client–patient relationship (VCPR). Subtherapeutic doses are used when pathogens are known to be present in the environment or when animals encounter a high-stress situation and are more susceptible to pathogens. Subtherapeutic doses are smaller than those required to treat established infections. They might also use compounds developed exclusively as production enhancers that have no therapeutic purpose. Although the U.S. Food and Drug Administration (FDA) defines subtherapeutic concentration as <200 g/t of feed, there is a wide range of concentrations below that for which different antibiotics are formulated into feeds and fed to different species.

As summarized in Cromwell (1991), there are three mechanisms of action through which antibiotics appear to enhance growth and production. The first involves direct biochemical events that are affected by antibiotics: nitrogen excretion, efficiency of phosphorylation reactions in cells, and direct effects on protein synthesis. The second involves direct effects on metabolism, including the effects of antibiotics on the generation of essential vitamins and cofactors by intestinal microbes and the way that antibiotics affect the population of microbes that make these nutrients. In addition, the feeding of antibiotics is associated with decreases in gut mass, increased intestinal absorption of nutrients, and energy sparing. This results in a reduction in the nutrient cost for maintenance, so that a larger portion of consumed nutrients can be used for growth and production, thereby improving the efficiency of nutrient use for productive functions. The third proposed mechanism of action is eliminating subclinical populations of pathogenic microorganisms. The elimination of this route of metabolic drain allows more efficient use of nutrients for production.

The goal of an efficient livestock operation is to maintain animals that are free of disease or injury, that gain weight well if they are intended for market, or that stay in optimal condition if they are kept as breeding stock. The producer