So-called unidentified growth factors have been reported throughout the history of poultry nutrition studies. Natural ingredients claimed to contain such factors are most often animal proteins or fermentation by-products (Summers et al., 1959; Al-Ubaidi and Bird, 1964; Dixon and Couch, 1970; Waldroup et al., 1970). Ingredients containing unidentified growth factors are claimed to improve chick growth and reproductive performance (Morrison et al., 1956; Touchburn et al., 1972). Bhargava and Sunde (1969) described a chick assay for quantitation of such unidentified factors.
The mode of action of these unidentified factors is far from clear, however. With the identification of vitamins and consideration of the significance of trace minerals, many nutritionists now disregard the importance of growth factors. That responses may still occur could relate to truly unidentified nutrients or, more likely, to changes in feed palatability and/or quality (Alenier and Combs, 1981; Cantor and Johnson, 1983), mineral chelation, or simple improvement in the balance of available nutrients.
Antimicrobial feed additives, although not nutrients in the sense that they are required by poultry, are included in diets to improve growth, efficiency of feed utilization and livability (Stokstad et al., 1949; Coates et al., 1951; Libby and Schaible, 1955; Milligan et al., 1955; Bird, 1968; Begin, 1971; Morrison et al., 1974). Antimicrobial agents are included in diets at relatively low concentrations (1 to 50 mg/kg), depending on the agent and stage of development of poultry. They are, accordingly, classified as additives and as growth promoters. Egg production is also frequently improved by dietary supplementation with antimicrobial agents (Carlson et al., 1953; Balloun, 1954; Andrews et al., 1966). The mechanisms by which antimicrobials improve performance are not clearly understood. Because antimicrobials do not stimulate growth of chicks kept in a germfree environment (Coates and Harrison, 1969), it is likely that stimulation of growth results from either suppression of microorganisms that may cause adverse effects or encouragement of other microorganisms that may have favorable effects on poultry performance.
There is some concern that feeding of low concentrations of antibiotics may favor the proliferation of antibiotic-resistant microorganisms, which could have serious consequences for disease control in humans or domestic animals. A study by the National Research Council (1980a) examined this concern and concluded that "the postulations concerning the hazards to human health that might result from the addition of subtherapeutic antimicrobials to feeds have been neither proven nor disproven." Continued monitoring of bacterial resistance in humans and animals has not provided clear-cut answers to this concern.
Constraints and regulations on use of particular antimicrobials in poultry feeds vary among countries and are subject to change. Detailed information on specific antimicrobial agents, levels of usage, and legal requirements for use in the United States and Canada may be found in the Feed Additive Compendium (published each year by the Miller Publishing Company, 2501 Wayzata Boulevard, Minneapolis, MN 55440) and in the compendium of "Medicating Ingredient Brochures" (Plant Products Division, Canada Department of Agriculture, Ottawa, Ontario, Canada).
For official information concerning Food and Drug Administration approval of antibiotics and other animal drugs, the Code of Federal Regulations (CFR), Title 21, should be consulted. Title 21 is revised at least once each year as of April 1. The CFR is kept up to date by the individual issues of the Federal Register. These two publications must be used together to determine the latest version of any given rule. Title 21 is published in six parts: Part 500-599 covers animal drugs, feeds, and related products and is available from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. The Federal Register is available from the Superintendent of Documents and includes monthly issues of the "List of CFR Sections Affected" and "The Federal Register Index."