This report will review the history of antibiotic growth promoter (AGP) use in the animal industry, concerns about development of antimicrobial resistance, and response in the European Union and United States to these concerns. A brief description of the history of legislation regarding feed use of antimicrobials in Denmark and the experience of animal producers following the 1998 ban will serve to illustrate the consequences on animal performance and health of withdrawing the approval for this use. The biological basis for antibiotic effects on animal growth efficiency will consider effects on intestinal microbiota and effects on the host animal and will use the germ-free animal to illustrate effects of the conventional microflora. The probability that no single compound will replace all of the functions of antimicrobial growth promoters will be considered, and methods to consolidate and analyze the enlarging database will be discussed.

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Trace minerals such as zinc, copper, and manganese are essential cofactors for hundreds of cellular enzymes and transcription factors in all animal species, and thus participate in a wide variety of biochemical processes. Immune development and response, tissue and bone development and integrity, protection against oxidative stress, and cellular growth and division are just a few examples. Deficiencies in trace minerals can lead to deficits in any of these processes, as well as reductions in growth performance. As such, most animal diets are supplemented with inorganic and/or organic forms of trace minerals. Inorganic trace minerals (ITM) such as sulfates and oxides form the bulk of trace mineral supplementation, but these forms of minerals are well known to be prone to dietary antagonisms. Feeding high-quality chelated trace minerals or other classes of organic trace minerals (OTM) can provide the animal with more bioavailable forms of the minerals. Interestingly, many, if not most, published experiments show little or no difference in the bioavailability of OTMs versus ITMs. In some cases, it appears that there truly is no difference. However, real differences in bioavailability can be masked if source comparisons are not made on the linear portion of the dose-response curve. When highly bioavailable chelated minerals are fed, they will better supply the biochemical systems of the cells of the animal, leading to a wide variety of benefits in both poultry and swine. Indeed, the use of certain chelated trace minerals has been shown to enhance mineral uptake, and improve the immune response, oxidative stress management, and tissue and bone development and strength. Furthermore, the higher bioavailability of these trace minerals allows the producer to achieve similar or improved performance, at reduced levels of trace mineral inclusion. (

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Conversion of 2-hydroxy-4-(methylthio)butanoic acid (HMB) to L-methionine (L-Met) was studied by using chick liver homogenates. The first step was found to be stereospecific with different enzymes for the D- and L-isomers of HMB. L-HMB was the substrate for L-2-hydroxy acid oxidase, a peroxide-producing flavo-enzyme found in peroxisomes of liver and kidney. The enzyme for D-HMB, identified as mitochondrial D-2-hydroxy acid dehydrogenase, had not been previously described in the chick. This enzyme was found in every tissue tested including intestinal mucosa and skeletal muscle. Thus, D-HMB could be used by any organ for protein synthesis, like L-Met itself. These results provide a biochemical explanation of equimolar incorporation of HMB and DL-methionine (DL-Met) into chick hepatocyte protein in that the two HMB enzymes can simultaneously convert both HMB isomers to L-Met while only one enzyme, D-2-amino acid oxidase, converts D-Met to L-Met.