Bioavailability and Antagonists of Trace Minerals in Ruminant Metabolism

Modern analytical techniques and instrumentation make it possible to accurately determine trace mineral concentration in feed and animal tissues. Unfortunately, however, such determinations provide no information on utilization of the minerals by animals. Utilization involves the concept of bioavailability and is critical in the selection of a suitable source of the mineral. In terms of trace minerals, bioavailability may be defined as the proportion of an ingested mineral that is absorbed, transported to its site of action, and converted to the physiologically active species (O’Dell, 1983). Traditionally, trace mineral bioavailability studies have been conducted at deficient dietary levels, usually with purified or semi-purified diets. The mineral sources are then added at graded levels and response criteria measured. In general, the bioavailability of a mineral element in a particular source is determined relative to its functional availability from a standard source. Use of a standard source allows expression of bioavailability in terms of relative biological availability (Miller, 1983). Bioavailability can be affected by a number of factors including animal species, physiological state, previous nutrition, interactions with dietary nutrients and ingredients, choice of response criteria, choice of standard source, and chemical form and solubility of the mineral element. For more detailed information on the effects of each of these variables and their effects on specific minerals the reader is referred to a recent Academic Press publication on bioavailability of nutrients for animals by Ammerman et a/. (1 995).