The Ionophore Resistance of Ruminal Bacteria and Its

The FDA approved the ionophore monensin as feed additive for cattle in 1976, and within a few years most feedlot cattle in the United States were being fed this antibiotic (Russell and Strobel, 1989). Feed efficiency improvements as large as 10% were documented (Goodrich et al., 1984), and in vitro studies and in vivo incubations showed that monensin was altering ruminal fermentation (Dinius et al. 1976; Richardson et al., 1976). Since this time two other ionophores have also been approved in the United States for use in cattle as feed additives (e.g. lasalocid and laidlomycin). Because ionophores are antibiotics, some groups have argued that they pose a public health threat and have called for a ban on their use. Indeed, the European Union has proposed a future (January 1, 2006) ban on the use of ionophores as growth promotants (European Commission Directorate-General XXIV, 1999). Some ruminal bacteria are highly resistant to ionophores (Russell and Strobel, 1989). The question then arises, does he ionophore resistance of ruminal bacteria confer an increased resistance to other classes of antibiotics?