Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/1996
Publication Date: N/A
Interpretive Summary: Most beef cattle in the U.S. are fed ionophores as feed additives. Improvements in feed efficiency and animal performance are the ultimate criteria of ionophore activity, but such studies are time consuming and expensive. Ionophores often decrease the ratio of acetate to propionate in ruminal fluid, but this measurement is not a sensitive index. Previous work kdemonstrated that ionophores caused an efflux of potassium from sensitive ruminal bacteria. In this manuscript, we demonstrate that potassium efflux constants provide an accurate and highly sensitive of assay of ionophore activity. Potassium efflux measurements may provide a tool for enhancing the activity of ionophores and improving animal performance.
Technical Abstract: When mixed ruminal bacteria from cattle fed timothy hay were suspended in a medium containing a low concentration of potassium, monensin and lasalocid catalyzed a rapid efflux of potassium. The ionophore-mediated potassium efflux was concentration-dependent, and it was possible to describe the relationship with saturation constants. Mixed ruminal bacteria never lost more than 50% of their potassium (Emax = 46%), and the concentrations of monensin and lasalocid needed to cause half maximal potassium efflux (Ke) were 180 and 140 nM, respectively. When cattle were fed 350 mg monensin/d, the ratio of ruminal acetate to propionate decreased from 4.2 to 2.9, and Ke of monensin was 8-fold higher. Monensin supplementation also caused a 2- fold increase in the Ke of lasalocid. Lasalocid supplementation (350 mg/d) had no effect on ruminal acetate to propionate, but it caused a 2-fold increase in the Ke values of monensin and lasalocid. Increases in Ke occurred almost immediately after ionophore was added to the ration, and the Ke values returned to their pre-feeding values within 14 days of withdrawal. Ionophore supplementation had no effect on the Emax values, and approximately 50% of the population was always ionophore-resistant. Because the Ke values of even adapted ruminal bacteria were low (<1.5 uM), it appears that a large proportion of the ruminal ionophore is bound non- selectively to feed particles or ionophore-resistant bacteria.