Effect of Monensin on Rumen Fermentation in Vitro and in Vivo

Abstract

Monensin, a biologically active compound produced by a strain of Streptomyces cinnamonensis, was tested in three in vitro ruminal fermentation systems. Monensin caused a shift in the proportions of fermentation end product acids with very little effect on total acid production. At 1 ppm in the concentrate substrate system, monensin increased propionic acid 50% and decreased both acetic and butyric acids. Similar changes were observed when monensin was tested in vivo in concentrate and pasture-fed fistulated cattle. In a subsequent non-fistulated cattle feeding experiment, 100 and 500 mg monensin/head/day increased the molar proportion of ruminal propionic acid from 31.9 to 41.0 and 43.5%, respectively, while the molar proportions of acetic acid and butyric acid were each decreased. Total ruminal volatile acid concentration was unchanged by monensin. Monensin maintained this effect on the ruminal acids throughout the 148-day experiment. Theoretical implications which suggest both ruminal and nonruminal energy savings by this modification of ruminal fermentation are discussed.