Effect of Monensin Fed with Forage on Digestion and the Ruminal Ecosystem of Steers

Abstract

Monensin, a polyether antibiotic, was blended with a forage diet at levels of 0, 11, 22 and 33 ppm and fed to steers to determine its effect on cotton fiber, dietary carbohydrate and nitrogen digestibility, on numbers of ruminal microbes, and on concentration of ruminal volatile fatty acids. No differences (P>.10) in cellulose digestibility from cotton were observed in response to monensin level when cotton fiber samples were incubated in vitro in ruminal fluid from these steers. Neither were differences in loss of dry matter detectable (P>.10) when cotton fabric strips were placed within the rumens of steers for 72 hours. In vivo digestion of dry matter, crude protein, hemicellulose and cellulose of the forage diet was not different (P>.05) among treatments. Total ruminal volatile fatty acid concentration was not affected by feeding monensin, but the molar proportion of acetic acid decreased (P<.01) from 66.7 to 61.3% and that of propionic acid increased (P<.01) from 20.1 to 26.1%. No other volatile fatty acids were affected. Neither the numbers of protozoa, total bacteria nor cellulolytic bacteria in ruminal fluid were affected by feeding up to 33 ppm dietary monensin.