Effect of monensin fed with forage on digestion and the ruminal ecosystem of steers.

Monensin 3 , 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 cot ton fiber, dietary carbohydrate and nitrogen digestibility, on numbers of rumihal microbes, and on concentration of ruminal volatile fa t ty acids. No differences (P>.10) in cellulose digestibility from cot ton were observed in response to monensin level when cot ton fiber samples were incubated in vitro in ruminal fluid from these steers. Neither were differences in loss of dry mat ter detectable (P>.10) when cot ton fabric strips were placed within the rumens of steers for 72 hours. In v/vo digestion of dry matter, crude protein, hemicellulose and cellulose of the forage diet was not different (P>.05) among treatments. Total ruminal volatile fa t ty acid concentrat ion was not affected by feeding monensin, but the molar proport ion 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 fat ty acids were affected. Neither the numbers of protozoa, total bacteria nor cellulolytic bacteria in rumihal fluid were affected by feeding up to 33 ppm dietary monensin. (