Monensin effects on digestion of corn or barley high-concentrate diets

We conducted two experiments to determine the effects of monensin addition on digestion of high-concentrate diets based on corn or barley and to identify any interactions between grain source and monensin addition. A replicated in vitro experiment with a 2 x 4 factorial arrangement was used to evaluate monensin addition (0 or 72 mg/kg in vitro substrate) and grain source (corn, Gunhilde barley [GUN], Harrington barley [HAR], or Medallion barley [MED]). Triplicate tubes for each treatment were incubated for 0, 3, 6, 9, 12, 18, 24, and 30 h. Rate and extent of IVDMD were determined. Four ruminally and abomasally cannulated steers were used in a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments to test the effects of monensin addition (0 vs 270 mg x steer(-1) x d(-1)) and grain source (70% corn vs 80% Medallion barley). Diets were balanced to be isocaloric and isonitrogenous. An interaction (P < .10) was found between monensin addition and grain source for IVDMD during 3 through 9 h of incubation. Monensin increased the IVDMD of GUN and MED, but it decreased the IVDMD of HAR. Corn IVDMD was not affected by monensin addition. Steers fed Medallion barley had greater (P < .05) microbial protein synthesis, rate of in situ DM and starch disappearance, ruminal and postruminal digestion of starch, ruminal total VFA concentrations, and total tract digestion of DM, OM, and starch compared with steers fed corn. Monensin addition decreased (P < .10) ruminal digestion of feed N and ruminal proportions of acetate and butyrate and increased (P < .001) the ruminal proportion of propionate. No monensin x grain source interactions were observed for the variables measured in vivo.