MONENSIN EFFECTS ON DIGESTION OF CORN OR BARLEY HIGH-CONCENTRATE DIETS

We conducted two experiments to determine the effects of monensin addi tion on digestion of high-concentrate diets based on corn or barley an d to identify any interactions between grain source and monensin addit ion. A replicated in vitro experiment with a 2 x 4 factorial arrangeme nt was used to evaluate monensin addition (0 or 72 mg/kg in vitro subs trate) and grain source (corn, Gunhilde barley [GUN], Harrington barle y [HAR], or Medallion barley [MED]). Triplicate tubes for each treatme nt were incubated for 0, 3, 6, 9, 12, 18, 24, and 30 h. Rate and exten t of IVDMD were determined. Four ruminally and abomasally cannulated s teers 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.steer(-1).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 sou rce for IVDMD during 3 through 9 h of incubation. Monensin increased t he IVDMD of GUN and MED, but it decreased the IVDMD of HAR. Corn IVDMD was not affected by monensin addition. Steers fed Medallion barley ha d greater(P <.05) microbial protein synthesis, rate of in situ DM and starch disappearance, ruminal and postruminal digestion of starch, rum inal total VFA concentrations, and total tract digestion of DM, OM, an d 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 propion ate. No monensin x grain source interactions were observed for the var iables measured in vivo.