Effect of magnesium and potassium on microbial fermentation in a continuous culture fermentation system with different levels of monensin or lasalocid

Two continuous culture fermentation experiments were used to determine ruminal microbial fermentation in the presence of monensin (Exp. 1) or lasalocid (Exp. 2) with different dietary concentrations of Mg and K. In each experiment, eight 500-ml continuous culture fermentors were supplied with late-cut coastal bermuda grass (80%) and cellulose (20%) as substrate during five trials with two levels of Mg (.07 and .14%), K (.6 and 2.5%) and either monensin (Exp. 1; 0 and 20 mg/kg) or lasalocid (Exp. 2; 0 and 20 mg/kg) in a 2 x 2 x 2 factorial arrangement of treatments (each trial served as a replication). Substrate was added every 8 h (5 g/fermentor) for 5 d and digesta samples were collected every 2 h for 6 h and at 20 h for volatile fatty acid (VFA) analysis on d 6. In Exp. 1, monensin decreased (P less than .05) the acetate:propionate ratio (A:P) from 2.98 to 2.22. A Mg X monensin interaction was observed for the molar percentage of acetate and propionate (P less than .05). There was a 4.8% reduction in acetate (mol/100 mol) when monensin was fed with .07% Mg and a 12.6% decrease (P less than .01) when fed with .14% Mg compared with diets not containing monensin. Propionate (mol/100 mol) increased 14% when monensin was fed with .07% Mg and 32% (P less than .001) when fed with .14% Mg compared with diets not containing monensin. Potassium did not affect VFA concentrations in the presence of monensin or Mg. In Exp. 2, lasalocid decreased (P less than .01) the A:P ratio from 3.53 to 2.86.