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Effect of Magnesium and Potassium on Microbial Fermentation in a Continuous Culture Fermentation System with Different Levels of Monensin or Lasalocid^1,2,

Texas A&M University and Texas Agricultural Experiment Station³, College Station 77843

Abstract

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 bermudagrass (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<.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<.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<.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<.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<.01) the A:P ratio from 3.53 to 2.86. There was a Mg x lasalocid interaction (P<.01) for butyrate (mol/100 mol). Lasalocid lowered (P<.001) butyrate (mol/100 mol) 35% when fed with .07% Mg and 53% when fed with .14% Mg. A lasalocid x K interaction existed for propionate (P<.10) and butyrate (P<.05;mol/100 mol). Lasalocid increased (P<.10) propionate molar proportion 30% when fed with .6% K and 41% when fed with 2.5% K compared with diets not containing lasalocid. Butyrate molar proportion decreased (P<.05) more when lasalocid was fed with 2.5% K than when fed with .6% K. These data suggest that previously reported inconsistent results with monensin and lasalocid supplementation may have been due to these ioniphores' interactions with other nutrients. Furthermore, these results suggest that the efficacy of monensin and lasalocid in altering the VFA concentrations of ruminal fermentation might be enhanced by optimizing other nutrients.

¹ Journal paper TA. 22356. Texas A&M Univ., Texas Agric. Exp. Sta.

² Reference to a company or trade name does not imply approval or endorsement by the Texas Agric. Exp. Sta.

³ Dept. Anim. Sci.