In vitro lactic acid inhibition and alterations in volatile fatty acid production by antimicrobial feed additives.

Batch culture fermentations were used to determine the effects of avoparcin, lasalocid, monensin, narasin, salinomycin, thiopeptin, tylosin, virginiamycin, monensin + tylosin combination, and two new ionophore compounds (RO22-6924/004 and RO21-6447/009) on lactic acid and volatile fatty acid (VFA) production. Ruminal fluid from cattle fed a high alfalfa hay diet was incubated with glucose for 12 h in a buffered medium to determine the effect of antimicrobial compounds on lactic acid concentration. Fermentations treated with antimicrobial compounds had higher final pH and lower L(+) lactic acid concentration. Narasin and salinomycin were more inhibitory than other ionophore compounds. Monensin and tylosin in combination was more effective than monensin alone. Among the nonionophore compounds, avoparcin was the least effective and thiopeptin, tylosin and virginiamycin were extremely effective in reducing lactic acid concentration. Ruminal fluid from cattle fed a diet of alfalfa hay and grain (50:50) was incubated with a mixture of sugars, casein and urea for 12 h in a buffered medium to determine the effect of antimicrobial compounds on VFA production. Generally, total VFA concentration was not affected by antimicrobial compounds except RO22-6924/004, tylosin and virginiamycin, which caused a reduction at high concentrations. Tylosin, monensin and tylosin mixture, thiopeptin and virginiamycin at high concentrations (greater than 6.0 micrograms/ml) increased the acetate proportion. All compounds increased the molar proportion of propionate. Tylosin and virginiamycin at high concentrations (greater than 6.0 micrograms/ml) decreased the proportion of propionate. Monensin and tylosin in combination had no effect on propionate proportion. Among the compounds tested, narasin and salinomycin were the most effective in enhancing propionate proportion. Ionophore compounds were more inhibitory to butyrate production than the nonionophore compounds. Batch culture fermentations may be used to quantitate the relative efficacy of antimicrobial compounds to alter ruminal fermentation characteristics.