In vitro effects of a thiopeptide and monensin on ruminal fermentation of soluble carbohydrates.

Using a purified diet and a mixed culture of ruminal microorganisms, we studied the in vitro effects of a sulfur-containing peptide (A10255) and monensin on ruminal VFA and lactic acid concentrations. After 22 h of fermentation, total VFA concentration was greater in cultures with A10255 and monensin (2.5 ppm each) than in untreated controls (84.1, 91.9, and 65.2 mM for cultures with A10255, monensin, and control, respectively). The molar proportions of propionate were more than 40% greater, and the molar proportions of acetate were about 15% less, in treated than in untreated cultures. The molar proportions of the remaining VFA were lowest in monensin-treated cultures, intermediate in A10255-treated cultures, and greatest in untreated cultures. Addition of A10255 or monensin also prevented the accumulation of lactate and maintained higher pH than in untreated cultures. In pure culture, growth of Streptococcus bovis was slowed by A10255 at pH 5.5 and 6.5. Growth of S. bovis was reduced markedly with addition of monensin at pH 6.5 and inhibited at pH 5.5. Growth of Lactobacillus acidophilus was inhibited more at pH 5.5 than 6.5 with monensin but was unaffected by A10255. The thiopeptide A10255 and monensin inhibited ruminal lactate production, but sensitivities of lactate-producing bacteria differed between compounds.