Evaluation of the effects of Kluyveromyces marxianus, a mixture of Bacillus subtilis and Bacillus licheniformis, or their combination on rumen microbial fermentation and methane production in vitro

Microbial feed additives may influence ruminal fermentation and methane (CH4) production, but their effects vary among microorganisms and inclusion levels. This study evaluated increasing inclusion levels of the non-conventional yeast Kluyveromyces marxianus (KLU), a mixture of Bacillus subtilis and B. licheniformis (BAC), or their combination (BACKLU) on in vitro rumen fermentation, gas and CH4 production. Ruminal fluid batch cultures were incubated in a randomized complete block design with a negative control (CTL), a positive control (monensin, MON; 10 mg/L), KLU (40 to 2000 mg/L), BAC (10 to 500 mg/L), or BACKLU (50 to 2500 mg/L). Compared with CTL, MON decreased gas and CH4 production, acetate and branched-chain volatile fatty acid (BCVFA) proportions, and ammonia concentration, while increasing propionate proportion. KLU had limited effects at ≤ 400 mg/L, although ammonia increased at all doses and BCVFA at ≥ 80 mg/L. At 2000 mg/L, KLU increased gas production, total VFA concentration, and valerate and BCVFA proportions, without affecting CH4 production. BAC had minimal effects on fermentation, although ammonia increased at all doses, and did not affect CH4 production. Responses to BACKLU were similar to KLU alone. Overall, KLU, BAC, and BACKLU did not reduce CH4 production.