The effects of specific Saccharomyces cerevisiae strains and monensin supplementation on rumen fermentation in vitro

In recent times there has been much concern among animal product consumers about the safety and use of antimicrobial substances in the production of food for human consumption. This has been driven by the ban of the use of antibiotics at subtherapeutic levels for food animal production in Europe. For this reason, producers are always looking at ‘natural’ alternatives to antibiotics to improve production from their animals. One such alternative is the use of yeast cultures of Saccharomyces cerevisiae in ruminant diets to manipulate rumen fermentation. Yeast culture fed to ruminants has increased production from beef and dairy cattle and sheep as well as stabilizing rumen fermentation under conditions such as rumen acidosis. Yeast culture has been shown to increase the microbial protein supply from the rumen by stimulating growth of bacteria in the rumen. Yeast culture may be used to alleviate the negative effects of rumen antimicrobials such as monensin on rumen microbial populations and fibre digestion. Four separate sets of experiments were undertaken. In the first set of experiments, the effects of 10 specific yeast cultures on the growth of 3 selected rumen bacteria was evaluated. The rumen bacteria evaluated were Ruminococcus albus, Selenomonas ruminantium and Ruminobacter amylophilus. It was found that only two of the ten strains of yeast tested were able to consistently decrease the lag time of the selected rumen bacteria. In the second set of experiments, the effects of yeast culture addition on a rumen fluid based batch culture fermentation was analysed by measuring the gas pressure produced by the fermentation. The results obtained were too variable to draw any conclusions from the data. In the third set of experiments, the effects of yeast, monensin and their combination were evaluated in rumen simulating continuous cultures. It was found that monensin increased the efficiency of the fermentation but decreased the total anaerobic bacteria. Yeast culture increased the total anaerobic bacteria. UNI yeast alleviated the reduction in anaerobic bacteria when combined with monensin. The last set of experiments were an attempt to develop an assay to measure the potential of certain yeast strains to stimulate rumen fermentation. The potential assays were based on the ability of yeasts to stimulate a growing culture of Ruminococcus albus. None of the assays attempted showed obvious potential as a future assay. From the study it seems that yeasts stimulate the growth of certain species of ruminal bacteria but not all yeast strains are able to do so. Yeast supplementation may be fed in combination with monensin in order to reduce the impact of monensin on the microbial populations of the rumen.