Ruminants by nature are well-adapted for digestion of forage and grains of a wide variety, thanks to ruminal microflora. Still, there are times when biochemical conditions taking over the rumen can prevent optimal utilization of the ration.
In these cases, certain microorganisms, often referred to as probiotics or direct-fed microbials (DFM), can help to stabilize the rumen environment, particularly when the cow is consuming a diet rich in readily fermentable carbohydrates.
Yeast stimulate rumen bacteria
DFMs are gaining attention for another reason: the growing interest in natural products for animal feeds. Lee Kilmer, Department of Animal Science, Iowa State University, points out that yeast cultures in particular stimulate growth of rumen bacteria in contrast to products such as antibiotics which are toxic to selective bacteria. Although the total microbial population is increased with DFM supplementation, the cellulolytic and lactic acid utilizing bacteria are stimulated preferentially, he notes. Other yeasts and yeast cultures, such as brewers yeast, have been used as supplements in animal feeds for years, primarily due to their high protein and vitamin content.
There has been considerable research on the feeding of yeast cultures to dairy cattle. Johanne Chiquette, researcher with the Lennoxville Dairy and Swine Research and Development Centre, Quebec, Canada, points to experiments there that have shown that probiotic addition (Aspergillus oryzae -AO, alone or in combination with Saccharomyces cerevisiae-SC) to the diet of dairy cows has increased daily milk production by 1.4 kg., or approximately 4 percent. Moreover, concentrations in volatile fatty acids were greater and ruminal pH lower in animals receiving AO alone or combined with SC before their meal, suggesting a stimulation of ruminal fermentation following the addition of probiotics.
There are a number of yeast products marketed worldwide that are available for ruminants. Peter H. Robinson, cooperative extension specialist, Department of Animal Science, University of California, Davis, points out that while the number of yeast products that have undergone evaluation in controlled research studies is somewhat limited, there is a widespread belief among dairy and beef producers, and ruminant nutritionists, that yeast products are beneficial because they enhance dry matter (DM) intake and overall animal performance." Since yeast products are relatively low cost, economic barriers to their use are minimal, he adds.
How do yeast products stimulate dry matter intake and improve animal productivity and performance? Robinson says there are several possibilities. One of the oldest hypothesis is that the yeasts are able to grow, at least for a short period of time, in the rumen where they can directly enhance fiber digestion and/or produce nutrients that stimulate growth of rumen bacteria, which do the bulk of the fiber digestion. He says another theory is that the yeasts utilize nutrients, such as lactic acid that, if allowed to accumulate in the rumen, could suppress bacterial growth and/or suppress DM intake by driving rumen pH down.
Robinson also points to a more recently suggested explanation: growth of yeast in the rumen utilizes the trace amounts of dissolved oxygen, particularly at the interface of the cellulolytic bacteria and fiber, thereby stimulating growth of rumen bacteria, to which oxygen is toxic. This particular explanation makes sense only if a portion of the yeast product fed is viable or able to actually grow, at least for a period of time, in the rumen. This has given rise to the ongoing debate of ‘live' versus ‘dead' yeast products.
It is also theorized that the yeast culture itself, which is created in the yeast fermentation process, provides a mixture of micro-nutrients which stimulates bacterial growth in the rumen, thereby facilitating increased fermentation of fiber and/or utilization of the end-products of fiber fermentation to prevent their accumulation in the rumen, Robinson points out.
There has been a substantial amount of controlled research on affects of yeasts and yeast cultures on rumen fermentation and performance of lactating ruminants, although most is focused on three specific commercial products, Robinson sums up. When the University of California examined the results of such research, the average milk yield was consistently at about 3.4 percent increase.