This is Part I of a two-part series – read Part II.

Over recent years, a whole category of additive products have exploded in dairy cow (and other food animal species) nutrition. Microorganism-based products such as yeasts, bacteria, fungi, enzyme products, etc., have become standards in dairy nutrition and are commonly found in a significant number of diets.

Current research and practice are showing increased applications at different stages of dairy animal production.

Much of this growth is related to the increased understanding of management of microbial population in both the rumen and the lower gut of the cow. We are learning the importance of the relationship between the microbiome (total microbial population in the animal or in each region of the digestive tract). Science is showing that this relationship is critical not only for improved nutrition but also for gut health, stress mitigation and overall animal well-being.

Some questions that commonly get asked about these additives include:

  • What are the differences in yeasts, bacteria, fungi, etc.?
  • Do these materials really work?
  • What kind of performance effects should be seen?
  • Which are the best products to use?
  • Can they be used in combination?

This article will attempt to answer some of these questions. But first, let’s discuss some definitions and explanations since there are many common misunderstandings.

What is the difference in a prebiotic and a probiotic?

Some confusion exists in the definition of these two words. Both are used to describe general categories related to the function of microbial products working in the animal’s digestive tract to perform a (hopefully) beneficial purpose.

In general, prebiotics are specific nutritional sources for the microbial population. The concept is that these fermentable ingredients will promote microbial activity, growth and reproduction, resulting in effects such as increased fiber digestion, increased microbial protein production, increases in volatile fatty acid (VFA) production or beneficial shifts in VFA ratios and support of ruminal pH when feeding higher levels of rapidly fermentable carbohydrates. Interestingly, some types of microbial products (i.e., inactive yeasts or yeast cultures) can perform a prebiotic role, serving as a food source for the ruminal microbial population.

Probiotics are the actual fed organisms themselves. Probiotics are often considered the various strains of bacteria but can include all classes of organism including the various forms of yeast, bacterial species and fungal products. The challenge the nutritionist and dairy producer encounter is: There are thousands of strains or types of bacteria, yeasts and fungi products. Some have significant performance effects, some are more marginal, and some show little, if any, effect. This can be dictated by ration types and production circumstances and other variables.

While there is a large body of research available for many of the products, the reported results have shown to be somewhat inconsistent. The use of a specific product requires a considerable amount of review to find the best product or product combination for the specific ration type, production group, production environment or other variable.

Product types


In general, the feed industry largely agrees that bacterial products must be alive/viable when fed. Thus, they must survive processing, storage, addition to the feed and feed mixing, and finally the gut environment to the target location. Commonly fed bacteria include Lactobacillus acidophilus (and other Lactobacillus species), Enterococcus diacetylactis, Propionibacterium freudenreichii and Bacillus subtilis.

In cattle, much of the research in feeding bacterial-based direct-fed microbials (DFMs) has involved feeding Lactobacillus-based DFM to young calves fed milk, calves being weaned or cattle shipped to feedyards because these conditions were often classified as times of high stress. Calves fed L. acidophilus have been reported to have reduced incidence of diarrhea and reduced counts of intestinal coliform bacteria. Earlier feeding data, while variable, reported improved growth performance.

Microbiologists have continued to work to understand the effects bacterial feeding can have in dairy feeding scenarios. For instance, Megasphaera elsdenii (ME) is the major lactate-utilizing organism in the rumen of adapted cattle fed high-grain diets. However, when cattle are abruptly shifted from a high-forage to higher-concentrate diet, the numbers of ME are often inadequate to prevent lactic acidosis. Research has shown that during a challenge with highly fermentable carbohydrates (starch), addition of Megasphaera elsdenii B159 prevented an accumulation of lactic acid and shifted ruminal fermentation beneficially.


Feeding yeast to cattle has been common for years. A variety of mechanisms have been suggested to explain changes in the activity of the rumen and improvements in performance when ruminants are fed different yeast products. For example, yeast may have a “buffering” effect in the rumen by mediating sharp drops in rumen pH, which follows feeding of higher-concentrate rations. As such, yeast may help to reduce excess lactic acid production when ruminants are fed higher-concentrate diets. While yeast products are not actual buffers per se, their function and activity appear to provide support to rumen pH.

At higher pH levels, however, when feeding yeast, research has found increased numbers of rumen cellulolytic (cellulose-digesting) bacteria and improvements in fiber digestion. One of the most common types of yeast fed, Saccharomyces cerevisiae has been shown to stimulate the growth and activity of rumen bacteria. The response is different somewhat depending on the exact strain of the yeasts.

There has been debate on the efficacy of specific forms of yeast products. The two broadest categories are generally seen as “live yeasts” and “inactive” or “dead” yeasts. Live yeasts are fairly well defined. A culture of a specific yeast is grown, collected, freeze dried and packaged. This yeast can then be rejuvenated as it is added to a ration or feed mix and exposed to the moisture and air. At this point, it begins its activity. While different claims are made, the general understanding of the effect of a live yeast in the rumen is: It will scavenge (consume) oxygen from the system, thus making the rumen bacterial activity more efficient.

Inactive yeasts come in a variety of forms, the most common being the yeast “culture” (YC). A YC is a blend of yeast cell materials, metabolites from yeast growth and respiration and the media the yeast is grown upon. These yeast cells are not viable, but the cellular material and the metabolites are believed to provide specific nutrients (prebiotic) to the rumen. This is the most common form of yeast product fed.

Another form of yeast which is purer, although still inactive or non-viable, is often referred to as a functional dried yeast (FDY). This purer yeast product contains little or no media on which the yeast was grown. An FDY is commonly sourced from the fermentation of cane molasses or other substrates to ethanol. The yeast is recycled through the fermentation process and the cells collected periodically.

The yeast cells are then processed (autolyzed, hydrolyzed, etc.) and finally dried and packaged. The FDY is similar to YC in that it can act as a prebiotic, providing specific nutrients to the rumen bacteria. However, because the product is largely made up of the total cells, it can also include cell wall contents such as mannanoligosaccharides (MOS) and beta glucans (BG), which are parts of the yeast cell walls.

Beside the debate on the type of yeast product to feed, there are questions on how to feed. Some products on the market guarantee high numbers of live yeast cells (e.g., 20 x 1010 colony-forming units per gram) with low recommended feeding rates (5 to 20 grams per day or less). Other products suggest that live organisms are not required for beneficial effects and that the end products (metabolites produced by the yeast cells) are the “active” ingredients.


Because of the length and depth of this discussion, we’ve needed to break this into two parts. In Part II of this series, we’ll discuss other probiotics and related types of products, their efficacy and cost-effectiveness.

References omitted but are available upon request by sending an email to email an editor.