Direct-fed microbials (DFMs) are defined as products that contain live (viable), naturally occurring micro-organisms such as bacteria and yeast. The term “probiotic” is commonly used to describe DFMs.

DFMs have been used in dairy rations for many years but, recently, there has been renewed interest in their use due to specific restrictions in the use of certain antibiotics. Most dairy farms repeatedly face challenges such as variable weather, feed conditions, environmental pathogens, etc., and the use of DFMs can be a viable strategy to overcome some of those challenges.

Bacterial DFMs

Bacterial DFMs are probably most associated with the term probiotic. By definition from the World Health Organization, and the Food and Agricultural Organization of the United Nations, probiotics are live micro-organisms which, when consumed in adequate amounts, confer a health effect on the host.

For a strain of bacteria to be considered as a probiotic, it must:

  • Maintain its physiological characteristics and remain stable during the manufacturing and delivery processes

  • Survive the physiological and metabolic conditions in encounters along the entire gastrointestinal (GI) tract (i.e., stomach acid, intestinal pH, bile salts, digestive enzymes, etc.)

  • Prove to be beneficial to the host

Many different genera of bacteria are used as probiotics and include; Bifidobacterium, Enterococcus, Bacillus, Lactobacillus and Megasphaera, as well as others.


Selecting for specific species and strains of these bacteria becomes important, as some strains within a species are very effective probiotics while others may not show the same beneficial effect. For example, within the Lactobacillus species, Lactobacillus  animalis LA51 has shown very effective probiotic properties in several dairy and beef studies.

Bacterial DFMs generally work in the intestine of the animal and have the overall effect of mitigating the pathogenic bacteria that populate the digestive tract. Bacterial DFMs can also have positive effects in the rumen as well. In the lower gut, there are several modes of action that bacterial DFMs use to reduce the pathogen load, and these can include:

  • Competition for nutrients
  • Direct antagonism
  • Competitive exclusion on the intestinal surface
  • Barrier function by increased mucus production
  • Immune stimulation

Once consumed by the host animal, bacterial DFMs populate the digestive tract and then change the environment to benefit themselves and the host animal, usually by lowering local pH conditions. Pathogenic bacteria tend not to like environments with lower pH, so the effect of the bacterial DFMs results in a reduction in pathogenic bacteria and improved GI function.

Because of this, animals absorb a higher proportion of nutrients from feed and expend less energy repairing the GI tract. More resources are available for productive purposes of the animal, which results in improved performance in the form of improved milk production, growth, reproduction, milk production efficiency, etc.

Yeast and yeast products

Yeasts are considered to be DFMs only if they are alive, whereas non-viable yeast products (i.e., yeast extract and yeast culture) are classified as prebiotics.

While less strictly defined, prebiotics are nondigestible substances that act as food for the gut microbiota, stimulating growth or activity of certain intestinal bacteria. A distinguishing factor is: Only active live yeast will have a guaranteed live cell count.

According to the Association of American Feed Control Officials (AAFCO), yeast products are defined as follows:

  • Active dry yeast (AAFCO 96.2): Yeast which has been dried in such a manner as to preserve a large portion of its fermenting power. It must contain no added cereal or filler and must contain not less than 15 billion live yeast cells per gram.

  • Torula dried yeast (or Candida dried yeast; AAFCO 96.7): Dried, non-fermentative yeast of the botanical classification (torulopsis) Candida  utilis which has been separated from the medium in which propagated.

  • Yeast culture (AAFCO 96.8): Dried product composed of yeast (Saccharomyces cerevisiae or Kluyveromyces marxianus) and the media on which it was grown.

  • Yeast extract (AAFCO 96.11): Dried product composed of yeast (Saccharomyces cerevisiae or Kluyveromyces marxianus) and the media on which it was grown.

  • Hydrolyzed yeast (AAFCO 96.12): Concentrated, non-extracted, partially soluble yeast digest accomplished by enzymatic hydrolysis of whole Saccharomyces cerevisiae cells.

The mode of action of yeast has been pretty well defined. Live yeast help to modify microbial activities in the rumen while also having an effect in the lower GI tract. Live yeast reduces the oxygen load in the rumen, which is protective of beneficial rumen bacteria.

In addition, live yeast will compete for sugars in the rumen that would normally be utilized by acid-producing bacteria in the rumen such as S. bovis. These actions result in an increased level of cellulytic bacteria and lactic acid-utilizing bacteria.

The reduction in lactic acid-producing bacteria in the rumen increases fiber digestion and helps to prevent sub-acute ruminal acidosis. This can lead to increased production because of the healthier rumen environment and additional nutrient availabilities.

Other yeast products, such as yeast culture and extract, may also have beneficial effects when consumed by the dairy cow. Some of these actions include increased microbial protein synthesis and fiber digestion. Studies with yeast culture products have sometimes resulted in increased dry matter intakes and milk production.

All yeast products that contain yeast cell wall material will have a prebiotic effect on the host animal by providing oligosaccharides and beta-glucans.

Oligosaccharides are carbohydrates that can be found in yeast cell wall material and act to bind pathogenic bacteria in the GI tract, causing them to be flushed out of the system.

Mannan oligosaccharide is probably the most recognized of this class of material, but others can include galacto-oligosaccharides and fructo-oligosaccharides. Beta-glucans are thought to directly stimulate immune activity in the host animal.

DFMs (probiotics) have a direct effect on the intestinal environment of the host and will compete against pathogenic bacteria. Yeast products have both a rumen and a GI tract effect.

Prebiotics provide substrates for the growth of beneficial bacteria and may also have an indirect effect on pathogenic bacteria and immune stimulation. Bacterial DFMs work complementarily with yeast and yeast products and can be a good strategy to help protect your dairy cow from the daily challenges seen on the dairy farm.  end mark

Karl E. Nestor Jr. is a technical services manager with Chr. Hansen