Rumen microbiology

Bacteria, protozoa, and fungi exist together in the cow’s rumen. Bacteria make up about half of the living organisms but do more than half of the rumen’s digestive work. Rumen bacteria are classified into fiber digesters, starch and sugar digesters, lactate using bacteria, and hydrogen-using bacteria. They cooperate together and cross feed.


Bacteria make up about half of the living organisms inside of the rumen. However, they do more than half of the work in the rumen. The bacteria work together. Some breakdown certain carbohydrates and proteins which are then used by others. Some require certain growth factors, such as B-vitamins, which are made by others. Some bacteria help to clean up the rumen of others’ end products, such as hydrogen ions, which could otherwise accumulate and become toxic to other organisms. This is called “cross-feeding”.

Classification of Rumen Bacteria

Fiber–Digesting (or Cellulolytic) Bacteria:

The fiber-digesters are some of the “fussiest” bacteria in the rumen. They are very sensitive to acid. When a cow has acidosis (pH<6.0), the rumen produces a lower proportion of acetate to propionate because the fiber-digesters who primarily make acetate are not working well. Also, high levels of rumen available fat (generally over 5% of the diet) reduce the growth of the fiber-digesters. The exact reason for fat’s negative effect on the fiber-digesters is not known. Some think that it reduces the microbe’s ability to move nutrients into and out of its body. Others think that the fat coats fiber particles making it difficult for the fiber-digesting microbes to get in to do their work.

Bacterial species: Ruminococcus flavefacians, Ruminococcus albus, Bacteriodes succinogenes, Butyrivibrio fibrisolvens

Growth Requirements: Cellulose, Hemicellulose, Pectin

Many Also Require: Ammonia, Isoacids (Branched-Chain Amino Acids), Starch, B-vitamins - Fermentation Products: Acetate, Butyrate, Hydrogen (H2), Carbon Dioxide (CO2)

Rumen pH Requirement: High pH (above 6.0)

Fat Tolerance: Low

Susceptibility to Ionophores (Bovatec and Rumensin): Some are susceptible

Reproduction Speed: Slow

Starch and Sugar-Digesting (or Amylolytic) Bacteria:

Starch and sugar-digesters make up a significant part of the rumen’s bacterial population. Generally, high-producing dairy cows are fed diets containing more than 30% starches and sugars, so these bacteria are greatly needed. Even if a cow is on an all-straw diet, the fiber-digesters still never account for more than 25% of the rumen bacterial population. Starch and sugar-digesters are still present, cross-feeding off of the fiber-digesters’ byproducts.

Bacterial species:

Bacteriodes ruminocola, Bacteriodes amylophilus, Selenomonas ruminantium, Streptococcus bovis, Succinomonas amylolytica
Growth Requirements:

Sugar, Starch, Peptides, Amino Acids
Many Also Require: Ammonia, B-vitamins
Fermentation Products:

Propionate, Butyrate, Acetate, Lactate, Hydrogen (H2), Carbon Dioxide (CO2)
Rumen pH Requirement: Tolerate a lower (more acidic) pH (5.7)
Fat Tolerance:

Higher than fiber digesters
Susceptibility to Ionophors (Bovatec and Rumensin):

Most aren’t susceptible
Reproduction Speed:

Faster than fiber digesters

Streptococcus bovis, “The Rumen Weed”

Streptococcus bovisis present only when large amounts of starch or sugars are fed and pH is low. It produces lactic acid, a stronger acid than many of the other VFA’s produced in the rumen. When conditions are favorable for Streptococcus bovis, it will grow explosively (doubling every 13 minutes). This type of growth produces rumen acidosis. Streptococcus bovis is controlled by ionophores. This is one of the major reasons for the favorable growth responses seen by the addition of ionophores to the diets of feedlot cattle.

Lactate-Using Bacteria:

Includes: Megasphaera elsdenii, “The Rumen Maid”
As mentioned above, some bacteria, such as Streptococcus bovis, produce a strong acid called lactic acid. Megasphaera elsdenii uses lactic acid to grow. This helps to clean up the rumen a bit and raise rumen pH, aiding the growth of the acid-intolerant fiber-digesters.

Hydrogen-Using (or Methane) Bacteria:

Under normal rumen conditions, hydrogen (H2) does not accumulate in the rumen because it’s used by hydrogen-using bacteria, such as Methanobacterium ruminantium.

Growth Requirements: Carbon dioxide and hydrogen
Fermentation products: Methane
The methane bacteria commonly produce methane in this way:

4H2 + CO2 ---------> CH4 + 2H2O


As much as 50% of the microbial mass in the rumen can be made up of protozoa. However, their role, as compared to the rumen bacteria, is not as significant. The protozoa are actually predators to the bacteria in the rumen --- they eat the bacteria for dinner! Protozoa are about 40 times the size of rumen bacteria.

The rumen protozoa produce fermentation end-products similar those made by the bacteria, particularly acetate, butyrate, and hydrogen. Rumen methane bacteria actually attach and live on the surface of rumen protozoa for immediate access to hydrogen.

Rumen protozoa eat large amounts of starch at one time and can store it in their bodies. This may help to slow down the production of acids that lower rumen pH, benefiting the rumen.

Rumen protozoa multiply very slowly in the rumen --- over 15-24 hours – as opposed to the bacteria that may take as little as 13 minutes to multiply. For this reason, the rumen protozoa hide out in the slower moving fiber mat of the rumen so that they aren’t washed out before they have a chance to multiply. Low roughage diets reduce the retention of fiber in the rumen and may decrease the number of protozoa in a cow’s rumen.

Rumen Fungi:

Fungi are known to exist in the rumen (up to 8% of the total mass) but they are poorly understood. They attach to feed particles and they reproduce very slowly. They may help out the fiber-digesting bacteria by doing some of the initial work of splitting fibrous material apart and making it more accessible for the bacteria. Higher numbers of fungi have been found in the rumens of cows fed very poorly digestible sub-tropical forages.


Hungate, R.E. 1966. The rumen and its microbes. Academic Press, NY.

Russell, J.B. 1988. Microbiology of the Rumen. Animal Science 607 Class Notes, Cornell University.

Van Soest, P.J. 1982. Nutritional ecology of the ruminant. O&B Books, Inc., Corvallis, OR.


Mary Beth de Ondarza

Mary Beth de Ondarza
45 articles

Nutritional consultant for the dairy feed industry at Paradox Nutrition, LLC.

Look to Paradox Nutrition, LLC for providing:

  • Controlled research studies on commercial dairy farms
  • Computer modeling with complete nutritional assessment and statistical analysis
  • Complete literature review of a nutrition topic
  • Nutrition writing for company technical bulletins or popular press
  • Dairy nutrition seminars with new, practical, and understandable information for consultants and dairy producers
  • On-farm nutritional consultation

Dr. de Ondarza received her Ph. D. from Michigan State University and her Masters Degree from Cornell University, both in the field of Dairy Nutrition.

Read more »

Paradox Nutrition

Paradox Nutrition

Paradox Nutrition, LLC is a nutritional consultation business for the dairy feed industry. Mary Beth de Ondarza, Ph.D. is the sole proprietor.

Read more »