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Ruminant Feeds Microbes, Microbes Feed Ruminant
A symbiotic relationship enables ruminants to utilize fiber and NPN


Ruminants have a unique ability to convert feedstuffs that are of low value to monogastrics or unfit for human consumption into high-quality food, fiber, and numerous other products that are beneficial to man. "Microbial fermentation" which takes place in the rumen and reticulum of the ruminant?s stomach makes it possible to extract nutrients from feedstuffs that have little nutritional value to monogastrics. The microbial population is comprised of microorganisms, such as bacteria, protozoa, and fungi. Fermentation is generally thought of as the process used to make various forms of alcohol. Technically, fermentation is the "enzymatic transformation of organic substances." In the rumen, as well as with alcohol production, microbes produce enzymes that cause fermentation.

The rumen provides an excellent environment for microbial growth and fermentation. The rumen is anaerobic (without oxygen) with stable temperature and osmotic conditions. Consumption of feedstuffs and water provide a steady nutrient supply. Waste products (undigested feed material, dead microbes, fermentation end products, and microbial wastes) are removed from the rumen by passage down the digestive tract or by absorption through the rumen wall. In return for the ideal environment, the microbial population supplies ruminants with nutrients derived from otherwise non-usable feedstuffs. Ruminants along with other mammals (including man) do not possess the enzymes necessary to digest fiber or to efficiently convert non-protein nitrogen (NPN) into protein. The single largest contribution made by rumen microbes is their ability to ferment fibrous materials. This fermentation provides energy, protein, and B vitamins to the animal. In a real sense the ruminant eats to feed the microbes, and the microbes then feed the ruminant. It is a classical symbiotic relationship.

Three major factors, which are interdependent, influence the amount and type of fermentation that occurs in the rumen:

  • Type of feedstuff entering the rumen

  • Microbial population

  • Retention time in the rumen

                  Rumen Microorganisms                             Four Compartment stomach of a ruminant

Type of Feedstuff
Ruminants are fed a variety of feedstuffs ranging from very poor-quality forages, such as cereal grain straw, to almost 100% grain diets, similar to a non-ruminant diet. The types of feedstuffs consumed by the ruminant influence the microbial population and feedstuff retention time in the rumen. Nutritional balance (energy, protein, minerals, vitamins) of the diet consumed by the ruminant will influence the microbe's ability to digest feedstuffs and overall ability of the microbe to live and grow. Almost all feedstuffs lack certain nutrients essential for optimum microbial growth. Proper supplementation of the base diet benefits the microbes, which enables more nutrients to be derived from feedstuffs resulting in improved ruminant performance.

Microbial Populations
The type of microbes which are most efficient at deriving nutrients from feedstuffs will be the dominant population in the rumen. For example, a forage-based diet will encourage a forage-digesting microbial population to dominate the rumen. Changing diets will cause a shift in the microbial population from one set of species to another. The change in microbial population is one reason why diets should be changed gradually. Rapid changes in type or quantity of diets can result in rapid changes in the microbial populations, which in turn can cause digestive upsets, such as acidosis or bloat.

Even though the rumen macro-environment is ideal for microbial growth, it is a hostile environment with fierce competition among individual microbes and species of microbes for resources. Microbes have developed interesting competitive strategies. Some microbes attach to feed particles and horde nutrients released by their enzymes. Some microbes attach to the rumen wall and benefit from nutrients supplied by the host. Others work to create an environment where they can prosper and other species cannot. The most economically important microbial battle is between the "starch digesters" and the "fiber digesters." These terms describe two large groups of microbial species that prosper on different diets. The starch digesters work to create a lower pH environment because they can tolerate lower pH than fiber digesters. Acute acidosis can result when acid production by starch-digesting microbes quickly rises during a diet change from a predominantly forage-based diet to a diet providing higher levels of starches or other easily degradable components.

The types of microbes in the rumen are extremely large and varied. In a single drop of rumen fluid there may be millions of microbes and hundreds of species. The number of microbes and the relative proportion of species define different microbial populations. Together, the microbial species function as a community with different species filling different niches. The niches are often defined by the substances utilized by a species and the end products produced. For example, earlier it was mentioned that fermentation is often used to produce alcohol. Some microbial species digest components of feedstuffs and produce alcohol as an end product. However, other species use alcohol as food to live and grow, and they produce other end products. This cascade of one species using the end products of other species dead-ends when volatile fatty acids are produced. The largest quantity end products of microbial fermentation are the volatile fatty acids
(VFAs -acetic acid, propionic acid, and butyric acid) along with methane gas. It is from these VFAs that ruminants derive the majority of their energy. Because ruminant metabolism is aerobic (with oxygen) they are able to further metabolize the VFAs into usable energy. Carbon dioxide and water are end products of the ruminant's use of VFAs.


Microbial fermentation has a large impact on the protein nutrition of the ruminant. Protein in feedstuffs fed to ruminants is usually described as ruminally degradable (or available) or undegradable (or bypass or escape). The undegradable portion passes through the rumen and into the lower tract "as is" where it is digested and used by the ruminant. The quality of the protein (amino acids profile) is important in these feeds because they directly impact the ruminant. A portion of the protein component (rumen degradable) of feedstuffs is broken down by microbes and incorporated into their own bodies. Microbes break down protein into amino acids, which are then split apart into ammonia and "carbon skeletons." Carbon skeletons are often utilized as energy, while nitrogen from ammonia is used by microbes to form amino acids and proteins for its own growth and reproduction.

Microbes can effectively utilize non-protein nitrogen sources (such as biuret and urea, which are ammonia sources) because they have the enzymes and other metabolic machinery to build amino acids and proteins from ammonia. The process of degrading and rebuilding proteins has positive and negative consequences for the ruminant. On the negative side, not all of the ammonia released from feedstuffs is captured by microbes. Ammonia can escape through the rumen wall into the blood stream, and eventually pass into the urine and voided. The amount of ammonia lost can be minimized by proper ration balancing (i.e. formulating rations based on microbe and ruminant needs, which will minimize the amount of excess ammonia). Another negative aspect is the potential for ammonia poisoning, which can occur when too much ammonia passes from the rumen into the blood stream. The potential for ammonia poisoning can be avoided with proper feed management and diet formulation. On the positive side, protein built by microbes for their own use is relatively high quality and provides solid protein nutrition to the ruminant when microbes pass into the small intestine. The added perk is that this high-quality microbial protein can be built from NPN sources. The ability of ruminants to effectively utilize NPN enables more cost-effective protein supplementation.

Retention Time
Retention time in the rumen also influences fermentation and nutrients available to the ruminant. During digestive upsets, such as diarrhea, feedstuffs move through the rumen rapidly and the microbes do not have adequate time to do their digestive work. Available nutrients pass through the entire digestive tract and end up on the ground. Low-quality roughages pass through the rumen very slowly and can remain after all the available nutrients have been released. Processing low-quality forages increases the rate of passage and allows more available nutrients to pass through the rumen. Feedstuffs with medium to slow degradation rates can have different ruminal availabilities depending upon retention time. For example, digestion of feedstuffs with medium rates of digestibility, such as oil meals, is high when fed with roughage diets which have slow rates of passage, but much lower when fed with grain diets because the ruminal retention time is different.

The approach to feeding ruminants is quite different than feeding non-ruminants because of rumen microbial fermentation. The benefits of microbial fermentation include the ability to utilize fiber (forages) and NPN sources. The negative aspects include some nutrient loss and digestive upsets that occur when ruminants are mismanaged. In ruminants and non-ruminants, optimum production occurs when properly balanced diets are supplied to the animal. Supplementation of base diets is essential to maximize microbial fermentation and to obtain the most nutrients from feedstuffs and greatest production from the ruminant.




ADM Alliance Nutrition, Inc. , a wholly owned subsidiary of the Archer Daniels Midland Company