Understanding the Rumen

            Ruminant herbivores, including cattle, sheep, goats, deer and many others, are unique animals in their ability to derive nutrients from forages and low quality roughages.  This ability has nothing to do with the animal’s digestive enzymes, but totally dependent upon the symbiotic relationship between host animal and microbial populations residing in the pregastric fermentation system called the rumen.  What separates ruminant herbivores from nonruminant herbivores is their ability to chew their cud.  The ability to regurgitate swallowed feed material for remastication provides the rumen microbes greater surface area to feed materials thus allowing greater extent of degradation.  Ruminant animals are the most efficient fiber digesting herbivores. 

            If the rumen system is functioning properly, it can provide a large proportion of the needed nutrients to support productive activities of the host animal.  The rumen microbial population converts consumed dietary substances into highly available microbial protein and volatile fatty acids (VFA) that can be used by the host animal for protein synthesis and energy needs, respectively.  In addition to the rumen microbial end products, a proportion of dietary protein, fat, and starch may escape microbial degradation and be directly available for digestion by the host animal.  Dietary nutrients that escape rumen degradation are commonly termed bypass protein, fat, or starch.  It is the combination of rumen degradation products and dietary bypass nutrients that support all body functions of the host animal.  The feeding program becomes most efficient when microbial products from rumen degradation can account for a greater proportion of host animal needs and minimize the need for additional dietary bypass nutrients.  To accomplish this, one needs to fully understand the interactions of dietary substrates with rumen environment and impact on fermentation end product production.  Within the last decade much research has focused on developing models that can accurately predict rumen output based on dietary inputs.  To this end, the Cornell Net Carbohydrate and Protein System (CNCPS) has provided great insight as to the inner workings of the rumen and thereby increased the potential for improved and more efficient dietary formulation.

Rumen Development

            The above described rumen fermentation system is a marvelous system, however, one needs to recognize that ruminant animals are not born with a functioning rumen.  The rumen system needs to develop anatomically and physiologically before it is capable of accomplishing extensive fermentation of feeds.  Both anatomic size and muscular motility as well as physiologic development are dependent upon the type of diet fed the young preruminant calf.  Relative size changes shown in Table 1 depict gradual increasing rumen and reduction in abomasal sizes.  These changes are based on optimum feeding programs. 

Table 1.          Relative changes in anatomic size of the four compartments of the rumen system associated with animal age.

Compartment	Newborn	2-3 Months	Adult
Reticulum	5 %	5 %	5 %
Rumen	25 %	65 %	80 %
Omasum	10 %	10 %	7-8 %
Abomasum	60 %	20 %	7-8 %

            In addition to anatomic changes, fermentation ability of the rumen needs to be developed.  First, microbial organisms must be inoculated into the rumen and then the absorptive surface must anatomically grow and develop metabolic activity.  Rumen microbial inoculation occurs very quickly with the calf’s exposure to feeds, environment and other animals.  This is usually never a problem under normal situations.  The second component of rumen physiologic development is papillae growth and metabolic activity.  Historically, it has been believed that consumption of forage by the calf would initiate rumen papillae development.  Research work from the 1960's as well as most recently has definitively shown that the end products of starch fermentation, namely butyrate and propionate, are the mediators of rumen papillae development (1,2,3).  A number of studies have shown how rumen papillae development can be suppressed by continued milk feeding without sufficient calf starter intake. What this means is the feeding of calf starter is critical to the rapid development of proper rumen function and starter consumption by the calf should be encouraged.  Contrary to popular belief, feeding of forages will not promote rumen development, but will stimulate rumen muscular activity.  Fermentation of forages generates predominately acetate, which has little stimulatory activity on papillae development.  For calves to make a smooth transition from the milk feeding phase through weaning, rumen development should be well initiated, which can be accomplished when calves are eating 700 to 900 g of calf starter per day for at least 3 consecutive days prior to weaning.

Applied Rumen Anatomy

            The rumen is actually only one chamber of a complex, pregastric fermentation system.  This is in contrast to the postgastric fermentation system found in horses and many other nonruminant herbivores.  The reticulum is a smaller fermentation compartment, anterior and intimately associated with the ruminal compartment.  The reticulum is primarily responsible for assisting in rumination contractions and distributing feed within the reticulo-rumen.  You may be more familiar with this compartment from its association with "hardware disease".  Heavy objects that are swallowed by the cow will drop into the reticulum.  This compartment is located next to the heart, just the other side of the diaphragm.  Sharp objects may protrude through the reticulum wall and puncture the heart or liver, thus causing "hardware disease".

            The rumen is the primary fermentation vat, being between 80 and 100 liters in volume in a mature cow.  Muscular contractions aid in the constant mixing of feed materials with bacteria laden fluids to promote fermentation and in the regurgitation of feed materials, which results in particle size reduction from chewing and stimulates copious production of saliva.  Salivary bicarbonate ion is primarily responsible for maintaining only a slightly acid pH in the rumen, given the tremendous amount of acids being produced during fermentation (Table 2).  Also as a result of the continuous fermentation process, rumen temperature is slightly greater than the cow's and can contribute to helping maintain normal body temperature during cold weather or making the cow more uncomfortable during hot weather.  The rumen has a specialized lining that contains many finger-like projections called papillae that absorb end products of fermentation, volatile fatty acids (VFA).  The cow uses VFAs for energy (acetate, propionate, butyrate), fat synthesis (acetate, propionate, butyrate) or glucose (propionate exclusively) production.  The rumen lining can be easily damaged by severe or prolonged declines in rumen pH, a result of excessive grain or insufficient fiber feeding.

                                    Table 2.    Characteristics of the rumen environment.

pH	6.7 - 7.2 optimum
Temperature	38 - 41o C
Bacteria	108 - 1010/ml fluid
Gas Phase	Anaerobic, CO2, CH4
Solid Phase	Fibrous Mat
Liquid Phase	Volatile Fatty Acids (VFA) Ammonia Minerals Soluble Protein
VFA's	Acetate Propionate Butyrate

            When the rumen is appropriately fed, it will contain a small gas cap, middle fibrous mat layer, and a lower liquid layer (Table 2).  The gas cap consists of carbon dioxide and methane, both end products of fermentation, which limits exposure of bacteria to oxygen.  These gases must be regularly belched out to relive pressure, otherwise a potentially life-threatening condition termed bloat may occur.  The fibrous mat layer is composed of long dietary 'effective' fiber, which will help stimulate rumination and ruminal contractions.  The tremendous number of bacteria found in the rumen are differentially distributed within the fibrous mat and liquid layers.  Beside the type of raw materials microorganisms require for metabolism, reproductive rate also determines where the organism will be found in the rumen.  Bacteria and protozoa that do not reproduce rapidly in relation to rate of passage through the rumen must attach to fibrous material if they are to remain in the rumen.  When 'effective' fiber is not adequately provided, these microorganisms will be wiped out of the rumen and will result in abnormal fermentations and potentially digestive upsets and 'off-feed' situations.

            The third ruminal chamber is the omasum, which is approximately the size of a basketball and located on the right side of the cow.  The omasum is responsible for regulating particle passage rate from the rumen and water absorption from ingesta.  Under normal rumen conditions, particles greater than 2 mm in size do not leave the rumen.  Very little other information is known about this organ.  When large fiber particles or whole corn kernels are found in the manure, this is a good indication of improper rumen function and should be further evaluated.

            The abomasum, or fourth rumen chamber, is similar to our own stomach.  Digestive enzymes and hydrochloric acid are secreted, which initiate breakdown of complex proteins and starches for further digestion in the small intestine.  You may be more familiar with this organ from the problem associated with its displacement in early lactation.  Left displaced abomasum (LDA) is a health disorder where the abomasum becomes atonic and distends with gas and passes under the rumen and becomes trapped on the left side of the cow.  This condition has been associated with high grain and low fiber diets in early lactation and may also be related to subclinical hypocalcemia conditions. 

Rumen Microbiology and Fermentation

            Over 120 different species of microorganisms have been identified in the rumen.  These organisms range from bacteria, the most abundant, to protozoa, fungi, and viruses.  Although there is a wide variety of bacteria found in the rumen, they can be loosely grouped into five major categories in addition to protozoa.  A basic understanding of the nutrient and environmental requirements of these different microbial groups is necessary to fully appreciate how feeding programs may impact rumen health.  Table 3 lists substrates, requirements, and end products for these different microbial groups.  One important concept to glean from this table is the observation that cellulolytic activity (i.e., fiber fermentation) occurs only at higher pH levels.

 

            A healthy rumen is one that has a balanced interaction between all groups of bacteria.  In abnormal rumen environments, usually one group of bacteria has overwhelmed all other groups and dominates fermentation activity.  For example, rumen acidosis is the result of feeding too much grain (sugars and starches), which allows starch digesters to overwhelm the rumen environment and eliminate cellulolytic activity.  This is the crux of the problem in dairy cattle feeding, providing

Table 3.           Characteristics of the different categories of microorganisms found in an anaerobic fermentation system.¹

Read Also

Class of Organism	Primary  Substrate	Specific Requirements	Primary Endproduct	pH Tolerance
Cellulolytic Bacteria (Fiber fermenting)	Cellulose Hemicellulose Pectins	Ammonia Iso-acids Cofactors	Acetate Succinate Formate, CO2	Neutral 6.2-6.8
General Purpose Bacteria	Cellulose Starch	Ammonia Amino Acids	Propionate Succinate Butyrate Ammonia	Acid 5.5-6.6
Nonstructural CHO Bacteria	Starch Sugars	Amino Acids Ammonia	Propionate Lactate Butyrate Ammonia	Acid 5.0-6.6
Secondary Feeders	Succinate Lactate Fermentation Endproducts	Amino Acids	Ammonia Iso-acids Propionate	Neutral 6.2-6.8
Protozoa	Sugars Starch Bacteria	Amino Acids	Acetate Propionate Ammonia	Neutral 6.2-6.8
Methanogens	CO2, H2 Formate	Coenzyme M Ammonia	Methane	Neutral 6.2-6.8

¹Adapted from Chase, L.E. and C.J. Sniffen, Cornell University.

sufficient grain to support milk production without excessive amounts that can suppress fiber fermentation, milk fat test, and rumen activity.

            A number of factors can influence rumen fermentation efficiency.  Most obvious is the role of diet on rumen pH and use of buffers to minimize the effect.  In some countries ionophores are used to manipulate rumen fermentation patterns.  Some recent research has shown that dietary mineral concentrations can influence rumen fermentation.  Dietary supplementation of zinc (Zn) to maintain a rumen Zn concentration of 7 ppm was shown to inhibit ureolysis and increase molar proportion of propionate (4).  High rumen Zn concentrations (14 ppm) reduced fiber digestibility.  Supplementing manganese (Mn) at 100 ppm in the rumen resulted in increased in vitro dry matter digestibility.  More research is needed to fully determine the role of dietary minerals in altering rumen fermentation capacity.

            Many rumen microbes are very sensitive to the presence of dietary polyunsaturated fats.  Rumen microbes will attempt to reduce the metabolic toxicity of polyunsaturated fats by saturating double bonds through a process of biohydrogenation.  Recent research has identified trans-10, cis-12 conjugated linoleic acid (CLA), a product of incomplete microbial biohydrogenation, to be associated with milk fat depression in dairy cows (5,6).  The presence of trans-10 CLA inhibits or reduces mammary gland denovo fatty acid synthesis.  The presence of large amounts of polyunsaturated fats in the rumen or small amounts with high grain feeding seem to promote the production of trans-10 CLA and produce milkfat depression syndrome.