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
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.
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