Nutrient Requirements: Cow and Rumen
Required Nutrients
All living
organisms require essential nutrients to support metabolic processes to keep
them alive. General classification of
required nutrients include: water, the
most essential, energy, protein, minerals, and vitamins. Minerals can be further subdivided into
macrominerals, microminerals based on the daily amounts (gm or mg)
required. Vitamins are separated into
fat or water soluble sources. Daily
requirements for these essential nutrients are a function of the cow’s body
weight and physiologic state (e.g., maintenance, growth, lactation, pregnancy)
as modified by environmental conditions.
Bacteria have similar requirements for maintenance and growth (i.e.,
reproduction).
Differences
between the cow and microbes are seen in where they derive their nutrients
(Table 4). As a consequence of a
pregastric fermentation system, consumed feeds are exposed to microbial
fermentation before being available for digestion and absorption by the host. In many instances, this process enhances
nutrient availability to the host animal.
However, an opposite effect can also occur. Exposure to microbial fermentation can result
in nutrient alteration to a form that is either no longer absorbed or has lost
its biologic activity.
As an
anaerobic fermentation system, the rumen is a highly reduced environment with
an abundance of reducing equivalents looking for a place to go. Highly oxidized compounds are prime
targets. Absorption of selenium (Se)
occurs only in its highly oxidized forms as selenite (+4) or selenate (+6) via
an intestinal sulfate transport system.
Selenium absorption from selenite, is more efficient in nonruminants
than ruminants with a total retention of 77% and 29%, respectively (7). Microbial reduction of Se to its elemental
(+0) or selenide (-2) forms renders it unavailable thus reducing Se absorption
in ruminants. High concentrate diets are
associated with increases in insoluble Se complexes, possibly due to changes in
rumen pH, redox potential, microbial populations or some combination.
Table 4. Substances which supply essential nutrient needs for the
cow and rumen microbial population.
NUTRIENT
|
COW
|
BACTERIA
|
ENERGY
|
VFA's
Glucose
|
Complex Carbohydrates
Sugars, Starches, Amino Acids
|
PROTEIN
|
Amino Acids
Microbial Protein
|
Ammonia, Amino Acids,
Peptides
|
MINERALS
|
Dietary
|
Dietary
|
VITAMINS
|
Dietary
Bacterial
|
Dietary
Synthesized
|
Rumen
interactions can also result in a reduced availability of other minerals,
namely copper (Cu) and magnesium (Mg).
Best studied relative to copper availability are the interaction of
molybdenum (Mo) and sulfate (SO4) in the rumen. High Mo and SO4 concentrations in
the rumen allow microbial synthesis of thiomolybdates, which can chelate copper
making it unavailable for absorption or utilization (8). Dietary Mg absorption efficiency is low and
occurs in the rumen by a sodium (Na)-potassium (K) ATPase system. Many dietary factors, including excess K,
nitrogen, Ca, sulfur and organic acids as well as deficient Mg and phosphorus
levels, can predispose an animal to a form of hypomagnesemic tetany (9). Another factor, a direct consequence of rumen
microbial activity, also plays an important role in the pathogenesis of this
syndrome. Grasses typically contain
significant amounts (>1% of plant dry matter) of sucrose and
trans-aconitate, an organic acid, during periods of rapid growth. In the rumen trans-aconitate is metabolized
to acetate or reduced to tricarballyic acid (10). One of the most abundant rumen bacteria, Selenomonas ruminantium, is primarily responsible
for the reduction of trans-aconitate.
This organism also prefers to grow when sucrose is readily available as
substrate. Tricarballylic acid is a very
potent chelator of calcium (Ca), Mg and Zn inducing increased urinary excretion
and reduced tissue status of these minerals.
Making more bugs - Microbial Protein
The cow
derives a majority of her energy and protein from microbial end products. In other words, the more we make the bugs
grow (reproduce), the less additional, more expensive feedstuffs we need to
provide in the cow’s diet. Why is
production of microbes so important to a ruminant feeding program? Microbes contain approximately 62% crude
protein, which is 80% true protein and 80% digestible. This is considered high quality protein. Microbial protein production alone can
support up to 25 kg of milk production (Table 5). The first goal of a ruminant feeding program
should be to maximize microbial protein production and then secondly, meet
additional cow’s nutrient requirements over-and-above those not met by
microbial fermentation end products.
This type of feeding approach would theoretically be the most economical
and efficient. So how do we get the rumen
microbes to abundantly grow without disturbing the rumen ecosystem?
Table 5. Microbial protein synthesis relative to daily protein needs
of the cow.
Efficiency of Microbial
Protein Synthesis
|
Daily Milk Yield
|
||
25 kg
|
35 kg
|
45 kg
|
|
gm
N/kg OM digested
|
%
of protein from microbes
|
||
20
|
49
|
42
|
39
|
30
|
73
|
64
|
59
|
40
|
98
|
85
|
79
|
Data from
M. Stern, in Dairy Herd Management, pg. 28, April 1997
Bacteria
require a number of essential nutrients for the synthesis of protein, similar
to that of the cow. However unlike the
cow, bacteria can use a greater variety of potential nitrogen sources to
synthesize amino acids, the building blocks of proteins. In addition, bacteria can synthesize both
essential and nonessential amino acids unlike the cow which needs to be
supplied with preformed essential amino acids.
Figure 1 presents an overview of the processes required to synthesize
microbial protein.
As can be
seen from Figure 1, microbial protein production is a function of rumen
available substrates, primarily carbohydrates and nitrogen (see reviews 11,
12). If any of the required building blocks
are in limited supply, microbial protein production will be determined by the
availability of the most limiting substrate.
Usually this is energy from carbohydrate fermentation. Energy production
(generation of ATP) will be dependent upon the available carbohydrate source
and its rate of degradation. Ammonia (NH3) may be provided from
nonprotein nitrogen sources, amino acids, peptides, or proteins where
utilization of a nitrogen source is dependent upon the specific population of
bacteria. For example, cellulolytic
bacteria can only use NH3 as their nitrogen source.
Microbial protein production is more
complex than just providing the necessary amounts of substrate in the
diet. The rumen is a dynamic system that
constantly has fermentation end products, liquid, bacteria, and particles being
removed via digestion and passage through the rumen as well as new substrate
added. So not only do we need to address
concepts of total substrate requirements, availability of substrate relative to
other substrates needs to be addressed.
We must be able to
predict
rate and extent of carbohydrate and protein degradation takes place in the
rumen. This is the critical component of
a dynamic modeling system for the rumen and requires more comprehensive and
complex feed analysis procedures
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