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.

Read Also

            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