The objective of fermentation is to produce energy as ATP for the bacteria to use for synthesis and their own growth. Volatile Fatty Acids (VFA) produced as a waste product of this metabolism are absorbed through the rumen wall and utilized by the animal as an energy source.
Aerobic metabolism (oxidation) yields carbon dioxide, water, and energy (ATP). Anaerobic metabolism (fermentation) yields VFA, carbon dioxide, and energy (ATP) for growth of the fermenting organism.
Basically, this is a three phase operation:
- Phase 1 Complex carbohydrates are degraded to simple sugars by microbial extracellular enzymes. These are simple molecules, the components of the original polymers. (This is extracellular)
- Phase 2 Simple sugars are absorbed by the bacteria and converted to pyruvate, a three carbon compound. (This is intracellular).*
- Phase 3 Pyruvate is converted to the final products which are the VFA. This happens intracellularly and the VFA are then excreted (released) by the bacteria into the rumen fluid for absorption by the host animal.
The intracellular intermediates are rarely found in rumen fluid.
Phase One: A wide variety of complex carbohydrates are present in the ruminant diet and are available for fermentation by the ruminal microbial population of bacteria, protozoa, and fungi. The first step in each case is to degrade the more complex molecule to its basic components.
Phase Two: These simple sugars are rarely detectable in the rumen because they are rapidly taken up by the microorganisms and further metabolized intracellularly through the Embden-Meyerhof pathway to ultimately form pyruvate.
Phase Three: The final phase is the conversion of pyruvate to the major end-products of fermentation (acetic, propionic, and butyric acid, methane) by a variety of pathways including intermediates which are not shown in the diagram. Other acids such as iso-butyric, valeric, and iso-valeric are produced by deamination of amino acids.
Individual Conversions: Fructans are degraded to fructose by the action of enzymes, which cleave the 2,6, and 2,1 linkages of the fructose polymer.
Sucrose is degraded to its constituent monosaccharides, glucose and fructose by the action of sucrase.
Starch is degraded to glucose via maltose, iso-maltose, and dextrins, by the actions of bacteria alpha amylase, maltase and iso-maltase.
Cellulose is degraded to cellobiose by cellulase enzymes (beta-1,4 glucosidases) and then to glucose and glucose-1-phosphate by cellobiase and phosphorylase enzymes.
Pectins, hemicelluloses, and pentosans are degraded to pentoses by various beta-1,4 hydrolyzing enzymes including xylanases and arabinases. These then enter the glycolitic pathway as fructose-6-phosphate or fructose-1,6-diphosphate.
Different bacteria require or utilize different substrates and produce different products. Therefore, the products of fermentation will vary with the relative composition of the rumen microflora – which bugs are present and in what numbers. The population also depends on the diet since this changes the substrates for fermentation and subsequently the products of fermentation. This then affects the environment in which the microflora live. Consequently, there is never a true steady state fermentation as it is constantly changing in response to these various stimuli.
The rumen microbial population is composed of a large number of species and these species differ in the substrates they ferment. Therefore, the diet you feed a ruminant will directly affect how well each population of microbes is able to grow and reproduce. These microbial species are also different in the products they release into the rumen environment. So, if one species is growing well, there will be a build up of its product within the rumen environment. Some microbial species actually produce waste products that serve as substrates for other species. Some microbial populations produce waste products that will alter the rumen environment in such a way that other species would die off. Consequently, there is never a true, steady state fermentation because conditions within the ecosystem of the rumen are constantly changing in response to various stimuli.
Fermentation pattern – the relative proportion of the volatile fatty acids.
[Ac] + [Pr] + [Bu] + [iBu] + [Val] + [iVal] = [TVFA]
The concentration of each acid is summed to get the total VFA concentration (moles/L or mmoles/L).
Then the individual acid concentration as a percent of the total is the molar proportion or molar percentage of each acid.
( [Ac] X 100 ) / [TVFA] = Acetate ( M% )
These proportions are the fermentation pattern and can be affected by many factors, particularly by the diet and the rumen environment.
Factors Affecting Fermentation Pattern
- Diet composition – F:C ratio
- Physical form of diet – esp. forage. (Ground and pelleted forage is more like a concentrate).
- Intake – restricted or ad lib – this changes the microbial population due to changing the pH.
- Species of animal – sheep vs. cattle vs. wildlife
The rumen environment includes, not just pH, but also factors such as the buffering capacity, the ionic strength of the rumen fluid, and the concentration of rumen ammonia.
The environment changes the microflora because some species can proliferate and some are inhibited by different conditions. For instance, low pH inhibits the cellulolytic bacteria and the protozoa, thus cellulolysis (fiber digestion) is inhibited by low pH.
Amylolytic organisms are not inhibited by low pH so they proliferate and drive the pH even lower.
- The effects of adding concentrate to a forage diet will vary with:
- Type of forage – alfalfa vs. grass, long vs. ground and pelleted (G and P)
- Type of concentrate – corn vs barley; starch degradability
- Processing of concentrate – whole corn, ground, steam flaked since processing increases starch degradability
- Percentage of concentrated added
- Total intake level – ad libitum, restricted