Rumen fermentation in ruminants

The unique feature of digestive physiology in the ruminants is the fermentative digestion  known as microbial digestion which occurs  in the rumen and reticulum. Microbial digestion of food also occurs in the three chambered ruminates, camel, lama and in the stomach of marsupials and in hippopotamus.

Anaerobic ciliate protozoa and non-spore forming anaerobic bacteria and anaerobic fungi are the major microbes in rumen.Some facultative bacteria are also present. The microbes have a volume of 3.6 % of strained rumen liquor of which 50% is contributed by cilitate protozoa and the remaining 50% by the bacteria.The number of rumen fungi is negligible but their activity is of great importance.

The bacteria and protozoa grow on the substrates of the food in the fore stomach of the ruminants.

Establishment of bacteria in young ruminants

Development of bacterial flora in young ruminants occurs at a very early age.

The nature and rate of development is affected by type of diet fed and degree of isolation of young from adult animals. Under normal conditions, bacteria of adult type establish at about 6^th week of age in young animals.

The ciliates may not become established in young animals unless they are maintained in close contact with animals harbouring them or are inoculated.

Rumen microbes digest all major carbohydrates of ruminant diet such as cellulose, hemicellulose, pectin and xylose etc. These group of carbohydrates are not digested by mammalian digestive enzymes. The fermentation of cellulose is a slower process and is incomplete in rumen, but the digestion of cellulose and hemi-cellulose is almost completed in the rumen.

Microbes derive their energy for their growth by fermenting cellulose, hemicellulose, pectin, soluble sugars, starch, the carbohydrates of major plant constituents .

Rumen microbes are responsible for the digestibility of about 70 to 85% dry matter of the diet in ruminants, results in the production of volatile fatty acids (Acetic, Butyric and propionic acids) gases like CO₂, methane (CH₄), ammonia (NH₃)with small amounts of N₂, H₂, and O₂ and microbial cells.

From carbohydrates, certain organic isoacids, NH₃ and minerals, the microorganisms can synthesise good quality microbial proteins and    B complex vitamins required for their growth and metabolic activities.

They also hydrolyses lipids, unsaturated fatty acids. Proteins and NPN substances of dietary and salivary sources get degraded by the microbes and the released amino acids, NH₃ are utilized for microbial growth and microbial protein synthesis. Thus the ruminant animal can be maintained on diets free of essential amino acids.

In animals maintained in green pasture, the number of bacteria is higher than those fed dry rations. When ciliate protozoa are absent, viable bacterial count increases. Rate and method of feeding also affects bacterial count.

The pH of the rumen liquid ranges from 5.8 to 7.0, which get decreased after feeding.

Rumen as microbial habitat

• Maintenance of constant temperature of 40⁰C
• Maintenance of constant pH of 6 – 7 by the HCO₃ & HPO₄ buffers of saliva
• Aqueous environment by continuous salivary secretion
• Continuous supply of substrates for microbial activity
• Mixing of the ruminal contents by rumino-reticular contractions
• Continuous removal of the fermentative end products.
• Abomasal secretions are dependent on volume of material flowing into it and pyloric distension. A rise in abomasal pH, short chain fatty acid level stimulate HCl and gastrin secretion. Parasympathetic nerves and gastrin also involved in abomasal secretion.
• Microbes derive their energy for their growth by fermenting cellulose, hemicellulose, pectin, soluble sugars, starch, the carbohydrates of major plant constituents .They result in the production of acetic, butyric propionic and lactic acids and gases like CO₂, methane and H₂.
• Abomasum receives a continuous flow of these materials and functions as a true stomach by secreting gastric juice. More of gastric secretion is from the fundic glands but the pyloric secretion is low. Gastric juice in the fundus region contain pepsin and HCl, has a pH close to 1.0. Pyloric secretions are slightly alkaline with little peptic activity.

Fermentation of carbohydrates

• Speed of  carbohydrate fermentation varies with their availability and solubility. 
• Soluble sugars are rapidly fermented but starch is less rapidly fermented.
• Cellulose and hemi-cellulose are slowly fermented.
• Starch is a glucose polymer with alpha 1, 4 glucose linkage.
• The fructosans are polymer of fructose units with beta linkage.
• In roughages, most of the  carbohydrates are structural and found in plant cell wall (cellulose, hemicellulose and pectin).
• Cellulose is beta 1,4 glucose linkage polysaccharide and hemicellulose is polysaccharide composed of beta linked xylose units and few hexoses.
• Pectin, found in cell wall and intracellular is beta linked galacturonan (polysaccharide based on galactose with uronic acid).
• The alpha linkage can be  readily hydrolysed  by amylases, whereas the beta linkage can be digested only by microbial enzymes, but not by mammalian enzymes. 
• Lignin, a phenolic compound, a constituent of plant cellwall is resistant even to microbial enzymatic digestion. Only a small portion of dietary lignin is digestible due to the action of rumen fungi.
• The lignin reduces the digestibility of cell wall  carbohydrates.
• Lignin and cellulose content increase with the age of the plant and temperature.  
• Fructosans are more in young grasses. 
• Xylose is the most abundant sugar in grasses.

End products of carbohydrate fermentation

Short Chain Fatty Acids (acetic, propionic and butyric acids).

• Isoacids like valeric, isovaleric, isobutyric and 2-methylbutyric acids.
• Gases produced are CO₂, CH₄ and H₂ .
• The CH₄ accounts for nearly 30 to 40% of the total rumen gas production and CO₂ accounts for 50 to 60%.
• The short chain fatty acids are available asrespective sodium salts. In cattle fed a mixed diet, the proportions of VFAs are acetate 60-65%, propionate 15-20% and butyrate 10-15%. The ratio of A: P: B ranges from 70:20:10 for high forage diets to 60:30:10 for high grain diets. Fluctuation in the VFA concentration occurs on daily ration.
• The normal total VFA content of the rumen ranges from 60 to 120mEq/L. The type of diet, quantity and quality of feed, health of the animal alter the concentrations of individual acids. Total VFA production is higher in animals fed high starch diet than high fibre diet. Propionic acid decrease in animals fed on hay.
• The rumen ecosystem adapts to the type of diet. Lactic acid production is increased in the animals fed with large quantities of easily digestible carbohydrates like starch, sugar or rapidly changing the diet from high roughage to high starch diet. 
• Un-dissociated acids do get absorbed from rumen and the rate of metabolism follows similar pattern of butyrate to propionate to acetate.
• Rate of absorption of volatile fatty acids increases when the pH of the rumen ingesta decreases.
• This helps in regulating rumen pH by removal of acidic materials and constant pH is being maintained.
• Loss of VFA results in accumulation of carbon di oxide( as bicarbonate and as CO2) and concentration of HCO3 ion increases as that of plasma.
• Lactate production is relatively low. High starch diet if fed too high cause accumulation of high concentration of lactate.
• This follows lactacidemia (high lactate in blood) due to absorption from the rumen.
• Metabolism of propionate in the ruminal epithelium increases as the concentration of lactate.
• Increased lactic acid formation in the rumen reduces rumen pH, leading to lactic acidosis. There is an increase in the number of Streptococcus bovis. This causes suppression of the growth of other types bacteria as they are sensitive to pH,causes rumen dysfunction. Dehydration results due to osmotic imbalance.

Significance of short chain fatty acids in ruminants

The propionate is glucogenic acids and provides glucose through gluconeogenesis by entering into the Kreb’s cycle at the level of succinate.

Acetate and butyrate contribute the energy needs of the ruminant animal by entering Kreb’s cycle as acetyl-CoA. They are ketogenic acids cause the formation of the ketone bodies, acetone, acetoacetic acid and beta hydroxy butyric acid. The ketone bodies serve as energy source in tissues like CNS and heart. The acetate is the precursor for milk fat synthesis.

Methods to improve ruminal fiber digestibility

• Physical and mechanical treatments
  1. Soaking with water
  2. Grinding and pelleting
  3. Increase the fiber intake
  4. Increase surface area for microbial degradation
  5. Irradiation by Heat/steam treatments
• Chemical treatments
  • Alkali treatment
  • Treating with Ammonia and urea
  • Chlorine compounds – Improves cell wall digestibility
  • Ozone – Decreases lignin content and increases cell wall contents
  • Sulphur dioxide – Solubilizes hemicellulose
  • Alkaline hydrogen peroxide – Solubilizes approximately 50% of lignin
• Treatment with fungi