discuss the major sources and fate of the following substances in ruminant digestion
I. propionic,ethanoic and butanoic acid
ii. methane
iii. carbon dioxide
iv. ammonia
I. Propionic, ethanoic, and butanoic acid:
The major sources of propionic, ethanoic (acetic), and butanoic (butyric) acid in ruminant digestion are through the fermentation of carbohydrates by ruminal microorganisms. These carbohydrates include sugars, starches, and nonstructural polysaccharides such as cellulose and hemicellulose present in the plant material consumed by the ruminant.
During ruminal fermentation, these carbohydrates are broken down by anaerobic microorganisms, primarily bacteria, into volatile fatty acids (VFAs). Propionic acid is mainly produced during the fermentation of sugars and starches, while acetic acid is produced from the fermentation of a wide range of carbohydrates. Butyric acid is primarily produced from the fermentation of cellulose and hemicellulose.
The fate of these VFAs differs in ruminant digestion. Propionic acid is an important precursor for the production of glucose in the liver via gluconeogenesis. Glucose is used as an energy source by the ruminant's body cells. Acetic acid is also partly converted into glucose, but a significant portion is used for fatty acid synthesis in the adipose tissue. Butyric acid is both converted into glucose and used for fatty acid synthesis.
II. Methane:
Methane is produced in the rumen during anaerobic microbial fermentation, specifically by methanogenic archaea. These microorganisms use the hydrogen gas produced during ruminal fermentation as a substrate to produce methane. The primary source of hydrogen gas is the breakdown of VFAs by rumen bacteria. Methane synthesis is an essential process to remove hydrogen gas from the rumen to maintain fermentation efficiency.
The fate of methane in ruminant digestion involves its release into the rumen, where it can be eructated (belched) by the ruminant. It is estimated that the majority of methane produced in the rumen is belched by the animal and released into the atmosphere, contributing to greenhouse gas emissions. A small portion of methane may be absorbed by the rumen epithelium and metabolized by methanotrophic bacteria before being excreted in the feces.
III. Carbon dioxide:
Carbon dioxide is produced in ruminant digestion through several processes. It is released during the fermentation of carbohydrates by ruminal microorganisms, similarly to methane production. Additionally, carbon dioxide is produced during the oxidation of VFAs in the rumen by bacteria. It is also produced during the process of rumen microbial protein synthesis through the breakdown of amino acids.
The fate of carbon dioxide in ruminant digestion involves its release into the rumen and its subsequent absorption by the rumen epithelium. From there, it can be transported via the bloodstream to the lungs, where it is excreted through respiration.
IV. Ammonia:
Ammonia is formed in the rumen through the breakdown of dietary proteins by ruminal microorganisms. The bacteria present in the rumen produce enzymes that allow them to break down the protein into ammonia. The primary source of dietary protein in ruminant digestion is from microbial protein synthesized by rumen bacteria.
The fate of ammonia in ruminant digestion involves its absorption through the rumen epithelium. From there, it is rapidly transported via the bloodstream to the liver, where it is converted into urea through the urea cycle. Urea is then recycled back into the rumen and used as a source of nitrogen for rumen microorganisms or excreted in the urine. The urea recycled back into the rumen is used by ruminal bacteria to synthesize microbial protein, completing the nitrogen cycle in ruminant digestion.