Formation of Microbial Protein in ruminants
Dr. Smriti Shukla
PhD student, Division of Physiology and Climatology IVRI, Izatnagar, Bareilly, U.P.
Pincode: 243122
Email id: shuklasmriti2415@gmail.com
Introduction
Dietary protein for ruminants includes nitrogen (N) occurring in true protein and non-protein. In the rumen, the true protein is degraded into amino acid (AA) and ammonia and then utilized by ruminal microorganisms to synthesize microbial protein. In the small intestine, more than 80% of rumen microbial protein is digested, accounting for 50–80% of the total absorbable protein contained there (Tas et al., 1981; Storm et al., 1983).
Microbial Protein Synthesis:
The bacteria, the protozoa and the fungi forming the ecosystem have different requirements of nutrients and metabolis. They all ferment the feed constituents (polysaccharides, sugars, proteins) to generate the ATP molecules required to keep their homeostasis and guarantee their growth. This process comprises the synthesis of monomers (such as the synthesis de novo of amino acids) and their polymerization (such as the elongation of the polypeptide chains).
Rumen microbes are able of synthesizing de novo the ten amino acids essential for the tissues of the mammals, as well as of obtaining this via most of the amino acids’ requirements. The synthesis of these amino acids is carried out from ammonia and simple carbonated skeletons, produced during the feed degradation. Thus, ruminants subsist and have modest levels of production when they only have NPN (urea, ammonia) as Nitrogen source in the diet. Ammonia is the central intermediary in the Nitrogen degradation and assimilation in rumen. The levels of ammonia in rumen range from 0 to 130 mg of N/100 mL, according to the Nitrogen source and the postprandial time. The concentration of this complex may exceed these figures, after the animals ingest fresh pastures. The optimum concentration for the microbial protein synthesis is between 5.6 and 10.0 mg of NH3/100 mL of rumen liquor when the energy availability does not limit the rumen ecosystem. The possibility of using the ammonia allows the rumen microbes recycling large amounts of urea from the intermediary metabolism of the animal, as Nitrogen source for the synthesis of microbial protein, when enough energy amounts are available. Other nitrogenous complexes may also be recycled through the saliva or the rumen lining such as the purine metabolites and the mucoproteins. This evolutionary adaptation of the ruminants reduces effectively the minimum N required and increases the time of survival of undernourished animals. When the ammonia amount is large due to the extensive degradation of the proteins, the surplus of ammonia is absorbed through the lining of the tract. Later, it is turned into urea in the liver to reduce the circulation of this complex by the organism because it is toxic to the animal. The urea produced may be recycled to the rumen to be used by part of the microbes or it is excreted in the urine of the animal with the consequent.
This process, known as urea cycle, is the result of the adaptation of the ruminants to the inefficient use of the proteins in the rumen to prevent the toxicity of the ammonia molecules and use the N released afterwards. Thus, the energy availability in the rumen permits its incorporation to the microbial protein. The synthesis of the urea molecules in the liver demands energy, thus, it is an expensive process and affects negatively the animal production because a part of the energy available for maintenance or beef or milk production should be taken to compensate the situation created by an excess of ammonia in blood. The rumen bacteria may also incorporate directly amino acids and peptides from the diet. The diminished concentration of the free amino acids in the rumen indicates that they are used readily, although the rise in the first hours after the feeding suggests that the proteolysis occurs at higher rate than the use of amino acids. Around 30 % of the N from the diet degraded in the rumen is incorporated to the microbial protein in the form of peptides and amino acids.
Fig: Digestion and metabolism of nitrogenous compounds in the rumen
Factors affecting the production of microbial protein in rumen:
- Carbohydrates: – carbohydrates are important for carbon Skelton in microbial protein synthesis. The sources of carbohydrates and proteins, the levels of voluntary intake, the feeding frequency, and the fodder/concentrate ratio in the diet are among the factors affecting the microbial protein synthesis. Also, there are the synchronization of the rumen functions, the fodder quality the rumen recycling of the microbes, and the antinutritional factors of the plants. The most important factor limiting the microbial protein synthesis in the rumen is the energy released in the rumen during the fermentation of carbohydrates to organic acids.
The sources of carbohydrates are classified into two groups: those rich in non-structural carbohydrates (sugars, starches), and those rich in structural carbohydrates (pectin’s, cellulose, hemicellulose). The characteristics of the source of carbohydrates affect the rate of microbial synthesis. The lower rates of microbial growth are produced when using cellulose as only energy source, but the degradation of the structural carbohydrates depends also on the amounts of lignin in the feed. The synthesis of microbial protein is increased sometimes by the inclusion of moderate quantities of carbohydrates readily fermentable in the diet because there is increase in the availability of substrates and the growth rate of the bacteria associated with the liquid phase of the digestion.
- Particle size: – By increasing the particle size of the fibrous fraction in diets with large amounts of energetic concentrates, there is increase in the efficiency of the microbial synthesis due to the improvement in the rumen conditions by the enhancement of the processes of rumination and salivation. Likewise, there is increment in the enhancement of the organic matter digestibility.
- Starch in diet: – The inclusion of starch in the diet of ruminants may affect in several forms the ruminant microbes, and the forecast of the final effect is not simple. The starches may have negative effects on the microbial synthesis in the rumen because their fermentation diminishes the rumen pH, affects the fibre degradation, increases the energy losses in the microbes, and declines the synthesis de novo of the amino acids. Not all the energy sources have the same effect on the microbial protein synthesis. It has been proved that the soluble sugars (saccharose, lactose and fructose) increase even more the microbial protein synthesis in the rumen rather than when using supplements of cereals rich in starch. The starches of oat and barley are degraded from 2 to 2.8 times faster than those of corn; thus, the energy availability for the microbial synthesis and the negative effect of these sources on the rumen conditions of fermentation varied considerably between one source and the other.
- Nitrogen Sulphur Ratio: –The mean of the estimates of the ratio of sulphur to nitrogen in microbial protein is around 0.07. The UK Agricultural Research Council has recommended that the requirement for rumen-degradable sulphur should be calculated by multiplying the rumen-degradable nitrogen requirement by 0.07 (i.e., equivalent to a N:S ratio of 14:1).
Conclusion
The rumen microbes are able of incorporating their amino acids and peptides to the diet and of using the ammonia to synthesize de novo their amino acids such as the ten amino acids essential for the tissues of the mammals. The synthesis of microbial protein depends upon different factors such as the sources of carbohydrates and proteins, the level of voluntary intake, the synchronization of the rumen functions, the rumen recycles of microbes, and the antinutrients of the plants consumed. The microbial protein has a relevant role in ruminants fed diets with high-fiber content and low level of Nitrogen. It is sometimes the only protein source for the animal.
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