Microbial Digestion System of Carbohydrates
Mammals do not possess enzymes to digest the plant cell walls ( structural carbohydrates). They digest them by fermentative digestion with the help of microorganisms. Herbivorous animals are of two types: 1. Pregastric fermented (e. g. goats, cows and buffaloes) and 2. Postgastric fermentors (e. g. swine, horse, guinea pig, rabbit).
In goats, microorganisms breakdown the higher carbohydrates, cellulose, pentosans and starch (protein as well) to monosaccharides and then fermented to VFA and methane. Further/ microorganisms synthesize essential nutrients such as B- vitamins, amino acids. Host animals provide space (rumen, large intestine) and other factors most favorable for microbes’ activity. This symbiotic relationship is developed to the highest degree in ruminants. Even insects such as termites have significant microbial activity. Important rumen microorganisms and their fermentation characteristic are furnished in Table.
The reticulo-rumen provides a continuous culture system for anaerobic bacteria, protozoa and fungi. The rumen microorganisms can be envisaged as operating together as so called consortia to attack and breakdown feeds. Microbial mass synthesized in the rumen provides about 20% of the nutrients absorbed by the host animal. Bacteral DM contains 100g N/kg, 80% of which is in the form of amino acids while the remaining 20% as nucleic acid N. Microbial fermentation of carbohydrates and proteins yield volatile and short chain fatty acids which provide 60-80% of the ME of ruminants on most diets.
Microbial digestion of fibre is extensive in the large intestine herbivore though it is not as efficient as in the rumen. About 25 to 30% of the total dry matter consumed by a ruminant may be made up of crude fibre. Goats are able to digest more than 50% of the crude fiber consumed in their rations.
Factors that Affect Microbial Digestion of Fibre
- Characterstics of the feed: It Influencies the kind and number of the microorganisms present. For example, addition of easily digestible carbohydrates, such as molasses, starch to the ration reduces the digestibility of fibre because the rumen bacteria prefer to attack the simpler carbohydrates.
- Characterstics of the roughage: Irrespective of its fibre content roughage has an influence on the nature of the bacterial flora and on their activity. Replacement of a straw with a hay stimulate microbial activity because hay supply the critical nutrients for microbial growth.
- Chimerical and physical nature of the fibre: The Complex polysaccharides of mature plants are less well digested than they are in young, growing plants. Mature plants have higher lignin content and this encrusts the cellulose and hemicelluloses and thus carbohydrates are not accessible for bacterial attack. That is how lignin is not only indigestible itself in the animal but it also lowers the digestibility of cell walls. The reduced digestibility may also be due to increased rate of passage from the rumen.
- Rumen environment: The following conditions must be satisfied for the digestion of crude fibre in the rumen. The feed consumed should remain in the rumen for sufficient length of time. The ingesta should be sufficiently moist for the microorganisms to thrive and for their enzymes to act. An optimum rumen temperature of 38-40° C. An optimum rumen pH. It should not be less then 6.2 for optimum cellulolytic activity. The internal rumen environment should be anaerobic. Regular supply of nutrient through feed should be maintained to serve as substrate for microorganisms. End products of microbial fermentative digestion should not accumulate in the rumen.
There are two constituents which contribute bulk or volume to a feed. They are water and crude fiber. Highly succulent feed like green legumes roots, tubers are bulky because of their high water content. Coarse feeds are bulky because crude fiber contributes to the major portion of the dry matter of such feeds. They are light and occupy a greater volume than concentrators. The weight per unit volume of feed is their bulk density.
- Bulk satisfies the appetite of an animal.
- Digestive tract function more efficiently due to distension and peristalsis. Distension is particularly accomplished by the fiber.
- Bulk has the property of breaking up the concentrates thereby presenting a larger surface area for the digestive enzymes to act on the concentrate feeds.
- Bulk has a laxative effect in the animal. This is due to the water absorbing capacity of crude fiber especially the hemicelluloses. A fiber which readily absorbs water and swells has more laxative effect.
A nonfibrous feed which absorbs a large amount of water is less effective, because it is largely digested and thus do not reach the portion of the G.I. tract occupied primarilyby feed residues. Of course, bulk is not the sole cause of laxative effect. Many feeds are laxative because of specific chemical substances contained in them which promote peristalsis.
- Bulk is promoted by ability to absorb water. Some fibrous materials, such as ager, absorb large quantities of water while others, such as regenerated cellulose, do not. Linseed oil meal, which is much lower in fiber than wheat bran, absorbs three times as much water and thus, in this sense, is a more bulky feed in the digestive tract.
Effect of too much bulk: Feeding a ration of very low bulk density lowers digestibility of the feed and productivity of the animal due to consumption of less digestible nutrients. Such ration may cause atony and digestive disturbances like impaction.
Effect of too less bulk: Rations with very less fiber decreases rumen activity and animals develop a craving for food initially. Later the animal loses its appetite gradually. Too less bulk reduces the water absorbing capacity of the ration. There is a decrease in the milk yield and fat per cent due to decrease acetic a acid and enhanced propionic acid in the rumen liquor.
Lambs grow normally on low fiber purified diets when sodium and potassium bicarbonates were included to serve as buffers. All concentrated ration consisting of rolled barley or ground maize supplemented with protein, minerals and vitamins gave satisfactory rates and efficiency of gain in finishing beef cattle. However, injury to the rumen epithelium and liver abscesses accur in some animals. Adding long hay prevents these pathological changes. Gastric ulcers are observed in swine on feeding finely ground (less then 600 particle size) feeds.
Lactating dairy goats tend to produce milk with a lower fat content as the crude fiber in the ration falls beow about 17% (35% NDF)of the DM. In humans low fiber diets are associated with appendicitis, cancers of colon and rectum, gallbladder disorders and ischemic heart disease. A human diets should have 8-10% fiber and human need 40 grams of fiber a day. Thus the indigestible materials also influences health.