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5.2 The Importance Of Rumen Fermentation

5.2.1 The Role Of Rumen Microflora

Ruminants digest much of what they eat by microbial fermentation in the reticulorumen (referred to as the rumen). The rumen should be appreciated as a large fermentation vat that is central to the health of ruminants. Fermentation produces fatty acids absorbed from the rumen for energy, microbial protein that is digested and absorbed in the intestine, and methane gas that is eructed. 

Digestion in the rumen proceeds efficiently under the following conditions:
  • The feed entering the rumen is suitable for fermentation.
  • There is a large, mixed and dynamic population of fermentative microorganisms adapted to the feed.
  • The microorganisms are bathed in continually moving fluid with pH > 5.9.
  • The products of the fermentation process are removed (by absorption or further digestion, with waste removed as gas and faeces).
When any of these conditions are not met fully, fermentation, and therefore digestion, within the rumen may slow or cease and a functional (as opposed to physical) blockage may occur. The animal will become inappetent until normal rumen fermentation resumes. 

For rumen microflora to adapt with minimal interruption to fermentation, minor changes in type and quality of feed may need introduction over several days, and major changes may need incremental introduction over several weeks. By understanding the important role of microbial fermentation in ruminants, one can appreciate why oral antibiotics and overeating can be so disruptive, why new diets should be introduced incrementally, and why transfer of rumen fluid from a healthy donor may sometimes be necessary for recovery.

5.2.2 The Role Of Fibre And Saliva

Fermentation in the rumen is greatly dependent on the presence of fibrous plant material in the diet. It is the chewing and rumination of fibre that provides the large quantities of saliva necessary for rumen buffering to maintain pH within narrow limits. Saliva is also an important means of recycling absorbed nitrogen to the rumen. Longer fibre leads to more chewing and more saliva. 

The abrasiveness of the fibre (also known as the “scratch factor”) helps prevent loss of rumen papillae or ruminal parakeratosis in which the papillae of the rumen mucosa become abnormally hardened, enlarged, and less effective at absorbing nutrients. The length, structural strength of fibre particles and palatability must be adequate to maintain normal chewing, rumination, production of salivary buffers, and stimulate rumen papillae. It is recommended that a palatable fibre inclusion is fed at 5-10 cm in length for cattle and an average of 2.5 cm length for sheep. 

Excessive intake of rapidly fermented carbohydrates, such as the starch in grain, can rapidly depress rumen pH. This occurs because the lactic acid produced from starch is produced much more quickly than the milder volatile fatty acids produced from cellulose (the carbohydrate in fibrous plant material). The increase in acidity may severely disrupt and sometimes kill off the fermentative microorganisms of the rumen.