Ruminant Physiology

Ruminants are distinguished from other animals by having a four-compartment stomach, comprising rumen, reticulum, omasum and abomasums.

Ruminant Stomach

The Rumen

Located on the left side of the body, the rumen makes up over 65% of an adult cow's total stomach volume. It is, in effect, a huge fermentation vat containing a soup of around 130 litres of chewed-up feed with large amounts of saliva and micro-organisms - primarily bacteria and protozoa. Floating on top of this soup is a fibrous mat of coarser solid material which acts as a filter. Feed particles are regurgitated and re-chewed until they are small enough to fall through the fibre mat into the rumen liquor below.

The rumen liquor commonly contains between 109 and 1011 bacterial per ml, together with 105 -106 protozoa. These break down degradable feed materials to produce Volatile Fatty Acids (VFAs), ammonia and a variety of long chain fatty acids.

Ammonia is used as a nitrogen source for microbial growth and VFAs absorbed from the rumen are a key energy source for the cow. Increasing the rumen-available energy content of the diet in the form of sugar and starch stimulates papillae growth, improving VFA absorption. While rumen fermentation allows good use to be made of fibrous feeds that could not otherwise be digested, it does mean only around 70-85% of the energy in the feed is available to the animal - 6-15% commonly being lost as gases (mainly methane) and 6-7% as heat.

The Reticulum

Small in comparison to it, the reticulum is a continuation of the rumen with a honeycomb structure. Microbial fermentation continues as the feed moves through the reticulum and into the omasum - a globe-shaped structure containing page-like folds of tissue from which water and some nutrients are absorbed.

The Omasum

Moving through the omasum, the mixture of feed and rumen micro-organisms becomes progressively drier. Excessive intake of minerals or low quality fibre (such as sunflower hulls) can cause compaction of the omasum.

The Abomasum

Finally the abomasum or 'true stomach' secretes hydrochloric acid and digestive enzymes to begin breaking down feeds that have escaped microbial digestion together with microbes

excreted from the rumen. From the stomach the digesta moves into the small intestine where most of the digestive enzymes are secreted to break down both feed and microbial nutrients into simpler nutritional building blocks. These are absorbed across the intestinal lining and into the bloodstream through small finger-like projections (villi) which increase its surface area

Bacterial fermentation of some undigested feed occurs in the final section of the digestive tract - the large intestine - which also absorbs both VFAs and water.

Rumen Dynamics

The contents of the rumen are continually mixed by the rhythmic contraction of its walls, a healthy rumen contracting around twice a minute. As well as bringing feed and bacteria into close contact with each other, the contractions move smaller, denser feed particles out of the rumen while bringing larger, lighter particles up to the fibre mat at the top surface for rumination.

Cows commonly spend 8-10 hours/day ruminating, the extent of rumination depending on the roughage content of the diet. The cycle of rumination involves four distinct elements:

  • Regurgitation - coarse material (the scratch effect) at the upper end of the rumen stimulates a bolus of feed (cud) to be returned to the mouth.
  • Chewing - each cud of regurgitated food is chewed many times to grind it down into particles small enough to pass out of the rumen.
  • Salivation - chewing stimulates the secretion of buffer-containing saliva (as much as 75 litres/day) which is mixed with the cud to stabilise rumen pH.
  • Swallowing - once the coarse material has been ground down sufficiently by chewing it is swallowed and sinks to the bottom of the rumen to pass into the reticulum.

When cows are resting (not eating or being milked) over 60% should be ruminating. If the diet contains adequate long fibre cows should chew at least 30 times (ideally 60) before re-swallowing.

VFA Production

Volatile Fatty Acids (VFAs) produced from microbial fermentation of feed carbohydrates in the rumen are the primary source of energy for ruminants. Three distinct volatile fatty acids are produced by rumen fermentation:  acetate, propionate and butyrate.

Under optimal conditions the acetate:propionate ratio should be greater than 2.2 to 1. High levels of acetate indicate a high fibre/low starch ration, producing a generally slower, more stable fermentation. High levels of propionate indicate a high starch/low fibre ration producing a faster rate of fermentation which can lead to reduced rumen pH, depressed fibre digestion and even acidosis.

Rumen Efficiency

With fibre-digesting bacteria thriving best at pH 6.0-6.8 and starch-digesting bacteria at 5.5-6.0, the best balance of fibre and starch digestion occurs at a rumen pH of around 6.0. 

Factors affecting rumen pH and fermentation efficiency include:

  • Forage to concentrate ratio. High forage diets stimulate higher rates of saliva production, better rumen buffering and greater acetate production which supports butterfat levels. Excessive amounts of concentrates, on the other hand, increase propionate production, decrease rumen pH, reduce feed intake and microbial production and depress butterfats.
  • Concentrate type. Starch-based concentrates may depress rumen pH to a greater extent than those based on digestible fibre.
  • Physical form of feeds. Grinding, pelleting, chopping or over-mixing can reduce the particle size of feeds to a level at which the rumen mat cannot be maintained, depressing rumination, saliva production and pH. Finely ground concentrates increase microbial fermentation (favouring lactic acid-producing bacteria, in particular), reducing rumen pH and increasing the risk of acidosis.
  • Higher feed intakes mean more material available for bacterial fermentation and higher levels of VFA production. The amount of saliva produced per unit of feed dry matter may also decline, leading rumen pH to drop.
  • Wet feeds can reduce rumen pH because less saliva is needed to lubricate the feed for swallowing. Rumen pH can also be adversely affected with very dry diets because of low intake levels.
  • Degradable fat levels. Fatty acids in vegetable and fish oils can coat fibre particles, reducing their digestibility. They can also be toxic to fibre-digesting bacteria. Grinding or extruding oilseeds tends to make these effects worse by rupturing cell walls and releasing oil into the rumen, while feeding whole oilseeds can reduce this risk.
  • Feeding method. Total Mixed Rations (TMR) can be beneficial in stabilising rumen pH, providing a balanced supply of nutrients for the most efficient bacterial fermentation, increasing dry matter intake and minimising feed selection. When high levels of concentrates are included, rumen pH may still be below optimum and fibre digestion compromised. If concentrates are fed separately, it is important to limit the amount to 3 kg/meal and avoid high starch levels and finely processed grains to prevent large fluctuations in rumen pH.

It is vital to appreciate the dynamic state of the rumen environment and the extent to which changing feeds or feeding systems can alter rumen conditions - for better or worse.