Ruminants are animals that have a four-part stomach and can digest the fibre (mainly cellulose) in plants. These parts are called the rumen, reticulum, omasum and abomasum. The large rumen makes up about 80% of the total volume.
Ruminants also have cloven hooves and have their top teeth replaced by a hard dental pad. The farm ruminants (cows, sheep, goats, deer) harvest their feed using a long "pre-hensile" (grasping) tongue, and regurgitate their feed for a second chewing - called rumination or chewing the cud.
A milking cow spends about 8 hours a day grazing, and the rest of her time is spent ruminating, idling, walking, drinking, resting and sleeping. There is usually an idling period straight after grazing and this is when initial rumen fermentation starts.
What happens in the rumen?
Feed is first harvested by the cow, chewed, and with the addition of saliva (70-100 litres/ day) which contains a digestive enzyme, is passed into the rumen in the form of a ball or bolus. Here, bacterial fermentation of cellulose takes place in this large organ which has many papillae or wrinkles on the inside to increases surface area. The rumen constantly moves in waves to mix the contents and expel the gas.
Methane and carbon dioxide are produced from fermentation and these gases escape as the animal belches. The cow uses its diaphragm to help put pressure on the rumen to help in belching. Bloat is caused by these gases becoming bound up in foam (caused by some plants such as clovers) and is unable to escape from the rumen.
Ammonia is also produced, but unlike methane and carbon dioxide which is wasted, it is used and converted into urea and bacterial protein. When the bacteria die the protein is made available to the animal.
The other products of digestion in the rumen are acids. These are not the same kind of acids as those found in the fourth stomach or abomasum. Rumen acids are fatty acids such as acetic and butyric - the ones you find in silage.
The rumen flora as well as breaking compounds down also build them up. They build up proteins which are digested by the animal when they die. They can also synthesise all the water soluble vitamins needed.
The bacterial flora in the rumen take time to grow and multiply, so it's bad practice to change a ruminant's diet quickly. Diet changes should be done slowly to give the flora time to change. If you don't, digestive upsets will occur and the cow will stop eating.
What happens in the other compartments?
The reticulum is a small bag at the front of the rumen. The inside walls have honeycomb-shaped papillae on them (the butchers honeycomb tripe), and it's here where heavy objects accumulate such as bloat boluses, and bits of wire. The reticulum is near to the heart and wire can penetrate it through the diaphragm.
On its way up for a second chew, a bolus is brought back into the mouth using the gas pressure of the rumen. Each bolus is chewed about 30-40 times before being swallowed again this time passing into the omasum.
The omasum is a bag with leaves inside it like a book. Butchers call it the "Bible" and its job is to finely grind the grass particles before passing into the abomasum where "gastric" or true acid (peptic) digestion takes place. Pigs and poultry only have this acid digestive system and cannot digest cellulose.
The oesophageal groove
This is a fold in the rumen quite near the entrance which is especially important in the young calf. When liquid hits the back of the throat, it triggers the groove to close so that the liquid goes straight into the abomasum for acidic digestion. The last thing the calf wants is milk in the bacterial fermentation part of the stomach.
The calf's rumen starts to develop when it starts to eat fibre which is usually about 2-3 weeks of age. Encouraging it to develop its rumen is important as it can then digest cheaper feed sources - meal, grass and roughages instead of milk.
In adults the groove can be triggered by some minerals. So if you mix a mineral supplement with a worm drench (which you want to go into the rumen), it may be triggered right past into the abomasum. Ask for veterinary advice if in doubt.
What about non-ruminants?
Pigs and poultry rely on strong digestive juices in their simple one-compartment stomach to break down their diet. The resulting products of digestion are soluble in water and are transported to the various tissues for either combustion or storage.
Horses are not ruminants but have a similar digestive system to cattle as they can digest cellulose. However it is done in the colon or hind part of the gut where the gases produced escape through the anus.