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Digestion: the mechanical and chemical breakdown of feed into small, soluble molecules that the animal's cells can absorb and use. Ruminant: an animal with a compound stomach of four compartments, namely the rumen (paunch), reticulum (honeycomb), omasum (manyplies) and abomasum (true stomach), that digests roughage with the help of microbes, e.g. cattle, sheep and goats. Non-ruminant (single stomach): an animal with one single, simple stomach that relies on enzyme digestion, e.g. the pig and the fowl.
| Feature | Ruminant (cattle, sheep) | Non-ruminant / single stomach (pig, fowl) |
|---|---|---|
| Type of stomach | Compound, four compartments | One single, simple stomach |
| Main type of feed | Roughage (grass, hay), high in crude fibre | Concentrates and more digestible feed |
| Who digests the crude fibre | Microbes in the rumen | Cannot digest crude fibre efficiently |
| Chewing of feed | Swallows quickly and chews the cud later | Chews the food immediately and thoroughly |
| Alimentary canal | Long, with a large rumen | Shorter and relatively simple |
| Compartment | Internal structure | Function and adaptation |
|---|---|---|
| Rumen (paunch) | Largest compartment; wall lined with small papillae | Fermentation vat full of microbes; the papillae enlarge the surface for the absorption of volatile fatty acids |
| Reticulum (honeycomb) | Inner wall with a honeycomb pattern | Traps coarse particles and foreign objects; works together with the rumen (reticulorumen) and helps to bring up the cud |
| Omasum (manyplies) | Many leaf-like folds, like the pages of a book | Removes excess water and presses the fine material through; the folds enlarge the absorption surface |
| Abomasum (true stomach) | Glands in the wall | The true stomach; secretes gastric acid and digestive enzymes for chemical digestion |
The function of each compartment and the role of the rumen microbes are tested almost every year, often with a labelled diagram where you must name the four parts and give their functions. Know both forms of each name (rumen/paunch, reticulum/honeycomb, and so on).
Remember: the ruminant first swallows quickly and keeps the hard work for later. Only one of the four compartments, the abomasum, is a "true" stomach; the other three are a microbe factory that turns grass into energy.
A comparison between the alimentary canal of the ruminant and the non-ruminant (in table or diagram form) is a favourite question. Focus on who digests the crude fibre and where the microbes are.
The ruminant initially swallows roughage quickly and coarsely. It lands in the rumen and reticulum, where microbes start to ferment it. Later, while the animal rests, rumination takes place: a ball of half-digested feed, the cud, is brought back to the mouth by regurgitation, chewed slowly and thoroughly, and swallowed again. Each time, the feed is ground finer, which enlarges the surface so that the microbes can work faster. Peristalsis (rhythmic muscle contractions) moves the material through the canal, from rumen to omasum to abomasum and then to the small intestine.
| Aspect | Young (milk-drinking) calf | Mature ruminant |
|---|---|---|
| Largest compartment | Abomasum (true stomach) | Rumen (paunch) |
| Condition of the rumen | Small, undeveloped, few microbes | Large, full of microbes, highly developed |
| Main type of feed | Milk | Roughage |
| Route of the feed | Milk is channelled past the undeveloped rumen, straight to the abomasum | Feed follows the full route through all four compartments |
| Ability to digest crude fibre | Virtually none | Efficient, with the help of microbes |
The rumen is a living, oxygen-free fermentation vat that houses millions of micro-organisms. They are classified according to their action:
Functions: the microbes ferment roughage into volatile fatty acids (the ruminant's main energy source), build up high-quality microbial protein, and synthesise B-vitamins. Requirements for normal functioning: an oxygen-free environment, a stable pH near neutral, a constant warm temperature (about 39°C), enough moisture, and a regular supply of roughage as substrate. A sudden change of feed disturbs the microbe balance.
Absorption: nutrients are taken up through the rumen wall (across the papillae) and the small intestine by osmosis (water), diffusion (small molecules along the concentration gradient) and active transport (substances against the gradient, with the help of energy).
Q: A farmer separates a two-week-old calf from its mother and feeds it only milk. A student expects that the calf's rumen should already be large and full of microbes. Explain why this is not the case.
Reason: A newborn calf lives only on milk, and milk is channelled past the rumen straight to the abomasum, where it is digested by enzymes. The rumen therefore receives virtually no substrate and stays small and undeveloped with few microbes.
Change: As soon as the calf starts to eat roughage, microbes colonise the rumen and the compartment grows until it becomes the largest and most active part.
∴ The rumen only develops when roughage is taken in, not merely with age.
Q: The same amount of grass hay is fed to a cow and to a pig. The cow takes far more energy out of the hay than the pig. Account for this difference.
Cow (ruminant): the cellulolytic microbes in the rumen break the cellulose in the hay down into volatile fatty acids, which are absorbed across the rumen wall and supply energy. Rumination also makes the hay finer so that the microbes work faster.
Pig (non-ruminant): the pig has no rumen with cellulolytic microbes, and its own enzymes cannot break down cellulose. Most of the hay therefore passes undigested through its body.
∴ The difference lies with the rumen microbes: they let the ruminant extract energy from crude fibre that is lost to the non-ruminant.
Direct one- or two-mark questions such as "Name the true stomach of the ruminant" (= abomasum/true stomach) or "Give ONE example of a non-ruminant" (= pig or fowl) appear regularly. Do not lose them.
Links with other topics
Distinction tip
Do not call all four compartments "stomachs" - they are four parts of ONE stomach, and only the abomasum is the true stomach. Do not confuse the location of the two types of digestion either: microbial fermentation happens in the rumen, while chemical (enzyme) digestion only begins in the abomasum and small intestine.
Remember the order of the four compartments and what each does with one sentence: the rumen ferments, the reticulum sorts, the omasum presses out water, and the abomasum digests with enzymes. The first letters (R-R-O-A) follow the path of the feed.
A cow's power lies not in her teeth, but in the millions of microbes that digest grass for her.- the rumen is a living fermentation vat
Two systems: the ruminant with four compartments (rumen, reticulum, omasum, abomasum) that digests roughage with microbes, and the non-ruminant (single stomach) that relies on enzyme digestion. Rumination makes feed finer, the rumen microbes break down cellulose that no mammal's own enzymes can tackle, and nutrients are absorbed by osmosis, diffusion and active transport. Know each part's name (both forms), structure, function and adaptation.
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