Rumen

What is the rumen?

Cattle have a stomach with four compartments. Three of them are forestomachs: the rumen, reticulum and omasum. Only the fourth, the abomasum, corresponds to the human stomach. The rumen is by far the largest part. In an adult dairy cow it holds 150 to 200 litres and fills almost the whole left half of the abdomen.

Billions of bacteria, fungi and protozoa live in the rumen. They ferment the feed, above all the cellulose from grass, silage and straw. Without these microbes the cow could make little use of forage.

How does digestion in the rumen work?

The cow eats quickly and swallows her feed roughly chewed. In the rumen it is mixed, soaked and later brought back up for rumination. While ruminating, the cow breaks down the fibres and produces a lot of saliva. A healthy cow ruminates for around eight hours a day.

The microbes break down the carbohydrates into volatile fatty acids:

• Acetic acid comes mainly from fibre and is the building block for milk fat.
• Propionic acid comes mainly from starch. The liver turns it into glucose for milk sugar.
• Butyric acid is an energy source for the rumen wall and supports its development in the calf.

These fatty acids pass directly into the blood through the rumen papillae. The microbes themselves move on with the digesta and are digested in the intestine as high-quality protein. So the cow really feeds her rumen microbes, and the microbes feed the cow.

Rumen pH

Fermentation produces acids. Saliva buffers them with bicarbonate, and a cow produces well over 100 litres of saliva a day. Normal rumen pH lies between about 6.0 and 6.8. A lot of rapidly digestible starch and little structure lower it. If it drops below 5.8 for several hours a day, this is called subacute ruminal acidosis (SARA). The fibre-digesting bacteria suffer most, feed efficiency drops and milk fat content falls.

Rumen development in the calf

A newborn calf functions as a monogastric animal. Milk flows through the oesophageal groove past the rumen straight into the abomasum. The rumen is small and has hardly any microbes yet. It only develops once the calf takes in calf starter, hay and water. Concentrate supplies the butyric acid for the growth of the rumen papillae, and forage trains the muscles.

In the past this led to early weaning and plenty of forage. Today it is clear that generous milk feeding in the first weeks raises later milk yield without slowing rumen development. You can read more in Metabolic programming of calves.

Common disorders

• Rumen acidosis: too much starch or sugar, too little structure. Signs are loose manure, fluctuating feed intake and low milk fat.
• Rumen alkalosis: too much protein or urea with little energy. Less common than acidosis.
• Rumen atony: the rumen hardly moves any more, for example with milk fever, because there is not enough calcium for muscle work.
• Displaced abomasum: a poorly filled rumen after calving gives the abomasum room to shift.

How to keep the rumen stable

The basic rules are simple. Feed enough structured forage and mix the ration so that the cows cannot sort it. Build up concentrate slowly after calving and split it into several small portions. Make sure the cows always have free access to clean water.

A good look at the herd tells you a lot. At least half of the lying cows should be ruminating. Manure should be porridge-like with hardly any undigested fibre. A well-filled left flank shows that the cow has eaten enough. A milking robot or sensor collar gives you the rumination time per day directly.

Around calving the rumen is under extra pressure, because the ration changes and feed intake is low. Good rumen fill during this period lowers the risk of metabolic problems. You can read how drenching helps in The advantages of drenching.

Frequently asked questions

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