R for Ruminant

It’s day 17 of this Farm Dictionary Advent Calendar and today, we’re delving into Ruminants

Right side profile of a brown Lincoln Red cow with a diagram of here stomach structure over the top of her stomach. Gold number 17 on red and gold bauble shape in bottom right of image.

Ruminant – Ruminants (inc. cattle, sheep and deer) are animals which have a four-compartment stomach made up of the rumen, reticulum, omasum and abomasum.

Honestly, I’m in my element here, I love learning about the sciencey stuff and how everything works. Ruminants’ stomachs and digestion processes are completely fascinating to me; I hope they are to you too!

Diagram of a ruminants stomach with each chamber filled in a different colour and labelled.
An Adult Ruminant’s Stomach

Ruminants, like cattle, sheep and even giraffes, are polygastric animals, which means their stomach is divided into compartments which all have unique functions. Whereas humans and pigs are monogastric animals, who have a simple, single-chambered stomach.

They’re herbivores and can get nutrients from plant-based foods, like fibrous forages and grains, by fermenting them in the first compartment of their stomach before the food moves on to be further digested.

Their name comes from the fact that to break food down further and stimulate digestion, they regurgitate fermented lumps of food, called cud, and rechew them into smaller particles before they can be digested. ‘Chewing the cud’ is a process called rumination, or cudding, and a cow will spend about 6 hours each day cudding- it looks like they’re chewing gum!

Apparently, the word ruminant actually comes from the Latin word ruminare, which means “to chew over again”.

So, let’s follow some hay through a cow’s digestive system…

Brown Lincoln red cows eating hay out of a feeder inside a shed.

After the mouthful of hay has been chewed briefly and made into a sticky ball with saliva, it’s swallowed and enters the Rumen.

Rumen – A ruminant’s first stomach compartment. The rumen is a huge, fermentation vat which digests food with the help of millions of bacteria, protozoa and fungi (microflora). It develops with age.

The rumen is mostly on the left side of an animal and is their biggest stomach compartment. An adult cow’s rumen can be as big as 150 to 200 litres- nearly the size of a big, metal oil drum!

It’s full of microbes that ferment the food and turn its nutrients into different types of acids to feed both the cow and the microbes themselves. The microbes help feed the cow but also become food themselves later on.

The bottom sacs of the rumen are filled with a sea of liquid where most of the microbes live. The top ones are filled with the gasses that are produced in the fermentation process. And floating on the liquid is a mat of undigested fibre where all the swallowed food first lands.

The rumen contracts and churns the feed, dunking it in the liquid and bugs, and spills some of the liquid with those bugs in, and some of the heavier balls of food, into the second stomach, the reticulum

Reticulum – A ruminant’s second stomach compartment. A continuation of the rumen.

The reticulum is smaller and looks like it’s lined with honeycomb. It absorbs small particles of food, liquid and microbes. And it has sensors that detect the big lumps and pushes them back up the cow’s oesophagus to be rechewed (there’s that cudding!).

Anything really heavy that’s swallowed, like mineral boluses, drops into the reticulum and stays there.

The rumen and reticulum pass food back and forth, like pass-the-parcel, until its all in small enough particles to be absorbed. From there, it goes into the omasum

Omasum – A ruminant’s third stomach compartment.

The omasum is lined with leaves of tissue that look like pages of a book. These pages grind the food down further as it moves through them and absorb minerals and most of the liquid.

The remaining food then moves into the Abomasum.

Abomasum – A ruminant’s fourth stomach compartment. Young ruminants use this for digestion when most of their diet is milk.

The Abomasum is known as the ‘true stomach’, it’s the one that’s most similar to a human’s stomach and it digests the food in the same way as ours does, using acidic gastric juices and enzymes to break the food down further before it moves into the intestines to finish the digestion process.

It takes time for the rumen to grow and develop- it’s quite amazing how, at the beginning of their life, a calf or lamb’s abomasum is their largest stomach compartment, but by adulthood, the rumen is!


The digestion process is so long that it’s no wonder cows only spend a few hours a day actually eating! (Although ours seem to eat 24/7…)

I hope this made sense and didn’t get too technical. Writing about it has really helped me understand this unique process further, and also given me some handy trivia knowledge- did you know there are over 150 species of Ruminants dotted around the world!?

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