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Nutrition foundation: Appreciating and understanding ruminants

Woody Lane Published on 30 September 2015

Are we lucky or what?

We raise dairy cattle. Usually, we are very busy with daily chores and don’t have much time to contemplate the larger world of mammalian physiology, but we might stop for a moment and consider this: These animals are ruminants, and we are indeed lucky to raise a creature ideally designed to harvest sunlight.



Sunlight … a free and unlimited source of energy captured by green plants and converted into leaves, stems and roots. Much of the world’s land mass is covered by plants we call forages (grasses and legumes and other broadleaf plants). Ruminants are astonishingly effective in their use of forages; they are walking connoisseurs of green plants.

In one sense, a ruminant is a mammal that has evolved to contain a large colony of microbes in a gastrointestinal sac just upstream from its true stomach (abomasum) and small intestines.

In another sense, we may consider that these rumen microbes have guided evolution to provide a house for them in a safe and secure environment, in a mobile four-legged platform that can provide a steady source of substrates and seek out new sources of food for them. Clever devils.

Officially, “ruminant” means one who ruminates, and “ruminate” means to chew one’s cud. (This most assuredly does not mean someone who chews tobacco.)

Of course, our common livestock species – cattle, sheep, goats, deer, bison – are all ruminants, but so are wildebeest, pronghorn antelope, water buffalo, gazelles and the smallest deer – the dik-dik. And the largest ruminant? The giraffe. Whoa – chew cud? That’s food for thought ….


Is a rumen absolutely essential for animals to eat forage? Not at all. Horses are not ruminants, and they do a fair job of grazing forages. And so do elephants, who would be largely offended if someone called them ruminants. (Have you ever seen an elephant ruminate?)

And so do capybaras, the world’s largest rodents, who live in South American rivers and really enjoy an afternoon of grazing green plants. None of these animals are ruminants. So if a rumen is not absolutely required for grazing, what makes it so special?

Because of its many functions, which all work together to support a ruminant’s ability to obtain nutrition from plants and compete efficiently in the evolutionary race.

Let’s consider the rumen’s four major functions.

Rumen function No. 1

The first, and primary, rumen function is fiber digestion. Plant cells contain fiber, which is one of the main characteristics that make them plant cells. In terms of tonnage, fiber also constitutes the largest amount of nutritional plant material on earth, which would make it a convenient food source if it could be used.

Fiber is actually a complex matrix of large molecules like cellulose, hemicellulose and lignin. Let’s ignore lignin because it is not digestible unless someone attacks it with strong acids, oxidizing agents or TNT.


The other two major fiber components – cellulose and hemicellulose – are only digestible if an animal has the cellulase enzymes that can split their internal molecular bonds. The bad news is: No mammals have this enzyme.

The good news is: Some bacteria do have this enzyme, and the rumen is essentially a large house for these bacteria. The rumen, therefore, is a place of fiber digestion where bacteria break down fiber in a process called fermentation.

The products of this fermentation are small molecules called volatile fatty acids, which are then absorbed by the animal and used for energy and other metabolic needs.

Also, ruminants chew cud, which is really a novel way of recycling fiber through a crushing machine (the mouth) so it becomes easier for the rumen microbes to digest it.

In addition, the anatomical position of the rumen – at the beginning of the gastrointestinal tract, upstream from the true stomach and the small intestine – means many products of rumen fermentation and all the bodies of dead rumen microbes ultimately flow downstream into the lower tract where they can be digested and absorbed by the animal.

All this points to one thing: Ruminants are designed to extract maximum energy from the most common feedstuff (fiber) in the most efficient way possible.

Rumen function No. 2

The second rumen function is to convert non-protein nitrogen into proteins. To be technical, rumen microbes can convert many forms of non-protein nitrogen (NPN) into amino acids which the microbes then use to manufacture their own proteins.

These microbial proteins ultimately pass into the lower tract where they are broken down into amino acids and absorbed by the animal, who uses them to make its own animal proteins. What is NPN?

Things like nitrates, loose amino acids and other nitrogenous compounds that plants accumulate in their leaves, sometimes in large amounts. Also (and this is very important) a common NPN feed ingredient is urea – the same urea we use as a fertilizer.

If we include urea in a protein supplement along with an easily available source of soluble carbohydrates like molasses, rumen microbes can convert that urea nitrogen into amino acids, just like they do with plant NPN compounds. If you read the feed tags on some protein supplements and also protein “licks,” you’ll often see the word “urea.”

The bottom line is: Ruminants can convert non-protein nitrogen – which humans cannot use – into animal protein, which humans can use. Converting urea and nitrates into milk and steaks and lamb chops – it’s something to ruminate on.

Rumen function No. 3

The third rumen function involves vitamins, specifically the water-soluble B vitamins: compounds like thiamine, biotin, niacin, pantothenic acid, etc. Just like our livestock species, rumen microbes need these vitamins for their metabolic processes.

But these microbes don’t worry about taking one-a-day vitamin pills, they simply manufacture their own B vitamins in the rumen. Eventually, these vitamins pass out of the rumen and are absorbed by the animal from its lower tract.

Yes, you read that right – we don’t need to include B vitamins in ruminant rations because the rumen microbes usually make enough for everyone. There is, uh, one minor wrinkle: vitamin B12. Although rumen microbes can indeed manufacture their own vitamin B12, this molecule is built around an atom of cobalt.

Without cobalt, the microbes cannot make B12. But in practice, we can easily overcome this problem by routinely including cobalt in our trace mineral mixtures. We don’t need to feed vitamin B12 to ruminants, just cobalt.

This strategy provides enough cobalt to the microbes, and they can do the rest. (There is also some research about feeding extra niacin and biotin to high-producing dairy cows, but those are special situations, and the jury is still out.)

Rumen function No. 4

Finally, the fourth rumen function is detoxification. The plant world is a dangerous place, and many plants, including some common forages, can contain all sorts of toxic compounds. But before those compounds can be absorbed into the blood, they must pass through the rumen, which can act as an effective barrier.

Rumen microbes can degrade or alter toxic molecules so they are no longer toxic or are less toxic. Some examples are: nitrates, some phytoestrogens from legumes, some mycotoxins from moldy hay, toxic amino acids such as mimosine from the Leucaena tree, etc.

Also, the rumen contains so much fluid it dilutes some toxins below the critical threshold that would cause toxic symptoms. In a larger perspective, the rumen helps make life safer for ruminants so they can continue to consume forages.

The rumen is indeed something very special. Fiber, nitrogen, vitamins, toxins – all facets of a very complex organ that gives ruminants a real survival edge in a ruthless world.  PD

Woody Lane is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through his website.