Around a third of a cow is its rumen. That’s stomach number one. It's huge - certainly taking up well over half of her abdomen. The rumen is where a lot of magic happens - performed by trillions of bacteria, yeasts, protozoa, archaea and bacteriophages (like viruses, except they parasitise bacteria). Let's just call that lot "the bugs". One tablespoon of rumen liquor has more bugs than the total number of human beings who have ever lived on Earth. And as the rumen is around 200 litres, that’s around 10-20,000 tablespoons-full.
These bugs all live together in a complex ecosystem which makes the Amazon rainforest look boringly tame. Together, they're called the "microbiome". They move around, they breed, they kill each other, they live peacefully on each other, around each other, and even inside each other. And it's like any other animal ecosystem - some are predators, some are prey and others are scavengers. The prey bugs tend to eat plant fibre - that’s cellulose, hemi-cellulose and even, to a degree, lignin (the woody bits). This fibre is all the bits of a plant that you and I would not be able to digest. These prey bugs (foragers, if you like) eventually get eaten by apex predators, which predominantly are the protozoa. The biggest of these beasts can actually be seen darting around rumen fluid with the naked eye. I’ve put up a little video showing the protozoa in a sample of rumen fluid.
Amazing aren't they?
Now, when you feed a cow, it's very different to when you feed a pig, or a dog, or a chicken, or a human - all known as “simple stomached animals” who don’t have a fermentation vat like the rumen, and so who can’t digest forage in the same way. In fact you're not feeding the cow at all, but instead you're feeding the rumen bugs. The cow then lives on the products of fermentation that these bugs eventually produce.
There's two categories of product. The first are the "volatile fatty acids” (VFAs). These are acetate, butyrate and propionate, if you want the detail. The cow absorbs VFAs into her bloodstream directly through the rumen wall and this provides the vast majority of her energy requirements. So, whilst we get our carbs (energy) from sugar and starch (bread, pasta, rice etc.), cows are powered by VFAs.
The second products of fermentation are the bugs themselves, and in particular the apex predators, like the protozoa. Eventually falling to the bottom of the rumen, the fluid here becomes a protein-rich soup which provides the cow the vast majority of her amino acids (protein building blocks) for muscle, milk etc. Our dietary equivalent would be steak, beans, nuts and cheese etc. You might see where I'm going with this: rumen bugs eat grass, which we can’t digest; they feed the cow with energy and protein; the cow feeds us with high quality and digestible protein, as if it has been magicked up from nowhere...
A lot of people know that cows have four stomachs: the rumen (1), the reticulum (2), the omasum (3) and the abomasum (4). The rumen is this vast fermentation vat - stick grass + water in it, and VFAs + a proteinaceous microbial soup come out the other end.
The reticulum is really just like a little pocket off the rumen, and for all intents and purposes we can consider it the same thing. So, on to the omasum, stomach number 3, not much larger than a fist, and whose sole function is to reabsorb and recycle a lot of the water from the rumen fluid (microbial soup), or else cow poo would be even more runny than it already is. Now we're on to stomach no. 4, the abomasum. This is about the size and shape of a rugby ball, and it is the "true stomach”, very similar to our own. In fact, from this point onwards, a cow's innards are much like our own. The microbial soup has to be digested: enzymes work their magic, and nutrients need to be absorbed, then finally out of the rectum comes the leftovers which makes up poo. Similar to us, cows have a whole set of other bacteria in their hindgut (large intestine), all of which do important jobs and keep the digestive system ticking along.
So that's a little introduction to the cow's digestive system. My next post will delve a little deeper into the fermentation process, and come back to one particular rumen bug, the archaea. Fascinating little critters that they are!
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