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The Daily Vet by petMD

The Daily Vet is a blog featuring veterinarians from all walks of life. Every week they will tackle entertaining, interesting, and sometimes difficult topics in the world of animal medicine – all in the hopes that their unique insights and personal experiences will help you to understand your pets.

How a Cow Works

Today I feel the burning desire to share with you the science behind the magnificent beast that is the cow. They really are quite modest creatures, standing there in the field so sedately chewing their cud, while inside, they are factories of microorganisms working away non-stop at breaking down grass and grain, producing volatile fatty acids, and of course, methane. What fascinating (and gassy) creatures!


To begin, it is true that cattle (also called ruminants) have four stomachs. To look at these stomachs anatomically, they sort of appear like one giant odd-shaped sphere, but there are actually four distinct spaces within this sphere that constitute four different parts of the digestive tract. Let’s investigate this unique anatomy in a little more detail.


As a cow grazes, she is primarily consuming cellulose, the building block of plant matter that is difficult to digest. Cows swallow large chunks of grass at a time and then later, usually while laying down, they regurgitate this grass back up in order to re-chew it a second time. This process is called ruminating. This allows the grass to be as physically broken down as possible by the mechanical action of chewing before entering the digestive tract. Salivary enzymes mix with this chewed grass, beginning the chemical digestive process even before the grass hits the stomach.


Once swallowed a second time, the grass enters the first of the four stomachs, the rumen. This is the largest of the four stomachs and can contain up to 50 gallons of fluid in an adult cow. The rumen is basically a large fermentation vat. It is filled with “good” bacteria, protozoa, and yeast that are permanent hitchhikers within the cow in a symbiotic relationship, since they are the ones responsible for breaking down the cellulose. In fact, when cows become sick, oftentimes these microorganisms die off. This can make the cow even sicker, and we need to force feed her microbes from a healthy cow to re-populate her gut — sort of like when we eat yogurt with live cultures whenever we have diarrhea or take antibiotics.


Anyway, let’s take a quick step into biochemistry just for a moment. You may wonder how the heck a large animal like a cow gets any energy from grass. The answer lies in these microbes. As they digest the cellulose by way of fermentation, their metabolic pathways produce chemicals called volatile fatty acids (VFAs). The cow uses these VFAs as a primary source of energy. There are three VFAs produced: acetic acid, propionic acid, and butyric acid. These VFAs in ruminants and other large herbivores play the role of glucose in monogastric animals like humans, cats, and dogs.


So, back to the anatomy. Once grass is in the rumen, it mixes with the other ingesta that is there. As it mixes around in the rumen, it will make its way to the reticulum, the second stomach. The reticulum is a much smaller out-pouching in the frontal aspect of the rumen. This stomach aids in the mixing of digesta but also acts as a catch area for foreign bodies, like stones, twine, or bits of metal such as nails that a cow may pick up while grazing or eating from a trough. A condition in cattle called “hardware disease” occurs when a piece of metal is swallowed and perforates the reticulum. Sometimes, the rumen and reticulum are referred to as a single entity: the reticulorumen.


Next, the ingesta enter the omasum. This, in my opinion, is the weirdest of the stomachs. A small round organ, the inside of the omasum has many thin leaves of tissue that help absorb water and help filter large particles back to the rumen.


The fourth stomach is the abomasum, also known as the “true stomach.” Here is where digestive enzymes made by the cow herself act to digest proteins and carbohydrates, much like our own stomach acts. After this last digestive step, food passes to the intestines, where most of the absorption of nutrients and water occur.


Sheep and goats are also considered ruminants (classified by size as “small” ruminants) and have digestive systems exactly like a cow, except of course their rumens don’t hold 50 gallons; more like two. Other grazing animals such as deer are ruminants as well.


Horses, on the other hand, have to be complicated and not adhere to the “if thou art an herbivore, thou shalt have a rumen” doctrine, instead being “hind gut fermentors” with a large colon that tries to do what the rumen does, but ends up being slightly less efficient. However, despite the inadequacies of a horse’s digestive system, I will forgive them for this one simple fact: they do not ruminate, which I believe would greatly decrease their elegance.


No offense to cattle, but seriously. A burping horse? I can’t quite imagine that in the show ring.



Dr. Anna O’Brien



Image: Almog Ziv / Shutterstock



Comments  4

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  • Anatomy Class
    08/10/2012 11:27am

    Wow! Bovine anatomy class first thing on a Friday morning. Fascinating stuff!

    So.. if there are problems somewhere in a cow's digestive tract(s), is it pretty easy to figure out which stomach is causing the problem?

    What keeps grass from entering the first stomach before it's chewed a second time? Does the cow swallow differently? How is the destination determined by the "chew-factor"?

  • 08/14/2012 12:21am

    Good questions! It is relatively simple to figure which of the four stomachs is the problem child when something is amiss. In the reticulum, it's usually hardware disease (a penetrating injury due to a swallowed foreign body); in rumen, there's bloat, LDAs, RDAs (right displaced abdomasums, even torsions, as well as general inflammation, called rumenitis); and cows can get gastric ulcers just like humans in their abomasums - a dark, tarry stool will help clue me in to this problem. I don't generally see problems in the omasum.

    As far as the chew factor, rumination in a bovine is pretty innate, and mostly controlled by the autonomic nervous system. The first go-around, forage is swallowed in bulk and does enter the reticulo-rumen (could be both or either of these two stomachs) and then later on, regurgitated. Some microbial digestion does probably occur during that first swallow, but it is at its most efficient after the second chewing, when the cow has broken the long pieces of cellulose down into more manageable bits for the microbes. The act of rumination is a pretty imprecise process - stuff gets swallowed, then brought up at a later time, probably mixed with newly swallowed stuff and older stuff. To me, the act of rumination in bovines takes the place of the crop in birds (the organ containing rocks to help with mechanical digestion).

  • Grassy, Gassy Creatures
    08/10/2012 08:30pm

    I echo OldBroad's sentiments.

    Wow! This is fascinating.
    If reader's are scanning the headlines and skipping this posting, they are missing out. This is truly educational. Dr. O'Brien do you treat many cows with hardware disease? Do they ever require surgery?

  • 08/14/2012 12:29am

    Thank you very much!

    Once in a while I will see a cow with suspected hardware disease. I will suspect this condition if a cow goes off feed, starts to lose weight, and passes the "grunt test": when you press down on her withers and she makes a grunt. Doesn't sound very scientific, but I promise you it's taught in vet school! It works because when a cow with hardware disease hunches down to get away from you pressing on her back, she feels the pain of the foreign body in her reticulum and this causes her to grunt. With cows like this, I will put a magnet in them to help pull out the foreign body (hoping it is metal, which it usually is). Other meds such as gastro-protectants, B vitamins, and rumen flora from another cow to help re-populate her own gut flora, are also used. I don't do surgery on these cows, as it is often not financially an option for the farmer. Fortunately, the cases I've seen have not been too horrendous and have pulled through. Even in vet school, I never saw surgery performed on any hardware disease cows - as I said, it's usually just not financially do-able for the farmer.

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