What’s a Cud? Something to Ruminate Upon
by Ben Jensen, Livestock Manager
Have you ever heard the old bit about cows lying down before a storm? Well…it’s baloney. Cows do lay down A LOT, though, don’t they? Why? Why do you always see horses grazing and typically not laying around? I’m going to tell you why, and it all has to do with the difference between the digestive systems of horses and cows. Here goes nothing.
You’ve probably also heard that bit about a cow having four stomachs. Guess what? Baloney! Well, not exactly baloney. A more accurate way to explain a cow’s digestive system is one big stomach with four compartments. We don’t need to worry much about the names or exact functions of these compartments, but I’ll tell you anyway: the abomasum, the omasum, the reticulum, and the rumen. The rumen is where most of the interesting stuff happens, so let’s talk about that part.
The rumen is where rumination (i.e. fermentation) occurs and is also where the name “ruminant” comes from. Cows, goats, and sheep are all ruminants, meaning they digest cellulosic, fibrous feedstuffs via a big fermentation vat in their digestive system (the rumen!). The rumen is inhabited by a vast ecosystem of micro flora and bacteria that facilitate conversion of feedstuffs into energy.
Now let’s think about a compost pile for just a second. There actually aren’t a whole lot of similarities between the composting process and the fermentation process, but let’s think about composting anyway, just for confusion factor. Does long, stalky hay, whole loaves of bread, and whole lobsters compost very quickly? Not really. Grind up all that hay, bread, and lobster, add some manure and sawdust, and you’ve got a great start for a nice pile of hot compost. Why? High surface area, of course! Small particles have relatively more surface area where microorganisms can attach and work their magic. The same principle applies to how a ruminant breaks down long stem hay and grass. A cow takes a bite of hay, grinds it up as best she can during “mastication” (fancy talk for chewing). When it comes to the cow, it’s all about QUANTITY of feed. They do not have prehensile lips. They have a big tongue and a big grinding plate and they take big, indiscriminate swaths of fibrous, cellulosic feed into their mouths and then just let their complex digestive system do the work.
Cows have a grinding plate? Oh yeah, cows don’t have top front teeth (didn’t you know that?). Instead, they have a hard flat dental plate and bottom teeth which is just right for a good grinding effect, like an old flour mill. You notice a cow kind of chews back and forth as well as up and down (oh you don’t? Well look closer next time!!), which helps facilitate the grinding of the feed. This exposes as much surface area as possible with mastication, but it’s not quite enough. Particles of feed only pass into the small valve leading to the rumen based on their specific gravity (Wiki it), so additional physical grinding and breaking down need to occur…
This is where the CUD comes in! This is why you see cows laying around, chewing their cud! A cud is simply a bolus of feed that is moved back up a cow’s esophagus via “reverse peristalsis” (there’s a Jeopardy answer for you) and back into her mouth to be further ground up. A sick cow will often “lose her cud” as the old cowfolk put it, meaning her digestive system is just not functioning correctly.
So there’s that. Now you are an expert in rumination. But why are those horses ALWAYS eating, and eating, and eating? Well, horses are not ruminants. Horses are monogastrics. Think of it like this: mono=one, gastric=stomach. A one-chambered stomach. Pigs and humans are also monogastrics. We have a pretty Plain Jane digestive system, relatively speaking. We eat something, it goes into our stomach and intestinal tract, our bodies use what it can, and out comes the rest in one form or another. Horses have those soft noses and prehensile lips (remember Mr. Ed?) to pick and choose the best bites of feed. Their stomach is only about as big as a coffee can. Gotta eat pretty much all day to maintain that large a body with that small a stomach! Horses tend to select a higher quality feed per bite as the fermentation site in a horse’s GI tract is their cecum, which would be our appendix. Our appendix doesn’t do much other than cause emergency appendectomies at 2am, but the cecum in a horse makes up 16% of their entire GI tract. And that is saying something because I have assisted in colic surgeries where the intestines are spilled for resection or untwisting and let me tell you, there is A LOT of intestine in a horse.
Now next time you drive through the farm and see the cows contentedly chewing their cud and the horses grazing away you’ll know what’s going on and why. You might even want to just stop and enjoy the view and forget all of this all together.