Bovine biology series
Part - 7 Rumen and reticulum
The rumen and reticulum
I begin this lesson with a story. When I was in graduate school at the University of Wisconsin - Madison, I was asked one fall afternoon of 1975 if I would clean out the rumen of a fistulated cow. "Well, of course," I replied. So I wandered over to the campus dairy and faced this 1350-pound cow standing in a well-lit room. There she stood in a chute that resembled something they use at the fairgrounds to clip show cows.
Beside her, on the left side, was this steel garbage can with a plastic liner. On wheels. The herdsman instructed me to wear a heavy rubber should-length breeding glove, then a plastic disposable breeding glove, then a Playtex glove on top of all that. He unscrewed the lid, so to speak, and my goodness, what a smell there was! He did not flinch at all as he used a paper towel to clean the opening.
"It's all yours," he said, and out the door he went.
Well, this cow at that moment turned her head around to me, gave me a looking over as if to say, "be careful, young graduate student!".
So I dug in.
About thirty minutes later, fighting extreme exhaustion on my right arm and a garbage can full of about 200 pounds of ruminal contents, I finished. Thus began my indoctrination into the realm of research work, the rumen, and the reality of being a lowly graduate student! I do not remember how many more of these I did, but always I will remember this one. And a 1350 pound cow, Holstein, number 365.
We visit the rumen again, 19 years later.....
In many cases such a lesson of the rumen includes the reticulum as well, which are both considered to be the forestomach, and will be termed reticular-rumen in this lesson. The omasum (also part of the forestomach) and abomasum will be discussed in next month's series (part 8).
Another way of differentiating between stomachs is describing them as non-secretory, as are the rumen, reticulum and omasum, and secretory stomach, which is a secretory part of the digestive tract, just as it is in non-ruminants.
Now then, the contents of the esophagus actually empties into the reticulum, into a portion called the reticular groove. It is here that the digesta begins its microbial breakdown, albeit insignificant when compared to the rumen. One primary function of the reticulum is that heavier swallowed items fall to the bottom of this stomach. As such, many call this the hardware stomach because here metals and sand and the grit of intake accumulate. Other than this purpose, the function of the reticulum and rumen is the same.
The rumen is divided into two sacks, the dorsal (top and the largest of the two) and the ventral portion (bottom). The rumen is situated on the left side of the cow's abdomen. The ventral portion of the rumen is lined with small numerous papillae. The dorsal sack, however, because it is at the top of the rumen, is largely smooth muscle. There are other layers of muscle serving the purpose of rumen motility (movement).
The reticular-rumen comprises about 85% of the total capacity of the four stomachs. The four stomachs comprise about 65% of the total capacity of the digestive system.
Let's talk about motility. This term defined refers to movement of digesta. The nervous system, as in almost all muscle-control mechanisms, directs rumen motility. How? Well, several ways. Low pH in the abomasum is indicated in slow rates of passage. Thus the secretory stomach, the abomasum says, "let's see, too much acid (pepsin primarily, for protein degradation), I'd better tell the brain to send a signal to those muscles surrounding the reticular-rumen, and get some movement of digesta down my way to make use of this acid." And so the brain signals the muscles to move, and the rate of passage is increased.
Another way this happens is stimulus from the omasum. This third stomach functions as a place where water is absorbed into blood volume, thus to some extent regulates passage. In a dry diet, the omasum signals the brain "let's see, this is dry material, and I need some water volume here, so I'd better signal the brain to send a signal for reticular-rumen motility."
Low blood sugar also can cause motility, in that low blood sugar is a condition that represents a loss of homeostasis, so a signal is sent to the brain once again. You get the picture.
One last discussion point here about motility. Common sense tells us that if the ration is normal or reasonable for roughage, then motility will be normal. If the ration is comprised of higher than normal roughage levels, then motility is increased, because rate of passage is slowed. Again, a signal by the abomasum (too much acid and thus low pH) or omasum (too little water absorption for blood volume maintenance) are implied here.
The contents of the reticular-rumen are stratified, that is, are layered with the lightest, less dense material near the top and the finely ground portion of the ration near the bottom of the stomachs. A series of rumen motility waves stirs this mixture, as directed by the nervous system and accomplished by the musculature around the reticular-rumen.
Another function of these two stomachs is regurgitation. As you know from previous discussions, this is the term describing the formation of a bolus that is moved physically back into the mouth cavity for cud chewing. Saliva is added too. In this way digestion is increased of the ration. This function is involuntary, although the cow can slow the rate if she so decides.
A third function of the reticular-rumen is (.....burp, excuse me) eructation, that is, the expelling of gas from the rumen. The gas is directed into the lungs by that little traffic intersection we visited last month....the pharynx. From the lungs this gas is expelled during normal exhalation. Of course if eructation is impaired, bloat occurs.
How much gas is formed? As much as 10 -15 gallons of gas can be formed per hour during fermentation of digesta. These gasses are normal end products formed during the fermentation. They are primarily carbon dioxide, CO2, methane CH4, ammonia NH3, and to lessor extent hydrogen and hydrogen sulfide gasses.
The reticular-rumen is also known for its function as a fermenter of carbohydrates. Here is how it happens.
The environment is anaerobic, that is, without oxygen. The fermentation begins as microbes, made up of bacteria, protozoa and yeast-like fungi break down the digesta mass into glucose, a simple sugar, from the soluble portion of the ration, and another sugar called xylose from the more complex roughage portion of the ration. These sugars are then oxidized into pyruvic acid by another set of bacteria. Further microbial action converts pyruvic acid into lactic, propionic, acetic, butyric and formic acid. The formic acid is a true waste product, and is further converted into carbon dioxide and methane and (burp) directed into the lungs.
Lactic acid is an intermediary acid and sort of sits here until it can be converted into propionic acid. It is the sum of these acids that are called the volatile fatty acids, the end product of reticular-rumen fermentation. In the normal ration of 60:40 grain:roughage the volumes produced are 25% propionic, 60% acetic and 15% butyric acid.
The acids are called volatile fatty acids because of the way they are easily absorbed into the bloodstream, an activity that does not require energy, and one that occurs quite easily. The acids are really molecules that can be described as simple lipids. Once in the bloodstream, their fate is as follows: acetic acid as a precursor to other lipid molecules, propionic acid as a precursor to glucose formation in the liver, and butyric acid, which is a precursor to a ketone molecule that can be used for energy in muscle.
Protein is degraded in the reticular-rumen as well, as bacteria can degrade that portion known as rumen-degradable protein. The end products are amino acids, which bacteria use to build other amino acids. Some amino acids are further degraded, or deaminated with ammonia as a byproduct. Ammonia can result from non-protein nitrogen in the ration as well, and urea can diffuse back into the rumen contents from the bloodstream as a way of leaving the bloodstream.
In summing up, the reticular-rumen enjoys a symbiotic relationship with microbes, metabolizing roughage and concentrates into volatile fatty acids, amino acids, some vitamins, and waste products. The volatile fatty acids are primary metabolites that maintain bodily homeostasis and supply energy for growth and milk production. The amino acids are required for maintenance of muscle tissue and the formation of protein in milk. The waste products are converted into gasses, and through the lungs are expelled. Microbes themselves become part of the digesta mass, travelling into the next part of the digestive tract.
As such, a cow, a ruminant, is a valuable part of the food chain. She converts roughage that we cannot utilize into that which we can. Milk and meat.
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