Bovine biology series
5 - Digestive system introduction
The beginning of the digestive tract
We have examined four components of the digestive system that may be collectively called the extramural glands. They are not a part of the digestive tract directly, but play a role in the storage, modification, synthesis and excretions of that portion of digesta that are metabolized. Said another way, if we may think about the total energy of any feedstuff or ration, that portion digested is the metabolizable portion. Some of us old timers use the phrase TDN. Of course youngsters in school now use the more accurate net energy terminology (an actual measurement, whereas TDN is mathematically determined).
So for the next few parts of this series we will examine the digestive tract itself, beginning with the mouth and ending with the anus. We know, for instance, that as the ration is fed, many changes occur along the way, physical, chemical and modified by microbes. Then in the feces what remains is that portion not absorbed by the cow or inhabitants of the digestive system. Did you know that the digestive tract is inhabited by microbes, very important little guys, that if they disappear we have major digestive problems, such as diarrhea or inadequate digestion of food?
We will touch some of these interesting subjects in future months.
We begin with the mouth, tongue and salivary glands. Each serve a vital role as old bossy reaches through the lockups, and what is this? Why, here is a mixed bag of feed running about 30% moisture, some long green stems, some yellow cereals and the white whole cottonseed. And there is some of this highly palatable soybean meal, brown distillers, and yes, my favorite of all, grass silage. It is freshly coated with brown molasses! Yummy! I'll eat over eighty pounds of this if you keep it fresh and clean and put a mister over my back when I am warm. Now if you don't mind, let me begin.....
The cows' mouth is really a complex set of muscles, bone and epithelial tissue. The mouth is comprised of lips, teeth and the tongue. They are responsible for grasping the feedstuff, be it in a feed bunk or the blades of grass in the pasture, placing an amount in the cavity of the mouth, then a bolus is made when saliva is added, this bolus is swallowed. This is a complicated procedure, in that just the right amount of feedstuff is taken into the mouth, and just the right amount of saliva is added. If these are not done properly, the cow finds herself having difficulty swallowing, and intake goes down.
We call this process prehension. When a cow grazes, that big tongue is really important. Why? Well, old bossy reaches or extends the tongue out of her mouth cavity. After wrapping it around a selected quantity of herbage, she withdraws the tongue back into her mouth, pulling the blades of grass along. The grass comes into contact with the layer of four incisor teeth on bottom of the mouth, and the dental pad on the top jawbone. There are no incisor teeth on the upper jaw, instead there is a dental pad, made up of a very hard and durable connective tissue. The grass is compressed by this vise-like action of the jawbones, and the cow moves her head slightly upward, severing the grass from its base.
She continues this grazing process until enough herbage is inside the mouth cavity. Then she lifts up her head, chewing a little while. What she is doing is gathering this feed into a bolus-like wad of feed. This is called mastication. Part of this chewing action also is the first physical action of the mouth, in which long grass fibers, or perhaps the long stems of that expensive eastern Oregon hay, are being shortened, broken apart, and readied for the trip into the rumen.
In especially the grazing mammals, mastication is a very important part of digesta preparation. Because not only is the feed physically changed, it is being moistened with saliva.
So, mastication serves two primary purposes: 1. Begin the physical breakdown of forage, and thus increasing the surface area for digestive process further along in the gut, and 2. The addition of saliva so the action for swallowing is aided by lubrication of the throat and esophagus.
The mouth, then, is a grinder. A feed bolus is ground by the sliding of the lower jaw across the upper jaw. Did you know that the upper jaw is wider than the lower jaw? Well, it is and that is why the movement of the jaw is from side to side. The chewing action of the mouth can occur on one side or the other of the mouth, but not both at the same time.
I remember my first tongue sandwich. Tasted like tuna fish. But I buried it in mayonnaise, and got though it OK. What is this tongue, anyway? Well, it is really a very strong muscle covered with a mucous membrane keeping it very moist. It is attached to the chin at the very base of the mouth.
Do cows have taste buds? Can they tell whether or not you have added molasses or have forgotten? Yes to both. Taste buds in the cow are found in the epithelium lining of the mucous membrane, specifically in the tissues called foliate, fungiform and circumvlate papillae. You might think of them as cells that occur every so often that have an exposed pore, where feed can come into contact. When feed does, the taste bud cell takes a bit of this feed, sending a signal to a nerve cell at a spot in the brain that says, oh yes, the sweetness of sugar. Must be molasses present. This is a pleasurable occurrence, so I will eat more. And so old bossy is no dummy. She tastes the desirable sweetness and continues eating. All is well.
Taste buds are found at the base of the mouth and at certain locations near the base of the tongue. In the cow there are no taste buds on the tip of the tongue.
What about the salivary glands? These are indeed important in the scheme of things.
First of all, the cow secretes a range of saliva volume dependent upon the moisture level of the feedstuff. This range in the mature lactating cow is 20 to 40 gallons daily. A man, for instance, secretes about half a gallon.
Saliva consists of primarily water, but also electrolytes (pH alkaline), and some protein substances. The production of saliva, accomplished in the saliva glands, requires energy. The substrates used for saliva production are of course the blood. In the saliva glands, the appropriate mix of water, protein, hydrogen ions, potassium ions, chloride ions, and sodium ions are accumulated in a water media.
The diet has great influence upon the amount and type of saliva produced. If a cow is grazing on that lush perennial grass pasture, lubrication of the feed bolus in the mouth is largely satisfied by the moisture content of pasture. The saliva, then, is a more concentrated mixture of protein and electrolytes; the total volume of saliva is nearer the twenty gallons daily, or even less than this depending upon the level of grain feeding. If old bossy is eating a very dry ration, say a total hay and grain ration, then the volume of saliva is increased tremendously because of the lubricating function of saliva in assisting the swallowing of this rather dry feed bolus in the mouth.
Saliva is a buffer, at least to the upper portion of the digestive tract, the rumen. That is, the alkaline pH of the saliva, 8.1, helps maintain the rumen pH at 6.2 to 6.4.
We can run into some real acidosis problems in the rumen without adequate buffering. One example of this is overfeeding of grain, in which the need for saliva is diminished because there is very little mastication or chewing. The ration is already ground, somewhat, so the cow simply does not require the chewing and subsequent release of saliva. Rumen pH drops and we have a sick stomach. We cannot forget the reason the rumen is there.....the digestion of roughage’s, forage, grasses and hays, and along with the wonderful rumen microbes that digest this fibrous feed into smaller portions the actions of the lower digestive tract can handle. We remember too that one action of a cow is cud chewing, in which the mouth is used to further the physical action of chewing. At this point more saliva is added, providing the buffering necessary for proper rumen function.
My first thought about saliva volumes was the tremendous need for water intake. We talked about that last month when we discussed the kidney. Here again, we find that the most important nutrient for a cow, and for all animals, is water.
From a practical standpoint, some of you have or are considering the addition of water to a very dry ration. This serves several desirable purposes, including aiding swallowing, getting water into cows, keeping dust under control, and easing the demand of power on the mixer drive-train. Additionally, water supplants saliva production as we drive cows towards higher dry matter intake.
Finally, a cow at rest, laying in a free stall or resting in a pasture paddock will spend a fair bit of time chewing her cud. What amount of time? Published reports indicate about 40,000 jaw movements in cows. At one per second, this amounts to eleven hours. So your cows should be chewing their cud almost half of their life; otherwise, there may be problem of inadequate roughage intake, which opens the door towards many metabolic problems. Take a look at this as you walk the barn. See if half of the cows, at least, are chewing their cud while at rest. In fact the figure should be more than half, because it is during this time of rest when the majority of cud chewing takes place.
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