Bovine biology series
Part - 10 Large intestine
The large intestine
After months of examining the digestive system, the end is near. Quite literally, for in this lesson we find ourselves learning about the large intestine. It is here that the finality of digestion occurs, that is, after the large intestine, whatever could have been digested, metabolized, altered by bacteria, and absorbed as nutrients for target organs, is lost once discharged from the body.
I have wondered about this statement for some time. In that perhaps as much as we know about rate of passage and digestibility, there is seemingly a fair bit of gross energy disagreed from the cow as manure nutrients. Around 30-35%, the loss is about one-third. I wonder why it cannot be 25% or 20%? When you call on the telephone and I tell you that feed cost per cow are $3.50 to $4.25 per day, then one third of this is $1.16 to 1.40. These are costs that appear unavoidable, as if in the digestion of nutrients through the digestive tract, the system is overloaded at all times.
But that is not the entire story of course. Remember that the most abundant organic (carbon) molecule on earth is glucose. But the great majority is linked by the bonds of lignin, that glue-like substance providing the stem of plants, the branches of leaves, and the tunnels of fiber through which water pulls nutrients from the soil into the vegetative part of a plant. The leaves. There is no mystery here. A hundred-year-old tree is made up of tremendous volumes of lignin, otherwise the tree would never reach its majestic height, filling the sky with branches and leaves and shade for us shorter folks. That is why upon the lumbering of this tree, the reclusive sawdust fiber cannot be added to the mixer wagon. The rumen bacteria just don't have time to break apart, or jackhammer the lignin away, exposing at last the sugar of glucose.
But eventually this does occur, doesn't it. The process of mineralization of organic material into a mineral of ammonium and then nitrate is a process by which the bacteria can undertake, albeit slowly if nitrogen is limiting. Why? Well, mineralization bacteria require nitrogen.
On the other end of the spectrum is the newly formed leaf of an orchardgrass tiller. Ah, the succulent leaves cows love to eat first, the grasping tongue of a mouth pulling away a leaf that is indeed only ten days old. Formed by photosynthesis, the carbon and oxygen of carbon dioxide and the hydrogen of water, a sugar molecule is formed. It is here, the leaf, that digestion of a plant is highest. Never have we altered the standard recommendation of harvesting plants at early mature days, just at the final point of vegetative growth. The plant, as if signaled by some internal genetic code, begins the reproductive phase of growth, and at this point, the energy of sun and water and soil nutrients is directed towards seed formation. For the plant's survival, lignin is increased in the stem ensuring an upright plant as the seed tissue arises from the core. I rather believe that in this way plants are ultimately making the sacrifice of growth, vegetative growth, so that the next generation can be provided the embryo of life.
Such is the case for us as well. Female’s reach a point of fertility, in which extension of the species becomes priority as growth is slowed or stopped. We know that cows, for instance, will commit to estrus only when that internal signal says, OK, time for fertility here, but I will stop growing as rapidly and probably will have to sacrifice some energy for milk production.
So it is that plants and animals are not so different after all. First growth, then reproduction, then maturation, then, well, things go several different ways, as we all know.
Goodness, did I get off the lesson!
In the ruminant the large intestine consists of the cecum, the colon, the rectum and finally the anus.
The cecum is a very small part of the overall digestive system, holding no more than about 2.5 gallons of digesta. It is in this small sack-like portion that some microbial digestion can occur. That is, perhaps in some rations the cecum will find itself able to increase the total digestible nutrients, thus reducing the loss as feces. Consider this.....if the rate of passage is slowed by increasing fiber intake, then the large intestine motility slows, and thus the cecum can get one last shot at digestion. This is good news, of course. Because if the rate of passage is increased due to a lack of fiber or have grain feeding, the large intestine cannot contain the voluminous liquid fraction and the brake is released, the feces is defecated often and is quite fluid in form.
So the cecum has some digestive function when the proper fiber level is present, slowing the digesta as the fiber slows and packs itself in the intestine.
The colon and rectum are a larger part of the digestive tract, holding as much as 7-8 gallons of digesta. The primary functions of these parts of the large intestine are slowing the rate of digesta and absorbing water and electrolytes. The colon and rectum secrete mucus into the cavity, enhancing motility as well as aiding in the mixing of digesta. At times you can see a shiny layer of this mucous on your breeding glove or on a fresh cow pie. It is slippery, of course, for the reason of lubrication.
Here are tremendous populations of Eschericha coli, the putrefying bacteria that produce toxic substances, all of which we find unpleasant from an odor standpoint.
The rectum is an extension of the colon and is where much water can be resorbed into the bloodstream, thus dehydrating the accumulating feces. This is significant in that without this function blood volume would be compromised. The body would dehydrate from a lack of water.....not a lack of intake but a lack of absorption in the digestive track and resorption in the large intestine, primarily the colon and rectum.
Diarrhea is characterized by rapid movement of digesta through the digestive tract brought about by a myriad of digestive and metabolic irregularities. Most are brought about by conditions in the diet that either alters the pH of the digesta or the degree of fiber. Certainly diarrhea is a major problem in young calves; most of which is caused by E. coli infections in calves that have not secured adequate immunity from high quality colostrum. E. coli are the first to inhabit the digestive tract, especially if the calf is born is an unclean environment (and realistically, these organisms are ubiquitous and cannot be completely avoided in a calving pen). These organisms become pathogenic by inhibiting digestion of nutrients as well as secreting toxins from which the newborn is incapable of dealing with unless colostrum is fed early on.
Diarrhea, while not as problematic in mature ruminants, can still be severe on animals without adequate rations or rations containing potential toxic substances. The digestive tract secretes minerals or excess liquid in order to dilute and flush out these toxins; the animal is dehydrated due to increased motility of the poisoned digesta, and severe weight loss of death can occur. Winter dysentery is an example of diarrhea in mature cows.
Finally the last part of the large intestine is the anus. As we all know, the anus is largely a sphincter muscle, a muscle under involuntary control until the nervation of the large intestine signals the brain and the anus opens. At this point the brain signals the various musculature around the colon and rectum and the anus is relaxed; defecation occurs. At certain times this may occur rapidly if the animal is scared or frightened. Why? As if a signal for readying the body for locomotion or movement, the bowel is discharged as the anus is relaxed and the muscles move the feces outside the body. The body now lighter, ready for faster motion, one less responsibility as blood flow is diverted to muscles and away from digestion.
The remarkable, adaptable body. What a wonder of digestion is the ruminant, that herbivore capable of cellulose digestion, the fibrous part of our organic world unusable by us except by the symbiotic digestion system of the cow....the rumen and her microbes.
Can it be that in order for our existence to be surely promulgated by the most important chemical reaction of all, photosynthesis, then it is the ruminant that makes available to the man and specie the conversion of these plants into our very sustenance? I think so, if we are in the grand scheme of managing this cow remembering one thing: that she is a digester of fiber and forage. This is above all the most important part of not only her digestive system, but also the very reason why the dairy cow is so integral to the carbon cycle that begins each day when the first ray of sunlight begins once again the vegetative growth of plants.
Now that we have the ruminant fed and fueled, we must help her convert this chemical energy into work. The movement of muscles requires oxygen almost all the time, so she must breath. So in the coming months we will examine the lungs and respiratory system, moving into the most remarkable transportation system every created.....the cardiac system and blood flow. In many ways the respiratory and cardiac systems are linked, so we will examine them in that way.
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