Home > Horse Care > Vet Anatomy and Physiology > The Digestive System of Peaches
The Digestive System of Peaches, The Goat
Peaches is an eight-year old unregistered Oberhausi-Saanen cross. We’ve owned her two months; she was purchased from a local breeder who was going out of business. She was the second-last goat to go because she was a family pet. She’s fit right in here at Squalor Holler, our Southcentral Alaska-based micro farm. She follows me around when I do barn chores, and comes when I call her.
I am currently milking Peaches twice a day, every twelve hours or so. A little over a month ago, we bred her to Rover, who is a Togenberg-Alpine cross. We were initially getting a gallon of milk a day, now we’re getting a about three-quarters of milk a day. Her breeder, Matt Shaul, has told us that production is probably down because she’s pregnant. So far, we’ve made soap, cheese, and yogurt from her milk.
Peaches is fed grain twice daily, a low protein mix that was formulated by her breeder. She has kelp and free choice minerals available if she wants. She’s fed local grass hay—I was giving her our first-cutting hay, but switched to the second cutting hay when my veterinarian informed me that lower protein hay would be more digestible, and consequently, both decrease weight gain and up milk production. I’ve been careful to be consistent with her diet. Radical alterations (like too much carbohydrate) can lead to illness, and even an alteration in the length of the hay (chopped versus whole) can affect the microbial production of gas, volatile fatty acids, and ammonia, resulting in the fermentation process coming to a complete halt.
What follows is an overview of how a goat’s digestive system works. You’re welcome to come along, as I follow a wad of hay on its perilous journey, from the bale to the compost heap.
It’s 8 a.m. and I’ve just milked Peaches. On her way back to the pen, she grabbed a wad of hay from the backside of the feeder, chewed, and then swallowed it. She has no upper dental pad, so she did what goats do, which was pull the hay into her mouth. There was increased salivary production at this point in time because of stimulation of the parasympathetic nervous system.
The wad of food then traveled down the esophagus, a muscular tube that extends from the pharynx or throat, to the stomach. The place at which the esophagus enters the stomach is called the cardia. Peaches, like others of her kind, has three forestomachs, which are respectively called the reticulum, rumen, and the omasum. Yet another stomach, the abomasum, is considered to be the true stomach. The forestomachs are compartments of differing sizes and functions. Swallowed material must pass through them before reaching the abomasum.
Peaches’ wad of hay entered the smallest and furthest back (or cranial) compartment of the forestomach, the one called the reticulum. It’s separated from the rumen by the ruminoreticular fold. The lining of my goat’s stomach has a honeycomb arrangement of folds. These four-to-six sided structures increase the surface area of the reticulum and increase the absorptive surface. The motility of Peaches’ rumen and reticulum are generally called reticulorumen contractions.
If Peaches swallowed the nail that holds the mineral bucket in place, it might lodge in her reticulum. If reticulorumen contractions followed, she could be subject to hardware disease, which is when the object penetrates the cranial wall of the reticulum. Because the reticulum is separated from the heart by the diaphragm and is a short distance away, an object piercing the reticulum can penetrate the diaphragm and pericardium (outer membrane sac surrounding the heart) causing pericarditis or inflammation of the pericardium. This is good for goat owners to know, because caprines are curious creatures who will nibble at, and take in just about anything.
As I, an omnivore with a monogastric stomach, was eating my breakfast, Peaches, who is a herbivore with a three forestomachs and one true stomach, was beginning to digest hers. That wad of hay she swallowed earlier was now in the rumen, a large fermentation vat that processes plant materials into useable energy and cellular building materials. The rumen is actually a series of muscular sacs partially separated from one another by long muscular folds of the rumen wall called pillars. The pillars can close off certain sacs and in this way allow for the effective mixing and stirring of rumenal contents. Two things occur in Peaches’ gut shortly after she swallows her hay wad. She regurgitates the bolus, which is now called cud. This is called rumination. (Often, as she ruminates, I see a far off look in her eyes.) Sometimes she eructates or belches. This is Peaches’ way of releasing built-up carbon dioxide or methane gas. Eructation is essential for dispelling excess gas created by the fermentation process. To my knowledge, Peaches has not experienced bloat, a condition in which too much gas is trapped in the rumen. This happened to her herd mate, Rover, this past summer, but this would now be a digression that I will save for another time. Suffice to say, bloat is bad, and intubating a young goat is no fun at all.
Rumen motility is generally controlled by the vagus nerve in the parasympathetic nervous system. The rate and strength of contractions are determined by 1) pH, 2) Presence of volatile fatty acids, 3) consistency of foodstuffs in rumen and stretch receptors, and 4) Feedback from the brain and other parts of the GI tract.
Peaches and others of her kind derive much of their energy sources and cellular building blocks from the fermentation of rechewed plant material by bacterial and protozoal enzymes. This is called fermentative digestion. Like all goats, Peaches depends on these microbes for her digestive needs. Rumen bacterial surfaces have cellulase enzymes that digest cellulose effectively and transform the complex carbohydrate structure of cellulose into simpler sugars: monosaccarides and polysaccarides. This is very important: The glucose sugar produced by this process is not immediately available to the animal. Instead, the glucose liberated from the plant materials) and other carbohydrate sources, like starch, is absorbed into the microbes and converted biochemically into volatile fatty acids.
The VFAs are byproducts of the anaerobic fermentation process and, if allowed to accumulate, decrease rumen motility. However, the Peaches absorbs the VFAs into the bloodstream, and the liver converts selected ones to glucose. Peaches then uses the glucose in her rumen to generate VFAs, which are then taken into the body and turned back into glucose by her cells. Her other absorbed VFAs are used to produce adipose tissue, milk fat, and other essential components required by the rumen body.
The cud that Peaches has been digesting next goes to the omasum, the entrance of which is off the reticulum. The omasum’s primary function is to break down food particles further, and to convey the ingesta (it’s no longer a wad) into the abomasum. In addition, the abomasum absorbs VFAs nor previously digested, removes bicarbonate ions from the ingesta, and absorbs some water from the ingesta.
By now, it’s late afternoon. The ingesta have moved into the small intestine. Here’s where most of the nutrients are absorbed into the bloodstream. It’s made up of three sections, the duodenum, or short section, the jejunum, the longest section, and the ileum. The short ileum enters the colon, or large intestine, and is separated from it by the ileocecal sphincter.
Electrolytes and vitamins can be absorbed intact into the small intestine wall; however, carbohydrates, proteins, and fats must first be chemically digested. After mechanical breakdown, the food in the intestine is chemically digested by the enzymes in the intestinal lumen, and by enzymes in the microvilli brush boarder. The cells of this brush border have digestive enzymes and carrier molecules embedded in their cell membrane; this allows for the digestions and absorption of nutrients, vitamins, and minerals.
The general functions of the large intestine (cecum, colon, and rectum) are to recover fluid and electrolytes until they can be eliminated, and store feces until they are eliminated.
The cecum, which is located at the ileocecal junction, is more developed in ruminants than it is in other herbivores. Some microbial digestion of foodstuffs does take place here.
The rectum is the terminal portion of the large
intestine, and the anus is comprised of internal and external muscular
sphincters that allow for passage of fecal material. At about 3 p.m.,
Peaches defecates, leaving a pellet-like pile behind her. I, who am the
chief pen cleaner, go outside, scoop up the waste, and put it in a bucket.
I then take the day’s waste, about three five gallon bucketfuls
of manure and uneaten hay and put it in the compost pile. Come spring,
I’ll mix it with house scraps, horse poop, and chicken poop. In
time, I’ll have compost, which I’ll mix it with garden soil.
The Digestive System of Peaches, The Goat