Transcript:

[0:00]

Hi everyone, welcome back to Mad Barn Academy — and if this is your first time tuning in today, then welcome! We hope to earn your subscription. I'm Dr. Fran Rowe, one of the veterinary nutritionists here at Mad Barn. Today's video will be part two of a two-part series on the equine digestive tract. If you haven't seen the part one video yet, go ahead and do that now — it's a quick video that covers the foregut, which will set you up for today's video on the hindgut. So, let's get started.

[0:33]

Our focus today is the hindgut, which encompasses the cecum, large colon, small colon, and rectum. So we'll start where we left off in part one — the transition from the small intestine, the ileum, to the first segment of hindgut. But before we do that, let's quickly chat about the concept of fiber fermentation, and what it actually means to be a hindgut fermenter.

[1:01]

Horses are herbivores — this we know — and thus they consume a plant-based diet. Horses evolved as hindgut fermenters, versus other herbivore species like ruminants — cows, sheep, goats, and related species — which ferment fiber in their stomachs, the rumen. But what does it mean to ferment fiber in the hindgut? The breakdown of fiber requires special enzymes that mammals actually don't produce. Instead, horses rely on microbes to do this breakdown for them.

[1:38]

There are billions of microbes that live in the GI tract — bacteria, fungi, and protozoa — and in the hindgut, these microbes specialize in the breakdown of fiber through a process called fermentation. Fermentation refers to a type of metabolism that occurs in the absence of oxygen, like in the gastrointestinal tract. Cellulose fibers — components of plant cell walls — are the main substrate for fermentation in the hindgut, though other substrates like starch and proteins can be used if they make it past the small intestine.

[2:21]

The main products of fiber fermentation are volatile fatty acids (VFAs) — including acetic, propionic, and butyric acids — which are either absorbed out of the gut and used throughout the body as a source of energy, or used directly by theocytes and cells of the gastrointestinal tract for energy (as with butyric acid). Waste products of fermentation include lactic acid, carbon dioxide, and methane.

[3:03]

All of that fermentation takes place in the hindgut. The first segment of the hindgut is the cecum. It takes up a large portion of the right side of the abdomen — essentially a large fermentation vat holding a large volume of water and digesta. The cecum is a unique structure because it is a blind sac — its entrance and exit are right next to each other — yet digesta will traverse the entire length of the cecum where it undergoes the first steps of fermentation.

[3:46]

The ileum empties into the cecum at the ileocecal orifice. Unique peristaltic contraction patterns move digesta down to the tip (apex) of the cecum and then back up to the base, in a mixing motion. Fiber may stay in the cecum for several days, undergoing slow fermentation. Digesta exits the cecum into the first segment of the large colon through the cecocolic orifice.

[4:22]

The main function of the cecum is microbial fermentation, along with absorption of nutrients produced by fermentation — namely VFAs, B vitamins, and vitamin K. A repeating theme throughout our hindgut discussion is impaction colic, and the cecum is no exception. It can become impacted with dry, fibrous digesta in situations where the horse isn't drinking adequately or motility slows. This can happen without severe signs early on — maybe just mild discomfort or reduced manure output — so monitoring water intake and manure production is key.

[5:43]

From the cecum, digesta moves into the large colon, which traverses both sides of the abdomen and holds about 22 gallons of watery digesta. The horse has four distinct large colon sections: right vs. left, and ventral (bottom of the abdomen) vs. dorsal (top of the abdomen). Digesta moves: right ventral → left ventral → left dorsal → right dorsal colon → small colon. The large colon’s main job is fiber fermentation, absorption of VFAs, and water reabsorption.

[7:02]

The hindgut acts as a water and electrolyte reservoir, but if too much water is drawn out, the digesta can dry and cause impaction. Adequate water intake keeps the gut hydrated and motile. Two anatomical narrowing points — the pelvic flexure and another in the right dorsal colon — are common impaction sites. The pelvic flexure is especially high-risk due to its sharp directional change and narrowing.

[8:59]

The small colon, primarily in the upper left abdomen, doesn’t ferment fiber. It resorbs water and forms fecal balls. Like other segments, it can suffer impactions. Last is the rectum, a short segment that stores and expels feces; it passes through the pelvic cavity above the reproductive tract (in females) and the bladder.

[9:46]

Altogether, the horse’s digestive process is complex. As in part one, I’ll end with examples of common hindgut conditions and diseases — links are in the description below. If you’d like videos on specific conditions, leave requests in the comments. Here are today’s references, and thanks so much for listening! Don’t forget to like and subscribe. Check out our other videos — we’re adding new content all the time. And be sure to review the extra links for more detail on hindgut diseases. Thanks so much — until next time.