Ask-a-Pro Response By Katy Mayhew, DVMcontact Doctor Mayhew: firstname.lastname@example.org
Hay there! As we head into the coldest months of Georgia winter, we are seeing a definite increase in the amount of hay our horses need to maintain weight with the loss of green pasture. Our beautifully stacked, cut and baled gold is finally moving out of the barn and into our horses! Hay is for horses after all…. or are horses made for hay? As we take a quick tour through the digestive tract of the horse, we will see how well designed they are for eating leafy (or sun-cured and baled) greens!
That’s right, you heard it straight from the horse’s mouth, and that is exactly where this tour begins. Horses use their lips to pull tender blades of grass out of the dirt and into their incisors to get a mouth full of green goodness. Other animals, like cows have muscular and flexible tongues and hard dental pads they use to pull food into their mouths. Horses are able to be very picky with their lips and can often sort through mixed grain or medications to only eat what they like. Additionally, horse’s cheek teeth (molars and premolars) work in a unique way to accommodate their fibrous diet. When people chew we move our jaw up and down to crush food between our molars. Horses chew side to side, or almost in a circle to grind their food. Chewing is not just important for preventing choke in horses, it also has a calming effect and promotes saliva production. Horses without enough hay or grass (collectively known as forage) can develop stall vices like cribbing, wood chewing and weaving. Once chewed, this fibrous plant matter becomes a soft ball or bolus and down the esophagus it goes. Saliva has several important jobs. First, it lubricates the food bolus and helps it travel down the esophagus smoothly. Once it gets to the stomach it helps to buffer stomach acid.
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The stomach is our next stop. Horses have two different areas of their stomach, the squamous (non-glandular) stomach and the glandular stomach. The squamous stomach is white and the glandular is a dark pink. The main difference is the glandular part of the stomach has special protective properties to prevent it from being damaged by the stomach acid it produces. The squamous stomach is more vulnerable and when acid splashes on it or the pH gets too low it can become corroded. This is a prime location for gastric ulcers to develop. The big hay bolus we are following has an important job to do here. Forage helps to increase the pH of the stomach acid also known as buffering (thank your high school chemistry teacher for teaching you something you can use in the barn!). The stomach acid becomes less caustic to the unprotected squamous part of the stomach. It also will create a floating mat on the surface of the stomach acid to help prevent splashing when the horse runs or jumps. Horses prone to stomach ulcers often do well if fed a small hay meal before being ridden as well as having constant access to hay or grass in turnout or their stall. While many horses need additional feed to provide nutrients to support heavy exercise or to maintain weight, grain-based foods will actually decrease stomach pH when it reaches the stomach acid and can increase the chance of ulceration. This is another reason forage should form the base and bulk of a horse’s diet!
After a quick swim in the stomach acid (generally 15-30 minutes) to inactivate bacteria and start digestion of the food bolus, we move into the small intestine. The small intestine is important for digesting and absorbing many nutrients in the horse’s diet. Horses can have 70 feet of small intestine! Our hay bolus zips along through the first two parts (duodenum and jejunum) before coming to a very important location. The ileum is the final portion of the small intestine and is the most muscular area of the entire organ. It is also sometimes slightly narrower and can be a location where impactions occur. In the Southeast these impactions are sometimes associated with thin, fine grass hay such as Coastal Bermuda. The most common history in horses suffering ileal impactions is an abrupt change in hay. It is very important to gradually introduce hay to your horse’s diet each fall and also to gradually introduce a new cutting or a new type of hay to allow the digestive tract to adjust.
Moving on from the ileum we come to a unique organ, the cecum. The cecum is a large sac that sits on the right side of your horse. When this organ is healthy and moving it will sound like a toilet flushing on the inside of your horse. The cecum fills with food (what your vet might now call ingesta) and begins a fermentation process. The fermentation allows horses to produce volatile fatty acids, vitamin K and B, and proteins which are essential to their metabolic processes and can only be obtained from fermentation of forage! Another biproduct of fermentation is heat. Forage consumption is one way horses can stay warm in the winter and is a major reason hay consumption increases when it is very cold.
We are getting close to the end of our tour, only two big parts remain for our hay bolus to travel through. This leg of our journey is only about 30 feet long, but may take days! In the large colon, our slushy hay bolus starts to be dehydrated. Horses absorb the fermentation products and water from their food in their large intestine and slowly it becomes more paste like. If horses do not drink enough water when on a hay rich diet, the large colon may allow the food moving through it to become too dehydrated, causing an impaction colic. Impaction colics are very common in the winter, especially after a cold snap. During the journey through the large colon, fibrous parts of hay and grass are broken down and digested. Finally, they move into the small colon which has the very important job of forming horse apples.
Our journey does not end here! The fecal balls (horse apples) have a story all of their own. You can tell if your horse is chewing well (is their big pieces of undigested hay in the balls?), if he is digesting all of his food (do you see whole oats or corn in his poop?), or if he is having a gut flora upset (diarrhea!). Knowing what normal manure looks like for your horse and how much he generally produces a day is very important for catching early signs of colic!
Horses have a very complicated and complex digestive system. While this brief overview hits on some of the more important factors when it comes to digesting grass and hay, the information available on nutrition and metabolism for horses is astounding. Below are two links to more on forage in horses and the actions of the hindgut (cecum, large colon and small colon). If you ever have questions about your horse’s diet or digestive system contact your veterinarian, extension agent or an equine nutritionist, they are all happy to help!