Equine Digestive Anatomy: The Stomach & Small Intestine

This week on the Friday with Finn blog, we take a step further into the equine gastrointestinal (GI) tract and take a closer look at the foregut. When discussing the foregut of the horse, the focus is on the stomach and the small intestine.

Telling horse owners that large meals can be detrimental to their horse’s gut, and that a forage-based diet is optimal is great – however, when you understand the anatomy, and what truly occurs in the GI tract of the horse, it tends to be more effective in influencing management practices.

The Stomach

The stomach in the horse is small. It comprises less than 10% of the total GI tract of the horse. Of domesticated animals, horses have the smallest stomach in relation to their body size. This favours the continuous movement of feed through their GI, as they are not equipped with the anatomy to support the intake of large infrequent meals.

If we look at the basic anatomical comparison below, it is evident that the gastrointestinal anatomy is significantly different. Not only is the stomach of the horse much smaller, but they also have a massive hindgut present that other monogastric mammals lack.

Image retrieved from: Furness, J. B., Cottrell, J. J., & Bravo, D. M. (2015). Comparative gut physiology symposium: comparative physiology of digestion. Journal of animal science, 93(2), 485-491.

The horse’s stomach secretes acid as well as some enzymes such as pepsin to begin the digestion process. However, the main role of the stomach is not to absorb the nutrients, it is to hold the food so that it can pass slowly into the small intestine.

Another unique anatomical feature is the two different regions that the equine stomach has. There is a glandular region as well as a squamous (also referred to as the non-glandular) region.

Image retrieved from: https://todaysveterinarynurse.com/wp-content/uploads/sites/3/2018/09/TVN1809F03Fig01.jpg

The squamous, or non-glandular portion of the stomach does not have the ability to absorb nutrients or secrete acid. However, the glandular mucosa is comprised of both gastric glands for acid secretion as well as mucus producing cells to protect the tissue from the very acidic environment.

A unique aspect of the acid secretion in the equine stomach, is that horses secrete acid constantly, it does not depend on feed consumption. This is why, when our horses undergo a fasting period it increases the risk of gastric ulceration – because acid is still being produced, therefore, the pH will be lower, resulting in a more acidic environment for the tissue to withstand.

The Small Intestine

The small intestine of the horse comprises about 30% of the GI tract, approximately 70 ft long! The passage rate of feed through the small intestine is fast compared to other regions such as the hindgut.

The small intestine is the primary site of digestion and absorption of protein, soluble carbohydrates, most minerals, fats, fat-soluble and water-soluble vitamins There are a variety of enzymes that aid in the digestion of the various nutrient classes including amylase, lipase, and protease enzymes. When we feed grains and concentrates to our horses, the goal is to have those nutrients digested prior to the starch reaching the hindgut as the hindgut is adapted to digest slowly fermentable fibres, not rapidly fermented starch.

When large starch-based meals are fed, the rate of passage through the small intestine will be faster as the stomach is unable to hold large amounts due to its small size. When this passage rate is too fast, there is less nutrient absorption in the small intestine, meaning a greater amount of starch will end up reaching the hindgut.

The passage rate (or mean retention time) of digesta through the stomach and small intestine is rapid, with the average time being 5 hours. However, when we look at the passage time for the hindgut (cecum and colon), the average is 35 hours!!

When large amounts of starch reach the hindgut undigested it will result in rapid fermentation and an increased production of lactic acid which drops the pH of the hindgut environment. This alteration in the hindgut ecosystem is commonly referred to as hindgut acidosis. More on this in the next blog post when we talk about the hindgut (cecum and large colon).

By Madeline Boast, MSc. Equine Nutrition

References:

1. Auwerda, P.M. (n.d.). Digestive Anatomy and Physiology in Horses. Iowa State University Extension and Outreach. Retrieved from: https://www.extension.iastate.edu/equine/blog/dr-peggy-m-auwerda/digestive-anatomy-and-physiology-horse
2. Hewetson, M., & Tallon, R. (2021). Equine squamous gastric disease: Prevalence, impact and management. Veterinary Medicine: Research and Reports, 381-399.
3. Van Weyenberg, S., Sales, J., & Janssens, G. P. J. (2006). Passage rate of digesta through the equine gastrointestinal tract: A review. Livestock science, 99(1), 3-12.