Equine Nutrition: The Evolutionary Origin of Hind Gut Fermentation (Part 3)

March 6, 2024

By Sarah Griffiths, DCH

Have you ever considered how horses came to be in our lives? From an evolutionary point of view, they are a fascinating species. Humans have only been in the picture for a tiny fraction of the time that they have been walking the earth. So what were they doing before we came along? Welcome to part 3 of this equine nutrition blog series where we are diving into the why and how of what horses should really be eating.

To recap: In Part 1: Are Modern Diet Recommendations Failing Horses?, I shared the disturbing statistics on the prevalence of equine gastric ulcers, colic and metabolic syndrome and why our industrialized feeding methods might be failing horses.

In Part 2: The Native Equine Diet: Exploring the Ancestral Forage of Horses, I share some of the research I’ve been doing into feral horse diet profiling and what horses actually eat in nature compared to what we are feeding them now. I also outlined the likelihood that horses have been able to adapt to rapid dietary and environmental changes in the last 100-150 years.

This week, we’re adding another piece of the puzzle: understanding the evolution of equine digestive system. What does it look like? When did it evolve and why? To answer these questions is to further understand what defines healthy equine nutrition.

Sadly, the digestive system of horses is still grossly understood. In mechanical/anatomical terms, we can explain it easily, however, the metabolic magic that happens inside this system is largely misunderstood. Modern equine nutrition recommendations are not honoring this highly evolved natural masterpiece. It’s my mission to help you understand it’s function from a higher level so you can feed your horses for resilience and longevity.

For the past 4 years, I have been studying the archeological, ecological, zoological and microbiological findings available on the evolutionary history of the horse’s digestive system. I’ve even had the privilege of viewing a horse dissection that gave me a whole new appreciation for the gut system of these special animals. I’ve made enough sense of the development of the equine gut to begin sharing it with you.

Evolution is a genetic survival mechanism that occurs out of a need for adaptation. It happens dynamically and in relativity to shifts in the host’s environment. During my studies, I went back in time to explore how horses evolved over the past 50 million years. Let’s dive in.

The Digestive System of the Horse

In terms of digestion, horses are hind gut fermenters. What does this actually mean? To understand it, let’s look at the digestive anatomy from different species including carnivores, omnivores, herbivores:

Photo courtesy of: Furness, J.B. (2022). Comparative and Evolutionary Aspects of the Digestive System and Its Enteric Nervous System Control. In: Spencer, N.J., Costa, M., Brierley, S.M. (eds) The Enteric Nervous System II. Advances in Experimental Medicine and Biology, vol

In relation to other mammalian species, it’s clear to see that horses have one of the largest cecal and hind gut digestive chambers. The purpose of this? Millions of years adapting to a diverse high fibre diet, requiring powerful digestive fermentation abilities. The cecum and hind gut of the horse is home to a highly evolved community of microbes that have been in symbiotic partnership with horses for millions of years. In the next chapter of this blog series, we’ll discover how this microbiome works and the differences that are being discovered between the gut microbiomes of wild and domestic equines. Spoiler alert: domestic horses aren’t doing well in this area….

What has the equine digestive tract evolved to process over the past 55 million years?

Horses have not always resembled their modern-day counterparts. During the Eocene epoch (56-33 million years ago), equids were small – about the size of an average dog. During this period, the environment was tropical and marshy in the areas of North America where these equid skeletons (Eohippus) have been found. Tropical conditions would have favored forage low in cellulose fibre (a complex carbohydrate). Their teeth also reveal that they had adapted to eating soft, less mature tropical plants that did not contain a significant amount of fibre. (1)

34 million years ago, the Oligocene epoch began along with a transition in climate to cooler seasons. Colder temperature resulted in the maturing of plants and significantly increased the fibre/cellulose content of the native forage. This is when the process of cecal and hind gut digestion became imperative to the survival of horses. Two new equids evolved – Mesohippus and Miohippus – with flatter teeth equipped to grind more fibrous foods and a larger body size to accommodate more digestive/microbiome surface area for the fermentation of a high fiber diet.

Equus (the modern horse) appeared approximately 4 million years ago during the Pleocene with an even larger body size and hind gut fermentation capacity. They also had flatter teeth than their descendants, indicating the ability to grind fibrous plant matter such as twigs and tree bark. These changes suggest that the climate remained supportive of fibrous forage in the environment for millions of years. Horses were able to specialize in eating a specific diet that did not compete with ruminant species.

The Dietary Significance of Cecal and Hind Gut Digestion in the Modern Horse

Horses evolved as cecal and hind gut fermenters – significantly different from other grazing animals like deer, cows and sheep (ruminants). As the climate changed during the Oligocene, it became cooler and drier, producing more distinct seasons and more mature plants with high cellulose content. Climate change coincided directly with the increase in the body size of early equids, increasing their digestive storage and microbial surface area – to get more nutrients out of highly fibrous plant material. This helped horses to maximize the sparse amounts of protein in their forage, conserve urea, avoid amino acid deficiency and work symbiotically with bacteria for essential nutrient synthesis.

Ruminants like cows ferment their food in their multi-chambered stomach over longer periods of time than horses. They don’t have a well-developed hind gut. Cows are known to have a high tolerance for metabolizing plant toxins and can survive on much less plant diversity than horses. Unlike ruminants, horses don’t house all of the same bacteria to synthesize the essential amino acids they need so diversifying forage helps them to avoid amino acid deficiency.

High Fibre Diets are Imperative to Equine Health

Given that horses have specialized to eat an incredible amount of fibre in their diet, it’s no wonder that today’s horses are suffering from epidemic rates of digestive disease. Equine anatomical evolution took over 30 million years to develop and we have dangerously modified their dietary intake in less than 150 years. Science reveals that significant anatomical changes like that of the equine digestive tract take at least one million years to evolve. (2) To suddenly change the diet in a 150 year time period is to increase the risk of diet-induced disease.

Today’s equine diet recommendations include:

  • one or two types of industrially produced grass and legume forage
  • ultra-processed feed stuffs from completely inappropriate food materials
  • synthetic vitamin and mineral rations
  • seasonal rotation is not even on the radar

It’s a far cry from the ancestral diet of horses.

Going back to the digestive disease statistics cited in the first part of this blog series, I believe that horses are suffering largely from nutritional insult. It’s time for some new ideas on how to keep horses healthy, not in terms of what’s convenient for us, but in terms of their ancestry.

In order to effectively address gastric ulcers, hind gut acidosis, fecal water syndrome and metabolic disease in horses, we need new tools – NOT immune-suppressive medications and industrialized diets. My mission is to help you understand how to feed your horses for health and longevity instead of for convenience. It’s way easier than you think and your horses will thank you! Keep your eyes on my blog and social media feeds to gain valuable insights and resources on how to feed your horses to honor their needs as a highly evolved species.

Stay tuned for the fourth entry of this series where we’ll dive into the differences that are occurring between the hind gut microbiomes of wild horses compared to modern horses. This will help us gain more clarity in the direction we need to take with equine nutrition.

Thanks for reading and stay tuned for more!


  1. Museeum of Comparative Zoology: The Evolutionary Strategy of the Equidae and the Origins of Rumen and Cecal Digestion, 1976
  2. Oregon State University: Lasting evolutionary change takes about one million years, 2011