Over millions of years of evolution, all aspects of the digestive systems of herbivores have undergone extensive modifications to be able to convert plant cellulose to energy. The horse is no exception.
Specialized teeth & muzzle
Horses have evolved with specialized teeth. The premolars and molars are enlarged and have ridged surfaces that act as a grindstone, breaking down the plant cell walls and making the nutrients contained within them available to the horse. The higher crowned, wider, and less angled incisors meet squarely and allow for improved cropping action. The amount of chewing required over the lifetime of a horse combined with the abrasiveness of soil and sand ingested while grazing means the teeth are prone to rapid wear, so a horse’s teeth continue to grow throughout it’s life.
Along with the changes in the shape and length of the chewing teeth and incisors, changes to the length and angle of the muzzle are evident when the skulls of prehistoric horses are compared alongside the modern horse (Equus Caballus). While there were subtle variations in muzzle angle and length in earlier stages of evolution, the characteristic long, angled muzzle of Equus Caballus first became evident in the Merychippus, which evolved from previous horse-like animals about 17 million years ago. It was also the first known grazing horse.
This elongated muzzle is advantageous to the horse because it enables it to maintain a view of the horizon and surrounding area while it’s head is lowered in a grazing position, enabling it to be constantly on the look-out for predators.
The fermentation chamber
No mammal is naturally able to digest plant cellulose due to the inability to produce the enzyme cellulase. To compensate for this, the guts of herbivorous animals are unique in that they have adapted special fermentation chambers. These fermentation chambers, the cecum in the case of horses, rhinoceros, and tapirs, contain cellulase producing bacteria that break down the cellulose in the plant walls and convert it to energy. However, the digestion of plant matter provides far less usable energy than animal protein so more of it must be consumed.
The importance of grazing
A horse would naturally graze for 16 to 20 hours a day, and can survive on grass and hay alone. However, modern management techniques largely prevent this from happening because many horses are housed in barns or paddocks with limited access to natural forage. Rather, it has become the norm to expect horses to adapt to a human-like eating schedule, with a portion of grain in the morning and evening, and hay in between. This diet is sometimes compensated with various supplements in an effort to maintain a healthy, balanced diet for the horse.
Having evolved over millions of years, the horse’s digestive system is not well-suited to the habits of well-meaning humans. Given the small size of the its stomach, it is essential that the horse eat small amounts of food frequently, as it is not equipped to handle large amounts of food at once, and doing so could have severe health consequences, including ulcers, colic, and laminitis.
Tips for digestive health
Ideally, the horse needs to graze almost constantly and failing that, it should be fed several small meals throughout the day on a set schedule. Basic guidelines for feeding and maintaining the digestive health of a horse are as follows:
- Ensure clean, fresh water is always available.
- Feed little amounts of food often and adjust according to the work load, temperament, and condition of the horse.
- Establish a routine and try to maintain the same feeding hours each day, allowing for seasonal variations to account for changes in daylight hours and time spent at pasture.
- Feed adequate roughage consisting of grass and good quality hay.
- A horse should have either grass or at least some succulent food such as carrots, turnips, beets or apples every day.
Bennett, Deb and Robert S. Hoffman. “Mammalian Species: Equus caballus.” American Society of Mammalogists. Published 3 December 1999. Retrieved from http://www.science.smith.edu/msi/pdf/i0076-3519-628-01-0001.pdf
Florida Museum of Natural History: Merychippus. Retrieved from https://www.flmnh.ufl.edu/fhc/merychippus.htm
Janis, C. (1976). “The Evolutionary Strategy of the Equidae and the Origins of Rumen and Cecal Digestion”. Evolution 30(4) December 1976. p. 757-774
Landels, J. 2012. “What’s on the Menu?” Academie Duello Accessed 2 Oct. 2016 http://www.academieduello.com/news-blog/whats-on-the-menu-2/
Landels, J. 2016. “The Grain of the Matter.” Academie Duello Accessed 2 Oct. 2016 http://www.academieduello.com/news-blog/the-grain-of-the-matter/
MacFadden, B. (1992). “What’s the Use? Functional morphology of feeding and locomotion.” Fossil Horses: Systematics, Paleobiology, and Evolution of the Family Equidae. Cambridge University Press, 1999, p.241
Texas Equine Dentistry: Equine Skull Anatomy, Interesting Facts. Retrieved from http://www.texasequinedentist.com/blog/2013/06/equine-skull-anatomy-interesting-facts/
University of Guelph. 2016. Functional Anatomy, Week 1 Presentation: The Gastrointestinal Tract – The Equine Digestive System