Evolution and species-specific feeding practices

Written by Mishel Metcalf for Hi Form Australia 2025

What does the term species specific really mean?

A diet that is species-specific is one that the animal has evolved to eat.  It is natural for its species or formulated to include only ingredients that are appropriate for that animal.

We don’t feed cats plants because they are carnivores.   We don’t feed dogs chocolate because this would make them very sick.  All species have their own unique and specific dietary requirements.

Over time we have become accustomed to looking at horse feeds based on the horse's use, such as racing, breeding, hard work, or no work, formulas. 

What we should be focusing on is the laws of nature and the physiological requirements of horses as a species.  It is possible to provide whole species-specific diets, whilst understanding each horse's individuality.  Horses require real food to support their digestion, which acts as its very own ecosystem.  Horses have eaten a 100% plant-based diet from the beginning of history until now.  Nothing has changed.

Evolutionary Changes

The evolutionary changes from then until now include an increase in size, reduction in the number of hooves, loss of the footpads, lengthening of the legs, fusion of the independent bones of the lower legs, elongation of the muzzle, increase in the size and complexity of the brain, and development of crested, high-crowned teeth suited to grazing. 

 Some of these features, such as grazing dentition, appear abruptly in the fossil record, rather than as the culmination of numerous gradual changes.

Horses were initially browsers, eating plants from trees and forages above.  I guess being only 50 cm high would make most of his forage located above his head.   

As the horse grew, so did its ability to both graze and browse for food.  By Merychippus who existed around 30 million years ago, the horses dentition was essentially changed from predominantly browsing to grazing.  This horse was approx. 100 cm tall.

4 – 5 million years ago Equus appeared which all modern equines evolved from.

From then until today, equines have evolved to eat plants.  They are classified as ungulates, which are large-bodied herbivores.  There are two types of herbivores, one is the ruminant such as cattle sheep and goats, the other is non-ruminant which includes horses, rabbits, hippopotamus, and rhinoceros.  

All of this group have a significantly sized large intestine and functional caecum where millions of microbes ferment and break down structural fibre into nutrients that can be utilised by the horse. 

The main difference between ruminants and non-ruminants is that ruminants are fore-gut fermenters and non-ruminants are hind gut fermenters.

As the horse evolved to eat tougher materials such as grass and herbage containing cellulous fibres, his dentition changed.  The teeth have developed very efficient grinding surfaces, and they continue to grow throughout life.

When the horse gradually began to move from forest areas out into the open plains. The physiological adaptations to this environment produced important changes:

1. The digestive system was developed to derive optimum nutrition and health from a variety of HIGH FIBRE, LOW ENERGY plains grasses and herbage. 

2. In common with other herbivores of the open plains, the horse became a 'trickle' feeder, large infrequent meals being inconsistent with a fast flight response from predators. The diet of the modern horse is often unsuited to these important physiological needs.

As we have seen, the horse no longer has access to multispecies swords, and he is probably fed only once, two or three times a day. In addition, when the domesticated horse requires more energy for work it is often provided in the form of cereal grains or other concentrate feeds, to which his digestive system has not fully adapted.  Understanding the consequences of feeding these is essential for the care of the horse.

The digestive process starts in the mouth. As the food passes through the system, it is broken down into a form which can be absorbed and utilized by the body. The horse's sensitive and prehensile lips with their attendant whiskers are very well adapted to locate, assess, and handle the feed he wishes to eat.  The lips are used almost like a hand.  The sensitivity, mobility, and power of the lips is extraordinary.

The front teeth, together with the lips and tongue, are used to get hold of the food and bring it into the mouth for grinding by the molars, or back teeth. The horse's powerful teeth reflect the coarse grasses he has evolved to eat, which require considerable mastication to break down the tough siliceous fibres.

The constant process of grinding such food has the effect of wearing the teeth down. For this reason, the horse has developed open-rooted teeth which continue to grow throughout his life.

The reason the modern horse must have his teeth regularly rasped is a reflection of the fact that modern food stuffs do not wear down the surfaces correctly. Proper maintenance of the teeth is vital to proper digestion.

The horse’s digestive system is designed to process small amounts of forage frequently.  A state of homeostasis occurs when appropriate amounts of energy are supplied for various life functions.  There is an entire ecosystem in the horse’s digestive processes that requires balance and harmonization to function adequately.   

Horses survived millions of years without us complicating things.  So why cant we just let them eat grass and live happily as their predecessors did?

Unfortunately, this is where the complexity comes in.  Paddocks and pastures around the world have become unsuitable for our domesticated horses.  Not through evolution of the horse but due to current and historical farming practices.

Introduced species of grasses can often play havoc on a horse’s digestion.  As I mentioned earlier, constant supply of high fibre, low energy and low nutrient forage is a species-specific diet. 

Most native grasses did not survive the introduction of high calorie, high sugar and high starch pastures, suited for rearing cattle and sheep for the milk and meat market.  These grasses were selected for their hardiness, meaning that they just keep coming back even after intense grazing.  Calories in vs calories out needs to be taken in to consideration.

As horses are housed in smaller pastures, they don’t have to travel great distances to receive the amount of forage required to supply energy to support their metabolism and bodily functions.  Less energy is expended to gather forage for food, causes another compounding problem when it comes to diet and exercise.   

The foundation of a species-specific diet for horses is fibre.  The type and quality of fibre needs to considered when formulating rations for horses.