The horse has evolved as a specialist herbivore and before domestication grazed grasslands, supplementing its diet with other plant materials. But, basically, horses eat grass; and the major nutrient in grass is fibre.
The modern horse’s gut is designed to digest fibre with two-thirds of the digestive tract devoted to breaking down fibre by fermentation.
Horses are ‘trickle eaters’ feeding continuously, small amounts of feed are constantly chewed, swallowed and acidified in the stomach to release nutrients prior to digestion and absorption in the small intestine.
Fibrous material is essential to create bulk in the gut, to ensure the gut contents are pushed along into the large intestine where the fibre fermenting bacteria break it down.
The horse’s gut depends on it being full to maintain its activity. By maintaining gut fill the whole process of transit is upheld and it is dependent on bulking for its wellbeing.
There are a number of processes involved in correct gut function which include gastric emptying, enzyme release from the pancreas and bile production, all of which impact on digestion and the subsequent environment of the gastro-intestinal tract. It also influences the populations of micro-organisms that inhabit the hindgut.