How to Accurately Feed Your Horse a Forage-Based Diet

In the hindgut of the horse there is a diverse population of microbes including bacteria fungi and protozoa. These microbes require a stable ecosystem and are sensitive to environmental changes such as a drop in pH.
It has been documented that when a horse is maintained on a foragebased diet the microbial population in their hindgut is more stable. When large grain meals are fed or when the passage rate of grains or concentrates is too quick too much starch will reach the hindgut. This is a situation in which the pH will drop due to overproduction of lactic acid. The resulting condition is termed hindgut acidosis. This digestive upset is often due to lack of forage and the overconsumption of grain.
When domestic horses are fed in a way that is supportive of their digestive anatomy there are clearly reported health benefits. Multiple studies have evaluated foragebased diets and their impact on gastrointestinal wellbeing. The commonly reported benefits include a more stable microbial environment in the hindgut longer chew time for forages vs grains a reduced risk of stereotypical behaviour and a reduced risk of gastric ulcers. Lack of forage in the diet also increases the risk of colic.
Evidence supports how imperative a foragebased diet is for horses. So how did the pattern of feeding large grain meals even start?
Historically a primarily foragebased diet was inadequate in meeting the energy requirements of working horses because of two contributing factors the hay being produced was often of lesser nutritional quality than the hay available today and horses had different job descriptions than most of todays horses. For example in days past horses were primarily used as work animals but many horses now live a more leisurely lifestyle.
Nutrient Requirements 
Horses require six key classes of nutrients water carbohydrates protein fats vitamins and minerals. A forageonly diet will not meet all these requirements. Often the hay will meet the carbohydrates protein and fat requirements especially for those easy keepers but the vitamins and minerals will need to be supplemented.
For example vitamin E is not stable in dry hay. Therefore if your horse is being maintained on dry hay and not supplemented with vitamin E he will likely be deficient. Trace minerals are another common deficiency when the diet is forageonly. Trace mineral content in the hay will often reflect what is in the soil. For example many geographical areas in Canada e g. Ontario