Simple System was founded on the principle of supplying you with the food nature intended your horse to eat. Horses evolved on the grassy, mainly treeless plains of Central Eurasia, and their digestive system is adapted to suit what grows in their natural environment: grass, herbs and shrubs.
Straw is the dried stalks of cereal grains - such as wheat, barley and oats. Although by definition members of the grass (Gramineae) family, cereals are the product of human engineering over thousands of years. They have been selectively bred for traits that improve cereals as a food source for humans. One of the traits is stiff straw that will not tend to break or bend, thus protecting the valuable grain that is used to make foods such as bread and pasta. Modern cereals are so far from their origin that they have lost many of their natural mechanisms (such as seed dispersal) making them unlikely to survive 'in the wild'.
Straw is very high in lignified fibre, meaning it cannot be fermented by the horse's gut microbes. To increase the feed value of the fibre in straw, it can be treated with sodium hydroxide to make Nutritionally Improved Straw (NIS). Sodium hydroxide, also known as caustic soda, is frequently used as an industrial cleaning agent. It is used in the manufacture of wood pulp and paper, as well as soap, detergent and drain cleaner. NIS was originally developed for cattle, not horses, as a cheap source of fibre.