As we started to use the horse more and more for work it was soon appreciated that the amount of roughage and grass that a horse could eat in any period of time was limited which meant that a large part of the day had to be devoted to eating which limited the time available for work.
This, together with the increasing demand for horses to perform at levels that require energy intakes above those able to be provided by their more 'natural' diet, has resulted in the addition of cereal grains and their by-products, as well as supplemental fat in modern equine diets. This has benefits but also potential disadvantages. These can be either nutritional or behavioural - for example the close confinement of horses, restriction of access to buddies and restriction of feeding to 'meal times' are all inimical to the horse. Further information is available in the paper The Natural Horse and Unatural Behaviour.