Proper gastrointestinal transit is only possible if the horse is given sufficient quantities of digestible forage (fiber).
For stabled horses, on wheat straw bedding, a minimum of 5 kg of hay is essential for horses weighing 500 kg. Horses on artificial bedding, due to respiratory disorders for example, should be given approximately 8 kg of meadow hay in order to cover their requirements in terms of dry matter.
Horses on artificial bedding will naturally consume approximately 3 to 4 kg of straw.
Straw cellulose is not as easily digested (lignin) as meadow hay (true cellulose).
Horses receiving "good quality meadow hay" will not fill up on straw, thus avoiding digestive problems (such as straw balls in the intestine).

Hay is therefore of paramount importance for stabled horses; hay should be reduced or withheld completely for grass-kept horses, depending on the quality (season) and quantity of available grass.
The later the hay is harvested, the more “woody” it will be, hence less digestible.
Hay should be stored in a dry, well-ventilated area; horses should never be fed mouldy or dusty hay, or hay contaminated by animal excreta (cats, mice, etc.).