Horses were designed to eat small amounts often. If at all possible, roughage should be available all the time. It not only allows the digestive tract to function properly, it gives the horse something to do besides chew down the barn and engage in other vices. If concentrates are fed, a general rule of thumb is to not feed more than 0.5% of the horses body weight in concentrate per feeding. The reason for this is that sugars and starches should be digested and absorbed in the small intestine. If larger amounts are fed at one time, much of the sugars and starches can get into the cecum where they can upset the normal conditions and result in colic and/or founder.
Example of amount and frequency:
2% of weight = 20 lb. (estimated total feed)
1% of weight = 10 lb. (minimum roughage)
0.5% of 10 lb. of concentrate = 5 lb.
This horse should be fed about 20 lb. of feed per day, 10 lb. of which should be roughage. He should be fed no more than 5 lb. of concentrate per feeding, so he should get at least two feedings per day. Feeding more often would be better, however, given the design of the equine digestive tract.