Let us lay this old horse tale to rest now. You cannot feed too much hay to most horses.
To some, this is so obvious, but to others who stall their horses or utilize dry lots because of geography or limited land, it may appear novel.
Horses should have the ability to graze or eat hay 70% of each day. This is a horses natural behavior, and it sets a horse up for better health, weight and less risk overall.
Ulcers, cribbing and colic will all be greatly reduced when a horse can freely graze or eat hay most of the day every single day, especially in a free roaming environment. Teeth also wear better, especially if a horse is roaming in large acreage and truly grazing.
Unless you have a horse with health concerns, like Metabolic syndrome, all the hay a horse will eat 24/7 is absolutely the most vital part of management you can see to in your equine(s). Grains, feeds, supplements and minerals are added to a diet of plenty of forage.
Unless your horse has confirmed health conditions that make it impossible, never ration hay and make sure your horse has pasture/hay/forage in front of him at all times.
Some people, especially in area were grains are cheaper than nice quality hay, opt to feed more grain in lieu of hay/forage, and that is never the best choice for most horses.
The average Horse will usually eat 2-3% of his body weight a day in feed. You may need to up this is your horse is working harder for any reason. Almost all of this should be in forage/hay/grass. So a 1,000 lbs horse needs 30lbs of very high quality hay a day, and this usually means 1/2 a bale a day if the hay is all excellent quality. You will need to up this amount if the quality of the forage/pasture/hay is mediocre, which is often is. Grains are usually feed at the rate of 2-5lbs a day. So you might feed 25lbs of excellent quality hay and 5lbs of grain a day to 1,000lb horse not under strenuous work daily in 2-3-4 feedings, ideally.
Plan for 185 bales of hay, at least, per horse, if you do not have 2-4 acres of flat, well maintained pasture that is managed very well and used in a rotational grazing program. If you have a well managed grazing opportunity, you can adjust the hay needs accordingly.
Never skimp on hay/forage/pasture. You are asking for health concerns if you do.