The horse is a grazing animal, designed to spend most of his waking hours eating. Feeding hay free choice can duplicate this to some extent, and it definitely has a favorable impact on behavior. Horses who are allowed to eat as much hay as they want are generally more relaxed and content, and 24-hour access to horse hay tends to reduce vices such as wood chewing.
However, before you open up the hay smorgasbord, there are some things you need to keep in mind about feeding hay free choice.
First, consider that fresh grass is about 90% water, 10% nutrients, fiber and minerals, while hay is the other way around. It's predominately dry matter. Horses on hay-only diets need about 3.6 liters of water for every 2.2 pounds (1 kg) of hay they consume. In hot weather, or when being worked, it's much more. If feeding hay free choice, it's imperative to make sure the horses have constant access to unlimited clean water to avoid problems with dehydration or impaction colic.
At first, your horses may seem to have insatiable appetites, spending nearly all their waking hours at the feeder. However, once the novelty of all-you-can-eat dining wears off, a normal horse will reduce its hay consumption to an amount needed to maintain a normal body condition score of between 5 to 6.
• When feeding free-choice hay, provide your horse with an endless supply of fresh water.
• If weight gain or insulin resistance is a concern, have the hay analyzed for sugar and starch content. (It should be less than 10%).
• Limit or eliminate your horse's grain intake.
• Monitor free-choice hay for freshness, keeping an eye out for contaminants such as foul bacterial growth, dust or mold.
If your horse continues to eat large amounts of hay and gains weight rapidly, he should be checked for insulin resistance. You may still be able to feed free-choice hay, but you will have to make sure that the forage you are feeding is very low in sugar and starch (less than 10% sugar and starch combined) by having the hay analyzed. Don't rely on guesswork here or you could have other health issues on your hands.
Excess weight gain may also occur if you continue feeding grain to horses that are being fed hay free choice, especially if they get little to no work. Most horses will not need any grain at all. Work with a nutritionist to determine what protein and mineral supplementation the horses will need, based on the hay type, and feed a pelleted supplement instead of grain.
A horse on free-choice hay may also take on a fuller look through the abdomen, but he should not develop a prominent "hay belly." A hay belly is a sign of parasitism or poor fermentation of the hay and is never normal.
If feeding free choice, every attempt should be made to find a suitable grass hay. The next best choice would be a grass/alfalfa mix. Alfalfa or grain hays (such as oat or rye) can be fed free choice too, and most horses will probably regulate their intake of these as well. But there are some problems with these. Alfalfa and oat hays are often somewhat higher in calories so will put weight on many horses. Extreme care is needed with the grain hays in horses that are known to be easy keepers and those who may be insulin resistant, because the soluble carbohydrate level in grain hays is much higher than in grass hays. This may also cause a problem with hindgut acidity.