All horses, regardless of their weight condition, should have forage at all times. Your horse’s digestive tract is designed to have forage moving through it consistently. Horses allowed to graze freely, will self-regulate their intake and eat only what they need to maintain condition.
If forage is restricted, the horse is likely to stay overweight, simply because it triggers a hormonal response that tells the horse to hold on to body fat -- the horse perceives this as “survival mode.”
Equine nutritionist Juliet Getty is a strong advocate for natural, free-choice feeding of horses. A common concern is that the horse will become overweight. Juliet addresses this concern in this ask-the-expert article. New window. 
Furthermore, an empty stomach is painful because the horse’s stomach secretes acid all the time, even when empty. The upper portion of the stomach does not have a protective mucus layer, making it vulnerable to acid erosion. Chewing produces saliva, a natural antacid, which protects your horse against ulcers.
Self-regulation takes time. A horse that is not accustomed to having hay offered free-choice will at first overeat. But once he gets the message that there is always hay available – that he can walk away and the hay is still there when he returns – then and only then, will he slow down his eating.
This adjustment usually takes about a week or two, but some horses take longer.
Important points regarding feeding hay free-choice:
Know the sugar, starch, and caloric level of your hay. The only way to do this is to have it analyzed. Make sure your lab offers results that relate to horses, not cattle. Equi-Analytical Labs is a good choice.
Once you get the results, add the Water Soluble Carbohydrates (%WSC) to the %Starch, on an as-sampled basis. This sum represents the Non-structural Carbohydrates (NSC). It should be no more than 12%.
The Digestible Energy (DE) is a measure of calories, and should not exceed 0.88 Mcals/lb (1.94 Mcal/kg).
Consider a slow-feeder. Slow feeders force the horse to work harder to get a bite of hay, thereby reducing consumption. These feeders are worthwhile as long as they are not a source of frustration.
Frustration can lead to excessive cortisol (stress hormone) production. When cortisol is raised, insulin is raised, leading to obesity and make produce a laminitis attack .
Allow your horse to become accustomed to the slow feeder by offering free-choice hay on the ground next to the feeder. Slowly offer more hay in the feeder and less on the ground, until the horse feels comfortable with the new feeding method.
Never let your horse run out of hay, even for ten minutes! If he runs out, even for short time, he will never get the message that there is always hay available, and he will continue to overeat.
Free-choice means 24/7 – all day and all night. The only way to know that your horse has enough hay to last all night is for there to be some left over in the morning.
Exercise does three things: burns calories, increases metabolic rate, and decreases insulin resistance. Encourage your horse to move around by offering hay in a variety of places. If you cannot exercise him regularly, walk him – even 10 minutes of hand walking per day will make a remarkable difference.
Offer insulin-lowering supplements. These include magnesium (plus chromium) as well as a source of omega 3 fatty acids (such as ground flaxseeds or chia seeds).
Horses fed free-choice forage will generally develop a “hay belly.” This is not fat – it’s gas! And it is normal. Walking around and more movement in general, will help.
Watch for the “magic moment.” That’s when your horse walks away from the hay, knowing that it is always available when he returns. He’ll start to eat less and, combined with extra movement, you will see a trimmer horse
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About the author
Juliet M. Getty, Ph.D. is a specialist in equine nutrition whose philosophy is founded on feeding a horse in sync with his natural needs and instincts. Dr. Getty is the author of the comprehensive resource, Feed Your Horse Like A Horse , and her articles and interviews often appear in national and international publications.