When a horse eats too fast it's said to be bolting its food. This most often happens with grains and other concentrates although it can happen with hay. It's a problem because horses that bolt their food are more susceptible to choke. Some horses that bolt spill a lot of food out of their mouths, to have it get lost in the bedding where it won't do the horse any good. Because your horse isn't chewing properly, it may not be extracting all the nutrition it can out of its feeds.
So, you need to slow your horse down some way, so that it chews smaller mouthfuls more slowly. Slower feeding lessens the chance of colic also.
Slow Greedy Hay Eaters
The easiest way to slow down a greedy hay eater is with a slow feeder. There are many types on the market and many you can build. These slow feeders make a horse work a bit harder to get its hay in smaller mouthfuls.
Grain Feeding a Bolter
Your horse may be more greedy if it is hungry, so try feeding hay first so that its stomach feels full. Some horses are afraid that another horse will steal its feed, so feed your horse away from other horses so it doesn't feel threatened. You can also try feeding much smaller amounts more often, if your schedule allows.
The simplest way to slow down a greedy grain eater is to wet the grain or concentrate until it's like a thick soup. This makes it impossible for the horse to get big mouthfuls, and it helps whatever the horse does get in its mouth slide down its throat easier.
There are times when this isn't practical, such as in winter when the feed may freeze before the horse has a chance to finish.
Some people find that a few big rocks in the bottom of the feed bin are enough to slow down a greedy eater. But some horses can be clever about flipping the rocks aside. If your horse is smart enough to get around the rocks easily, try screwing down tough balls, like the indestructible ones made for dogs, or other tough, non-toxic, non-splintering pet toys to the bottom of the feed tub.
Try feeding from a wide flat bin, rather than a deep bucket, so the grain is spread over a wider area. The horse won't be able to plunge its nose in to get a big mouthful. Sprinkling grain on the floor might result in a lot of waste if the horse tramples it and scatters it into the stall bedding. And, on dirt floors, feeding off of the floor might mean your horse is ingesting soil, which can lead to sand colic.
There are several products you can buy that will slow down a greedy eater. One called the Pre-vent Feeder looks like a feed tub with a muffin tin at the bottom. The horse is forced to pick the food out of the depressions in the bottom of the tub, and can't get very much with each mouthful. A horse toy like the Nose It, or the Amazing Graze Treat Dispenser can keep your horse busy, while slowly dispensing food.