Based on your description of the events and what you were feeding, it sounds like your horse has a tendency to bolt his feed, and what you saw was a "choke" - a large lump of feed passing down his esophagus that he managed to swallow without actually choking on it. Here are some things you can try to help prevent an actual choke from occuring:
1) Have the vet check his teeth. If he his teeth were floated 3 months ago they may be fine, *or* the vet may have overlooked problems on the very back molars. I've had horses that were "just floated" that still had hooks on the rearmost molars.
2) Eliminate or greatly reduce the amount of non-hay feed you give to him. Most horses don't need any grain or nuts at all - they can maintain their weight just fine on hay as long as they are offered enough good quality grass hay (and pasture). Grain was necessary in the days when horses worked hard for a living - plowing fields or pulling heavy loads for 40-80 hours a week. Those horses could not maintain their weight by eating hay alone.
However grain is NOT a natural diet for horses. Feeding grain is a primary cause for many problems such as ulcers and colic. (Colic is the #1 killer of horses.) Horses who are fed no grain have far fewer digestive problems, fewer colics, fewer chokes, etc. Because of the relatively light work-load for riding horses today, they do not need anywhere near as many calories, and most can easily get all their caloric needs from hay or non-grain supplements (hay pellets, beet pulp, etc.).
3) Use a feeder that slows him down so he can't bolt his food. There are many options for this. Some people put large smooth stones in the horse's feed bucket so the horse has to nose around between the stones to get the feed. This keeps the horse from being able to easily gobble down large mouthfuls that can lead to choke. Another method is to put the feed into a special feeder ball with a few small holes and put the ball into a large feeder (a 100 gallon water tank works well for this). The horse noses the ball around and small amounts of grain fall out of the holes. It takes a lot longer for the horse to work all the feed out of the ball. This greatly improves digestion too, as the feed is consumed more slowly.
4) Add water to the feed, to soak and soften pellets and nuts, and make the feed into a mash. Mashes are far less likely to cause choke than dried feed.
5) Feed hay first, and only offer grain or other hard feed after the horse has had at least 10 minute to start in on the hay. This helps reduce the "hunger" urge to eat quickly, as well as being better for digestion.
Here is a link with more information:Treating Choke in Horses
If his symptoms persist you need to have the vet examine his esophagus. The vet can use an endoscope to see if your horse has damaged his esophagus with prior (unseen) episodes of choke, or if he has some other problem that is causing an obstruction in his esophagus.
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