Anyone have any clue as to what this could be (HORSE)? URGENT!!!?

Call the vet or get her to the equine hospital now, and do not give any medication. Banamine doesn't treat colic. It kills pain and masks symptoms, and is given after the vet examines the horse if needed to inhibit absorption of endotoxins. At this point, the vet needs to get there and examine your horse. Whether or not the barn manager fed or not is completely irrelevant. You are the owner and the one legally and morally responsible for obtaining medical attention when your horse is obviously in distress hour after hour for this long. And if you know that the horse is being mismanaged at this facility, isn't it time to move your horse to a better one?

A gallon of feed? Besides it being a measurement based on volume instead of weight, that just sounds like a butt load of grain for one horse, especially if it's given at one time. Start weighing your feed based on your horse's weight and distributing it in meals less than 3-5lbs at a time. That would be a good place to start.

Colic is probably the tops, though choke is also an option. She may have even eaten a poisonous plant while out since you say that this happened after putting her back in pasture after the ride.

I'm going to vote that it is either choke or she's eaten something foul, and in these cases if you can't get a vet ASAP then they will either resolve themselves or, sadly, get worse. In a worst case scenario, in which you feel like the horse's life may be at stake -- which choke and poisoning certainly can do -- call your vet. If an object is obstructing the airway, choke could kill a horse in a short period of time.

A good vet has an emergency line and an on call doctor. I live out in Backwater Hicktown and our vet has an on call doc who will return calls as quickly as he can regardless of the hour. That should really say something; if your vet doesn't have an on call service where they can help you in an emergency like this, you should definitely find a new vet.

Add: Edit because I need to learn how to spell.

When my horse had that issue, it was colic. However, there was also a time when she wasn't eating simply because she needed to get her teeth done. I don't think there is anything you can do except try to keep her comfortable until the morning when you call the vet. Are you sure you can't get a vet out sooner?

Stop researching symptoms and call the vet. now. honestly, what is with horse owners these days? -_- It's like vets are a mythical creature... CALL THE VET IMMEDIATELY.

Vets make evening/emergency calls; Atleast, they did last week..

~Bailey.

Horse went off feed and doesn't feel good, but you didn't take her temp. or call the vet immediately?

Good god.

Vets do make evening calls too...

Vets make farm calls. Just sayin'.