It's been so long since I looked at his papers, Impressive is about all I remember. LOL It doesn't matter to me anyway. Joe is a gelding, he's not a performance horse, he's just supposed to hang out and be a happy horse.
Just got here. Sounds more like some sort of poisoning from what they ate. Either some bad weeds, or check the hay at the stables to make sure it is ok. Sometimes hay can come from a field where there is some kind of bad weed that accidentally gets into the bale.
If your horse won't take his meds without grain or concentrates, don't use more than a cup of grain. Sweet feed is the best since it is sticky and the crushed pills will stick to it. If you are using a dry grain, sweeten it with a litte Karo to help the meds stick to it. Here are some other ways to give a horse meds that he doesn't want. LOL
Are the meds pills? If so, you can crush them with a tablespoon or grind them up in an old coffee grinder (get one at a thrift store) and dissolve in hot water. Add just a tiny amount of Karo syrup and drench with a drench gun. I use the 20cc one I have for worming my sheep. Since you are only giving a little warm water with sweetener and the meds, you don't need a giant drench gun.
If you do not have a drench gun, you can cut an apple in quarters, put a slit in it with a knife and stick the pill in that. The horse will crunch the pill up in the apple without even knowing it. DH's old TWH was on arthritis meds for 2 years and we found that she would pick the pill out of her Senior feed but would eat it no problem in the apple quarter. I just cut apples in quarters and kept them in a sealed container in the fridge for daily use.
Third option is making a mash if your horse will eat it. One of our horses hated mash so we couldn't use this method. Crush the tablets, add to warm water to dissolve, add some cider vinegar and Karo syrup and add to the mash and feed. Don't use too much mash only about a cup or 2 to make sure the horse will eat all of it.
The reason I suggest Karo instead of molasses is that molasses can cause constipation since it is very high in iron.
What kind of hay are you feeding that you are feeding grain with it? We feed no grain to our horses but that is because we are in so Cal and have high quality alfalfa. Adding grain with the alfalfa is too rich a diet. If you only have lower protein grass hay I understand you have to add more protein in a concentrate feed. Sounds like the vet must feel your horse is getting too high protein a diet and wants to keep him on more of a roughage diet to help his gut recover.
FYI for poisoning you can buy charcoal paste in a tube (like a wormer tube). Charcoal will go through the stomach and can trap and flush some poisons from the system. Needs to be identified and charcoal used fast though.