Why can't my 16 year old quarter horse gain weight?

Have you had routine bloodwork run to make sure there isn't an underlying medical condition? Have you treated for ulcers? Is he getting harassed by herdmates? There are many many issues that can cause weight issues in horses. I would say that you've started well, but a vet and chiropractic check may be in order to make sure there aren't any other medical issues. If those clear, your vet and other local horsemen would be your best bet as they would likely know more about not only the medical condition of your horse, but also the herd/social condition, the environment, the roughage quality, etc... Best of luck!

Older horses often have problems keeping weight on. Is it a problem to feed him twice a day, because if he gains when you do this it makes sense to keep it up. We fed our 29yr old according to what he could manage as he had bad teeth and kept him round and shiny by changing his diet. If your chap has a good appetite it is likely that he isnt getting enough nutrient from his forage. Poor pasture and poor hay are usually to blame so analyse your soil and hay to see if the problem lies there. Soaked sugar beet is good for putting weight on poor horses, and can safely be fed with the other ration up to half a bucketful a day.

maybe she has a tape worm inside of her body somewhere.

might have worms........have a vet come out to look at the horse

Why dont u call a vet u love da horse so money aint a factor is it x

Well you have obviously done the worm thing. I would want to know two things, is this a young horse and perhaps it is growing and will look out of proportion for a while. If no,, does this horse share a feild with other animals and if yes is there condition okay.. All so is this horse out side and perhaps not handling cold weather. Horses are odd and the smallest ailment can make them loose weight.

contact your vet, he might have worms or some other "disease" If he doesn't have worms, he might just be that way, so I would give him weight builder or something or ask your vet to perscribe him something.

If he's gaining when you feed them pellets, why aren't you keeping him on them? Senior horses have different nutritional needs than other horses. I suggest looking at senior horse feeds. They are formulated for the older horse and have different quantities of fat, protein, and vitamins & minerals for the senior horse.

I agree with the other answers and I would also consider te weather. If it is cold, the food you feed will be used to keep him warm rather than put weight on, so make sure he is kept warm and well rugged up.