Here is what I have learned about senior horses that are fed right, teeth in great condition, get all the hay and pasture they want but still won't gain weight:
They've got ulcers
My 26 yr old has gastric stomach ulcers.
My 25 yr old with Equine Metabolic Syndrome has hind-gut ulcers.
Try putting your senior gal on brewers yeast. I started using Diamond V, at the vet's suggestion, on three of my horses. Diamond V© - The Trusted Experts in Nutrition and Health
It has completely taken care of the runny bums on my 26 yr old Arab. The true test will be when he comes in from pasture tonight. We got a good and much-needed deluge of rain last night. He generally has the runny bums after eating the next day's grass.
My 25 yr old with hind gut ulcers is not on the Diamond V; he is on Succeed and will soon be switched to EquiOtic. I almost lost him to major colic and I'm not taking chances.
I pay $19.99/50lb bag of Diamond V YC at my local feed store.
My vet's instructions were to feed the Walkers two tablespoons two times daily.
The Arab is only 13.3H and gets one tablespoon two times daily.
I've had horses for more than 53 years and this goes to show one is never too old to learn anything. Ulcers in both forms are becoming more and more common in horses these days.
It can almost be expected a senior horse who is not gaining weight but the Owner has done everything right and all they possibly can, has ulcers.
It's well worth $20/bag to add brewers yeast to the horse's daily diet. From all I have read, there is no toxicity as the horse will expel what it doesn't need.
Good for coat and hooves, as well.