The best way to get weight on a horse is pasture. If that's not possible, beet pulp is high in fat and will put weight on quickly. Other than that just keep free choicing her hay and her weight should pick up.
Beet pulp is not high in fat. It's actually very low in fat and sugars (unless you buy beet pulp w/mollasses), and high in fiber. It's FULL of calories, which would also be good to put weight on. It's between hay and grain when it comes to calories per pound
Rice bran is also excellent at putting weight on a horse. I have found that with hard keepers it is actually cheaper to add this to feed rather than giving the horse excessive feed. Follow the instructions on feeding though becuase too much of rice bran is not good for the horse. It has been a while since I have had a horse on it so I don't remember exactly how much you are not to exceed a day- I "think" it is a pound. The rice bran also puts a gorgeous coat on a horse!
What is she eating right now? Her diet could be causing her to lose weight. I know with my "hard keeper", it's a LOT harder to keep weight on him if I use sweet feed or any feed that contains corn, oats, or molasses. Once I took him off all those things, he was no longer a hard keeper!
Dental issues can also cause a horse to lose condition. Senior horses often need more frequent teeth floating.
Follow the instructions on feeding though becuase too much of rice bran is not good for the horse. It has been a while since I have had a horse on it so I don't remember exactly how much you are not to exceed a day- I "think" it is a pound.
It's 2 lbs a day for the max amount. Most horses do well on one pound though.
Rice bran and flax seed are supposed to be good for weight gain, along with making their coats shiny and helping them build lean muscle. I've never used them myself but I've been told they work great. Also, I would be wary of some senior feeds because a lot of them are packed with sugar. Read labels carefully.
My trainer gives her 33 year old horse Safe Choice pellets and it's worked the best out of all the grain she's tried. You just have to find what works best for your individual horse.