This is a subject near and dear to my heart, so bear with me! LOL!
I have a 13 yr old hard keeper. When she first came to me, I had no idea what a hard keeper was! I was such a newbie (still am in many ways). She had recently lost a foal so I think she still maintained some of her baby weight, even though I know now that it was much. Over the course of the next year, she lost weight and condition. It wasn't until the following spring (one year after she arrived here) that I realized how bad she was. I spent the next many months trying to find a way to reverse the issue. Our #1 problem is that not only was she a hard keeper but she's an incredibly picky eater. Many of the "standard" things that are often recommended for weight gain she simply would NOT eat. Would in fact, completely ignore everything in her bucket if an the smallest amount of the offending matter were in there.
We have tried the high calorie feed by Purina called Ultium. We tried the add-on feed supplement Amplify (also by Purina). We tried beet pulp (with and without molasses), canola oil, rice bran oil, to name a few. After spending more than a few $$ only to waste them because she refused to eat them (and many of these things came in 50lb bags only!), I was reluctant to try anything more that I couldn't at least "sample" first.
We finally have found that she will eat Purina's Omolene 300 (16% protein) without too much fuss (2lbs 2x a day). Because there's always concern that an ulcer might be at play here, we now sprinkle on a small serving (1/4 scoop 2x a day) of powder Ulcer Guard. We also found she liked SmartPak's Smart Gain 4 weight supplement...also 1/4 scoop 2x aday (which we could TRY first). She gets pasture time a few days a week, up to 3 hours on her own to eat her morning and evening feedings without interference from the rest of the herd. She is a very slow eater and despite being the boss mare, she rarely will fight for her feed. I give her an alfalfa mix hay while she's separated.
She went from having a body score of between 2-3 in my opinion, to just at a 4. I would *love* to get her to a 5, but I'm trying to be satisfied. I still feel that her hip bones and her ridge line are too pronounced but at least they're improved.
Its always a good to have their teeth examined in these cases. Our horse had some definitely issues that were making it her hard for her to eat at all (very poor lateral movement).
I'm including some pics but I need get some better ones that show her hips and spine ridge better, but you'll see that this is the best I've gotten her to despite giving her tons of opportunity to intake a good deal of calories each day. I realize that she's not nearly as thin as your mare, but this might give you an idea of what it took for me to get her to this point.
I'm a big fan of using a "pound" measuring tool vs. a cup as this is how most foods are made to be measured. Your profile doesn't say where you are, but I wouldn't worry about her getting too plump with winter coming on unless she has a history of laminitis/founder that you're concerned with. I prefer to send my girls into to winter months with some padding so they can withstand the colder temps better. They're happier when they don't have to be stalled or blanketed. That extra pad helps us be able to do both.
