older horse getting thinner

Question
I am feeding her a senior horse pellet twice a day of about almost a scoop each time and about a 1/4 bale of hay along with free grazing of grass up to recently when the grass stopped growing.  We now have her more in the stall with the weather being colder and feeding her more hay, probably about 1/2 a bale per day.  But she still looks so thin. Her hip bones are sticking out and her ribs are all visible.  I have seen 30 year old horses look meatier that this.  So I can't believe it's cause she older.
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-----Question-----
I bought a 23 year old horse who just seems to be getting thinner and thinner no matter how I feed her.  I had her checked for worms and she is fine.  She is skin and bones and does have some arthritis, I am really worried about her.  What can I do?
-----Answer-----
first of all,what are you feeding and how much?

have you had her teeth checked?

you might try putting her on some pro-biotics to help digestion

she is probably just getting old. 23  is pretty good

Answer
if she is eating 1/2 bale of hay a day (how large is the bale and what kind of hay)along with the feed, she is obviously not digesting her food to get the full advantage of the nutrients.

she may need a feed with more carbs than the senior pellets and she may need more of it. experiment with small increases and add a pro-biotic supplement