I'm glad the thread was reopened. Hopefully we can help educate the OP so she can improve the horse's care and look out for her welfare.
OP, as you can conclude from the responses in this thread, your mare is grossly underweight. Until she puts on a few hundred pounds and her health is up to par, please, for her sake, stop riding her. Not only does she have no energy to spare for carrying someone on her back, but no saddle would fit her back in its current condition without causing her serious pain.
The first step is to have her teeth floated and a vet recommend how to go about starting a deworming regimen for her. Without good dental work, she's not going to digest any of what she eats. So while you may not see much food spilled on the floor, she's probably not digesting it properly, and upping her feed or increasing the quality won't do anything for her except keep her mind occupied. If her hooves haven't been touched in the past month or two, please have a knowledgable farrier work on those as well.
Once you start eradicating any parasites that are in her belly and her teeth are properly floated, you need to assess her diet. You say she's eating a bale of hay a day right now. If this is true, there is something serious going on from a medical standpoint and she needs to be vetted ASAP. If she's not actually getting this much or pasture mates are stealing it, you'll need to find some QUALITY hay. My preference would be a mixed grass hay given free choice, though you'll need to transition to this slowly to allow for her metabolic and digestive systems to adjust.
She also needs a good grain. There are a lot of fantastic pelleted feeds on the market. My personal preference for putting on and maintaining weight is Nutrena SafeChoice. My hard-keeping thoroughbreds are both fed 6 lbs of this a day. I also recommend beet pulp. I used shreds, but you need to make sure you soak them before feeding--if they're not soaked prior to feeding, they will expand in the horse's system and could potentially lead to an impaction. I feed 4 quarts dry (equates to 6 or so when soaked) to my TBs. If your mare is a finicky eater, you can buy beet pulp with Molasses. I use Standlee Hay Co. shreds.
Spread her grain out into as many feedings as possible throughout the day. Optimum would be four. Make sure she has clean, unfrozen water available at all times.
If you don't have the resources to bring this mare back up to par, please give her to someone who can. It's not fair to let her suffer, and she is in very bad shape right now.
ETA: I was managing a barn full of horses (scheduling hay/shavings/farrier/vet/dentist, doing barn repairs and daily care/turnout) at the age of 10. I came to the forum two years later and found infinite resources available. I've learned an incredible amount in the 3 1/2 years I've been a member here, and I'm so grateful the people here were willing to be patient with someone so young. Please listen to what they have to say--there are so many knowledgable people willing to help you.
Last edited by equiniphile; 02-20-2013 at 11:10 PM.