For a horse who is basically on rest feeding that much feed is a lot of food to be pushing in..
A diet high in fat more than 8 %, moderate protein of 10 - 14% at most and the fiber higher in values is what you wish to find.
Add in you look for as low a NSC % which is starch and sugars combined is usually the safer choices to make to feed.
Hard keeper horses do well with a food that is rich in calories derived from fats...is aka cool calories, friendlier to the intestinal tract of the horse.
I happen to feed Purina Ultium feed because it is
the highest calorie feed per pound fed on the market today, period. My horse is truly a hard-keeper and requires extra calories or he is a walking skeleton...horrible.
TC Complete is better than most in calories per pound...with suggested fed 1 - 1.5% so if the horse is supposed to weigh 1000 pounds fully weighted then 10 - 15 pounds of this food from what I read if they are fed no hay/pasture at all. Sadly, there is nothing mentioning amount fed when fed hay/pasture to as a guideline. The 20.6% NSC though is high and can create issues for some animals, ie horse diabetes for some.
When you feed a caloric rich recipe you often feed less in amounts and meet the requirements of daily needs the animal thrive from...
Read you feed bag backs carefully to determine how much food is needed and remember in your case you are feeding 2 types of feed so are doubling gross amounts so do be careful you not overfeed...
It is not only calories that would concern me but the daily vitamin/minerals amounts can be overwhleming and knock a animal delicate intestinal system out of whack.
For example...my horse is just shy of 16 hands, a Thoroughbred with very high metabolism and is eating what your combined amount of feeds fed are...my horse at optimum weight tops the scale over 1200+ pounds.
Feeding a all-stock feed is not
best for the horse as the recipe is made for many animals who have different requirements to thrive but more generic in recipe where you want specific.
If you are feeding multiple species of animals, honestly it is best to feed specific feed for each species so the daily minimum requirements are met properly.
Depending upon what the turnout of the prior barn was like the horse may not of been able to get enough quality forage to eat... Was she turned out by herself or with others, was she chased by more dominant horses, was the grass 6" long or longer and acres of it as you will discover horses are very picky eaters and don't eat everything "green" by any means...
Watch your horse very carefully, take tape measurements around her girth line, take pictures and use them for comparison so you see if she is gaining or losing weight.
Learn to critique your horses appearance by the Henneke Scoring system for horses you can find from many web sites as there are many and all offer different pictures and slightly different written descriptions.
A horse with a strained or problematic stifle I would not honestly want fully weighted or fat as it is more strain for the animal to move around...if she was my horse I would want a score of a 4 till she heals...then once she is sound, fully healed you can start a work regiment to recondition the animals muscles and then add more weight to her frame if it is needed...
A under-muscled horse can often resemble to thin so do be careful how much weight you think this horse needs gained.
That should get you started on researching...