She doesn't look the least bit skinny to me, she looks racing fit. I'd turn her out to pasture, and reduce her grain to a normal amount. Even trying to put weight on, you can't feed that kind of quantity without having issues - and stalling just compounds that.
The more you continue to feed excessively and keep her cooped up, the worse your problems are going to get.
I have a lot to do with racehorses out of training. They come here fit from racing and people will think them thin when they are fit.
I never have any problem getting horses to put on weight unless there is a metabolic disorder.
I will let the horses have plenty of turn out time. A relaxed horse is a happy horse and even if it is winter when there is not much and little or no goodness in the grass, they will soon start to gain weight with good quality hay and small feeds.
When in work they still get turn out time, mentally they are relaxed so there is less tizziness about them. I tell them that when they are here they are riding school ponies, not racehorses.
A horse that is stabled for majority of the time needs regular work, not just in an arena which can get boring for them, but taken out for long rides of at least two hours. Walk trot and cantering.
If a horse gets 'portly' then it gets a little less hay and more work. Relaxation is the way to go.
When she was let down when she came off the track, she did gain weight and there were no ribs or pointy hip bones. Then she was put out in a field where she got picked on and lost a lot of weight, so I know she can hold more weight. She is still getting the same amount of food because or vet said to wait until she is out of heat and see if her behavior changes. I have been riding/lunging her every day and when she gets worked up, I just talk to her to calm her down. This usually works, but if it doesn't we go in the indoor arena where there are no distractions and just walk for a while. Some where I read that you shouldn't do small circles with horses off the track because it reminds them of the start of a race and they just get more worked up. Does any one know if this is true?
When she gets worked up do not pull back on the reins but guide her into a circle and just let her go around. It might be a bit fast but as you guide onto a smaller circle with your inside rein so she will slow down. Sit heavy making it uncomfortable for her then when she is trotting do the same until she is working at a decent trot on a small circle and then got around the arena.
Racehorses look for a pull on the reins so it is vital you do not do so to get her back to you.
Let her learn that fast is not the way to go - the pull does not happen and so it is no good looking for it.
Sorry but the vet is wrong in saying to keep her on the same feed. Vets know very very little on what to feed a horse. Not many have actually worked with horses to see what effect different a ratio of feed to work makes on them.
Less feed and add a cup of corn oil to help her gain weight. Personally I think she looks fine - just fit.
Thanks Foxhunter, your tips about what to do when she gets worked up worked great today. I went on a group trail ride with 3 other horses just to show her around and she hated being in the back. She was antsy and trotting sideways for a while and then she backed a bit. Once she realized we weren't running she was fine :)