Find a high protein ration balancer. For example, Tribute Essential K. You feed less grain while it provides more nutrients. However, it does come at a higher price point.
I have a young thoroughbred mare who is a very hard keeper. She is currently on 4.5 lbs of Tribute Essential K split into two feedings a day. This ration is for a horse in intense work. While she is ridden for 45-60 minutes 4-5 days a week, no jumping, I have her on this amount because she is a hard keeper, not because she is in intense work. She also gets 4 heaping scoops of beet pulp, soaked.
I add 30cc of Golden Horseshoe Fish Oil daily for extra Omega 3 & 6 fatty acids for long term weight management.
Plus unlimited hay inside and outside.
*Side note: ulcers are common in thoroughbreds, especially if they have not been off the track long. This can cause weight gain issues and best to have a veterinarian take a look at your horse. My thoroughbred becomes stressed easily and will develop ulcers. She is also given 60cc of Aloe Vera gel daily to sooth the gut. Aloe Vera gel is not an ulcer cure.
Throughbreds are notorious for being hard to keep weight on I would consult with a vet.
Your feed may not be balanced, have a high enough amount of forage or covering all of your horses requirements, I highly recommend making a FeedXL account and running your feed through that, it will give you all the information you need and show the gaps in your horses diet :)
My TB gained 16okgs in a few months, on a coconut product called "Cool Stance" added to his meal=(consisting of different cubes and oats covered in molasses), he received 3 cups of meal a day. He also received 4 oathay hay nets along with a slice of Lucerne a night.
It’s probably the grain just doesn’t work for your horse. Research other food options and give them a try. Also, every horse is different! Some might need more grain than others, some might need less. We have a thoroughbred at my barn and she eats more than anyone else at the barn but she is still skinnier than many of the other horses who only get like a fourth of what she gets! Maybe it’s a thoroughbred thing?
My mom had this problem with one of her horses and so she tried some different foods and the one that worked was oats and barley mix. Along with some soaked hay pellets and beet pulp