sorry, the word escaped me when i wrote it! I meant concentrate feeds (ie cubed!) my bad!
I have upped his feed to 6kgs a day which most people think is a lot but I have spoken to a few people who have been in the same situation with their OTTBs and they agreed he needs more especially due to his size. He needed to put on weight when I got him and has since lost quite a bit around his bu, and topline.
I have also put him onto what is called "Acid Buffer" for his ulcers and I will keep him on that for minimum 6 months. To that I have added a supplement which was recommended to me by the locals who have OTTBs called Mass Gain which is 1 cup a day and is said to really help them pick up weight.
Someone else told me of a local health shop where the owner has helped many horses by putting together a mix of certain natural products to cleanse their systems so I am going to go see her this week and see what she puts in it.
I avoid corn at all costs. It just makes them hot and I don't need that. He is a very calm and chilled horse, Id like him to stay that way!
He has good quality hay available all night and a net in the mornings as well and I am trying to get my hands on some good quality lucern. This month the BO is providing Teff for them all on top of their normal hay.
When we got our 16+hh OTTB about 11 years ago, he lost quite a bit of weight before I figured out how much he actually needed. I've forgotten the amounts, but forget about what you are used to feeding normal horses and throw mass quantities of food at him!!
The metabolism of the average OTTB is running so high, that they inhale feed. They need TONS of calories and high quality calories if they are going to stay looking good. Once their body adjusts to their new workload, their needs will moderate but most OTTBs remain hard keepers.
That's only my opinion and I'm not a vet and I've been known to be wrong. My best advice is keep high quality hay in front of him at all times and feed the absolute best feed you can afford in gigunda amounts with a top dressing of oil or rice bran.
In our case, we also needed to add Biotin+ as a hoof supplement. He had really thin-walled thoroughbred hooves that would NOT hold a shoe. He now has hooves with nice thick walls that hold shoes quite well. Clips help also.
ETA: If your grazing is not very good, consider the fact that most horses on the track NEVER get to graze. They exist on top quality hay and feed. The nutrition your horse is getting from low quality grazing is probably nowhere near what his system is used to. You need to take this into consideration when you are figuring out his feed.