What country do you live in?
Tbs and other hot blooded horses have high metabolisms. Adding feeds that are high in sugar and/or starch increase the metabolism, making them burn more calories, making them lose more weight...
The best way to break that cycle is to STOP feeding any grains or horse feeds. I have had great luck with the following diet for TBs and TB crosses who were billed as "hard keepers". Once their weight was up, they could maintain on a lot less food because their metabolism was returned to "normal".
- Free Choice grass hay or Timothy hay, as much as the horse can eat, at LEAST 25 lbs a day
- Alfalfa pellets (lucerne) 3-5 lbs a day
- Fat Source (I prefer Flax or stabilized Rice Bran, 1 cup)
- Vitamin/Mineral supplement (like Select II or GrandVite)
- Probiotic (like Probios Powder)
Once the horse horse is at a good weight, reduce the Alfalfa pellets to 1.5-3 lbs a day, cut the fat source in half, and remove the probiotic.