Your situation sounds a little like mine. I just moved back to the NE after being in Maryland for the last seven months. My guy lost weight while on grass pasture, as we’d started to reach that time of year when the grass was losing its nutrients but the barn didn’t want to add hay. Since he’s been back up north, he’s put on weight again and is filling out. He’s a hardkeeper in general, so I have a few weight builders.
First, he gets free choice hay. More access to quality forage is the best thing you can do. You can also look into adding a hay net to slow him down and help with absorption.
For grain, my guy gets Safe Choice Senior and Nutrena Enpower Boost. The latter is a rice bran pellet, so if you’re already branning, you may not need it. He also gets Hay Stretcher mixed in with his grain. I have never noticed much difference in hay cubes versus the pellets.
Additionally, I feed pure Flaxseed Oil. I get mine from Santa Cruz Animal Health, which seems to be the most affordable. Be careful when shopping for Flaxseed Oil, as a lot of them are cut with soy or other oils. Soy in particular can cause allergic reactions in some horses.
Finally, I feed Tri-Amino for building muscle mass. Some vets like it, some say the science is still out. I’ve noticed a difference in his topline since starting it. Granted, we also do a lot of conditioning work, so who knows how much it plays a role. But it certainly isn’t hurting, so I continue to feed it. He also gets a probiotic to help his gut. Again, I noticed a big difference since we started it.
Personally, I haven’t had much luck with beet pulp, Coco Soya, or corn oil. But every horse is different. NeighLox Advanced has also been recommended to us. If my guy hadn’t started to pack on the weight after the move, I would’ve tried it. Supposedly it helps with nutrient absorption and can be beneficial to hardkeepers. Again, that’s just word of mouth, as I haven’t used it on mine.
Good luck! Some of these TBs can be really difficult. And don’t forget to consider whether ulcers could be affecting him. They’re an all too common problem in many TBs.