My TB horse is really underweight any tips 2 fatten him up

my horse is really underweight he goes out on really long grass in the day,has plenty of haylage. has a feed of alfa A and some stuff thats used for fatting up cows.he used to be on Baileys topline bt it sent him stupid n didnt really put any weight on.he also is exercised for half an hour everyday.any tips to fatten him up?

Hi I bought myself a very underweight Oldenburg x Trekhna. I know that to have a healthy horse and for him to gain weight I need to feed him plenty of fibre. But before that I needad to sort out any underlying problems so I had his teeth checked on the day of his vetting I wormed him the day I got him and then I looked into what to feed him.

He is turned out 24/7 in a 10.5 acre field with good quality grass, I feed him on happy hoof and Fibre beet twice a day, vitamins and oil. I have had him for nearly 3 weeks now and he is steadily putting on weight. Fibre beet is a great thing to add to his feed because it is what it says on the packet and is great for horses who are underweight or dont have alot of turnout and it is great if your horse heats up you need to be carefull that they dont pile on the pounds too quick with it though a little bit goes a long way with that stuff! I think he would probibly be better turned out 24/7 that way he'll never run out of food give him his feed still and put haylege out so he can get some goodness from that too. If he's warm at night he'll be fine and at least he's not standing for hours in a stable with nothing to eat!