Best grain/feed for weight gain.

OMG montysmom that's terrible :(

Rice bran and a senior pellet is good for weight gain. And nothing can compare to grazing on fresh grass or 24/7 hay if little pasture is available. A complete supplement is good too. Whatever they are lacking will be replaced by this and it will help everything work properly and allow the body to do the best possible with the food they are getting.

Worming is essential too if you havent done that already

I just rescued a very underweight 8 yr old Thoroughbred and asked my Vet what I should feed her to put on weight. He suggested Senior XL. I give her that and let her graze and also have a round bail of hay. I feed her a small amount in the morning before work and some more at night when I get home (i work 12 hour shifts). I did this with another horse I rescued and he put his weight back on within 6-8 months. He's 20 yrs old now and looks beautiful. Just a suggestion.