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People have to know at least generally where you are - what state at least? It doesn’t matter if someone recommends the “best” grain - if you don’t have access to it, it’s useless. You’re not going to find Poulin in AZ, and won’t find LMF in Massachusetts. Even of you say “I’m in the Southeast”, you may not be able to get Seminole because it’s not in all the SE states. So we at least need that.
But given that Purina and Nutrena are probably 2 of the most widely-available feeds, go with Purina Ultium. Yes, it’s going to cost more per bag. But it’s one of the highest calories and highest fat combination feeds, so it’s not like you’re looking to feed 10lb of the stuff. BUT, that does depend on acknowledging that you HAVE TO feed more hay. You just do.
Check www.hayexchange.net to see what you can find. Ask around and see if any barns need hay and if they’d like to get some trucked in from out of state which, believe it or not, can sometimes be cheaper per pound given the volume. The volume is too large for most facilities at once, but if it can be shared, everyone wins.
If it’s not to much trouble. I would prefer to find a grain that isn’t super expensive and that won’t get used up to quickly either. And of course, that’s good for weight gain.
I’m not sure you understand how all this works
A horse needs what he needs. How fast a bag lasts entirely depends on the quality of the feed and how much the horse NEEDS. You can’t feed 2lb of a feed to make it last longer and expect weight gain. You can’t go with a cheap feed only to have to feed 10lb of it for weight gain and think it will last long.
Forget cost of a bag. Look at the cost of a typical feeding/serving. Cheap feeds often need to be fed at higher amounts, than higher quality feeds that cost more per bag, so the cost per serving ends up much more in line with each other.
Concentrates are almost always a more expensive source of calories, and healthy, than forage. And now you’re asking about fat supplements, which really is something that should only be considered when the horse can’t/won’t eat more hay, and won’t/shouldn’t eat more concentrates, and you’re not at either of those right now.
CAN you pour more cheap feed and fat into him and get weight gain? Probably. But I guarantee it’s not going to be a healthy horse with that little hay and that many poor quality calories from concentrates.