If you can, I would seriously look into getting a 1 month subscription to FeedXL
. It'll tell you if your horses are getting what they need or need more. It's truly fantastic.
At first I thought it was all smoke and mirrors but it's truly not. I finally got my mare's diet "perfect" and she is soooo much happier and friskier than she was to begin with. She's 27 so I can't even imagine how much feedxl could help a younger horse.
I would steer heavily away from BOSS/corn oil. Both things are not "the best" for horses. Horses can get some nutrition from them but not the full amount (I don't know the exact details) and personally, I want whatever I feed to be directly 100% nutritious, not 50% nutritious. Also, corn oil is super inflammatory so if your 18 year old has any arthritis, CO could exacerbate that. :/
All your horses look to be in really good weights to me (and I seem to like my horses FAT haha) so I probably wouldn't be worried about adding more weight. I'd just focus on maintaining what's there.
Personally, I'd get them all on a ration balancer (like Purina's Enrich 32, here's info on finding one, also this is a superrr informative ste - read it: Ration Balancer
) and add beet pulp (it's your choice to soak or not, especially with the pellets - it's been proven that the previous "need" to soak BP was larger unfounded) as needed.
As an example of a balanced diet, so you can visualize, my girl gets:
1lb hay pellets
Free choice "Nature's Essential's FreeBalance 12:12" mineral supplement
2oz flax seed (whole seeds, they don't go bad+horses CAN digest them enough to get nutrition)
1oz Red Cell
Plus a variety of painkillers etc for her eye issues
And 24/7 crappy pasture, wearing a grazing muzzle for half the day
Anyway, try to get FeedXL for a month -it'll probably save you so much feed $$- and you don't have to get the "multiple horses" one. You can just get the single horse one and re-enter differing information... :)
And read that link.