Of course he's picky! It's never that simple ;). 4 qts daily isn't a huge amount of feed, I consider it my "healthy maximum" for a horse Arab sized (meaning if they still need weight at that point I'll start looking elsewhere lol but I'm comfortable feeding that). If he does better with this feed then I would simply up the amounts he gets, 1/2 per day is barely anything especially if he's a little thin, you can definitely do 1/2 twice a day and still have him on a very minimal grain ration.
There are hay supplements designed as grain pellets that may help (like the big hay stretcher but made in grain sized pellets so don't need soaking...if you can find them!) If he's not showing a positive change on something cut it out. I assume you do NOT mean alfalfa when you said "pelleted processed feeds"? You wrote them back to back so just checking. Fully agree that forage should be first! I assume he's getting free choice good quality hay, and your mare is getting more restricted quality hay :). I haven't fed alfalfa pellets in awhile but assuming they are pretty small then no they don't need to be soaked, large pellets should be soaked, it really just depends on the style and your horse, mostly it's a choking risk. I think I've only seen alfalfa pellets in the larger style. You can also experiment with soaking (just damp, soupy, etc) or maybe he would like the cubes soaked. You could try alfalfa hay or something too if you haven't already, that might be very beneficial for him. I know that can be hard to get in some areas, it is here! But a flake or two a day would be plenty.
Have you ruled out possible health issues? Ulcers? I find it very odd that he has proven to be such a hard keeper despite feeding established quality high fat feeds. Not that his weight is poor, like you said better then fat, but you would expect him to fluctuate a little more when fed such different things! Unless you're continuing to feed at minimal doses... I just feel like there's a missing puzzle piece here, hmmmm....
Scales are great, my head works in amounts lol but most basic pelleted feeds (including the safe choice) are about 1 qt = 1 lb.
I don't like rice bran as I think there are better options (beet pulp, alfalfa, etc) but it may be a good choice for a horse that won't eat those things. Might be worth a shot for him. You could also look into keeping the grain amount the same but adding a top dress maybe calf manna or something specifically designed for weight like omegatin or amplify, that would likely be the direction I would try next for him (aside from getting some alfalfa into him, I think that would be a good thing!)
All of this was very helpful! I can adjust the amount of feed he is getting tomorrow when I visit the barn. We will see what an additional half quart does for him.
Tractor Supply in my area carries the Dumor beet pulp, alfalfa pellets, and timothy grass pellets, and also Standlee products. I am willing to try hay pellets out, it would just be tough getting rid of a 40 lb bag if he turns his nose up!
As for hay, both of them have free-choice hay on netted, slow feed round bales. Their pasture isn't great grass, nothing seeded by any means, it is under a lot of tree cover and whatever grows naturally. My BO bushhogs all of the crappy stuff down as it needs, so it is taken care of so they don't run out of anything to munch on. She also rotates pasture, it's just not GREAT grass. The hardest part about feeding him flakes of alfalfa as a supplement is that he is pastured with two other geldings, and he no longer is pushy enough to not share a hay pile. My BO may consider separating him long enough to eat a flake or two of alfalfa, but she is a very busy lady and I don't want to pile too much more onto her.
I had the vet out last winter for an a sheath swelling issue, and she commented that his overall health is very good. Every vet that has seen him isn't too concerned about his slimness, as everything is completely normal with him. I haven't considered ulcers as he has never shown the typical signs; no mood changes, discomfort, problems working... and he can be an honest horse when something is bothering him. I could look into dosing him with an ulcer medication, but he isn't showing any signs.
I have considered a top dressing for his feed, and I have had friends have success with Calf Manna. One of my 'requirements' to any additions is that they can be added to baggies that I mix his joint supplement, cool calories (until they run out), and loose salt with. I really don't want to be a boarder that hauls tubs and tubs of additions out to the barn, and would like it to stay as simple as possible for my BO!
Also, I could've included his workload in his description. Before I came to college, I was riding him 3-5 times a week, and competing pretty often in speed events, jumping, pleasure, etc etc. Since being up here, I showed him last summer in the same type of events and this summer all we have done is trail ride or do light arena work occasionally maybe two to three times a week. Since last summer to this summer, his weight has been constant, even with the differing workloads.
I don't know if his barrel conformation may have anything to do with it, but anyone that has ever seen him has commented on how large of a barrel he has. Sometimes from a distance it looks like he is a round, pregnant mare rather than my very not pregnant gelding. Who knows!