I use Nutrena Black Bag high fat feed. I board 20 horses and this past year got in a bunch of hard keeping TB's. This has helped keep the weight on them better than anything else I have used. Also for the REALLY hard keepers, I gave them Nutrena Boost as well.
I also like the Nutrena High Fat Performance Pellet in the "Black Bag". Since I have the label in front of me...
First 3 ingredients are Beet Pulp, Flaxseed, and Rice Bran
It also has pre and pro-biotics. Hard to find anything about it online. I switched to it from Purina Ultium which I liked as well at 12% Protein, 12.4% fat, and like 18.5% fiber. But it did not have the pre and pro-biotics. I wasn't worried about that, but it's about $5 cheaper a bag than the Purina. That's important when you feed a bag every 6 days to one horse.
I, too, am a Pennfield Fibergized fan. Toby gets his top dressed with an amazing amount of Cocosoya, as well.
I have also fed Enduro a lot, but I think the Fibergized is more "low carb" than the Enduro. And people would FREAK at what I used to feed Vernon because of his allergies (Ultra Max! Yum!!! Toby got it, too).
Beyond a shadow of a doubt: Nutrena "Black Bag". I have a OTTB that was on TC senior, plus flax oil, plus everything else, still ribby. Started feeding the magic "black bag", and now we are hunter fat and sleek. Strange coincidence, I also put the old man on it and his COPD is markedly better. Maybe the lack of alfalfa? Less mold issues?
I've been feeding Legends' Performance Pelleted feed (12% protein, 10% fat and 18% fiber) for my herd (tiniest taste for for the roll polly Shetland, 3 lbs daily for my "regular keeper" OTTB, and 6 lbs daily for my very hard keeper OTTB). Everyone has done fabulously on it and it is my go-to high fat, low-carb feed.
My guy does best on his current mix of Triple Crown Low Starch (13/6, 13% NSC) and Legends Omega fat supplement (12/25). I had him on Fibregized Omega over the winter, but at the first hint of grass (early this year), he got very muscle-tight and tense, so we (BO and I) put him back on the Low Starch. He's never been tested for anything, but he is very demonstrative about his diet not being quite right and low starch/high fat keep him happy.
Failure is always an option*
*As long as you figure out what you f'ed up and fix it! -Me
Seminole show and sport. The wellness line is all low nsc but i *believe the show and sport is the highest fat. 10 or 12%? I fed my ulcer prone hard keeper until he got fat
We love the Seminole too. We have a horse with dental problems, so our "go to" feed is Seminole Wellness Senior Mix. It's not as high in fat as the Show And Sport, but it's easier for horses with dental problems