Opinions as a supplement? What horses benefit from it the most, what does it target?

I haven't fed it recently but years ago I used it to help put weight on and keep it on with great results.

I love it, I have a 3 yr old and a 30 yr old and I feed it to both of them for extra calories. Both are hard keepers. They maintain their weight, have healthy coats, no tummy issues, and they LOVE IT... as in they look at me funny if it's not mixed in with their regular feed. I've fed it for years, my opinion is that it's a great low-cost option to add to your feed program for a hard keeper/ picky eater/ growing/ broodmare/ senior etc. My advice would be go through a 50lb bag or two and take before and after pictures. :)

I fed it mainly for the protein

I think calf manna is a great supplement, its great for horses that been underfed (straved} its just a great booster to help build a poor horse back up.

I used Calf Manna a few years ago on my two year old because she was short and kind of lean. I cant say much about the height (because she's a whopping 14.1 at almost 6 years old LOL) but it defintely helped her fill out in areas where she needed it. After about 6 months of being on it she stopped eating it though so I stopped giving it to her. But I liked the results!