When I found this article, I felt I'd found the answer to what my horses were missing. I fed a minimal 'species appropriate' diet (ie 24/7 low sugar grass hay, CA Trace, BOSS, chia. herbs, sometimes oats, salt) for a good while and often felt that my horses lacked "bloom". My horses don't have access to good pasture- too dry here.
I implemented this protocol from Feb - Dec 2022 after finding this article (it made so much sense to me). Toplines improved, hooves improved, mane and tail growth increased. Coat quality oddly enough got worse, not as shiny. I did a 3 mouth round of Succeed with all my horses as well (VERY expensive, but a great product). I tweaked things at the end of December 2022 and reintroduced Platinum Performance GI, Equinety XL (amino acid supp), vitamin E, and oil, increased alfalfa even more, and I switched one horse to whey protein after she started having weird hormonal issues that stopped when soybean meal was discontinued. Everyone is now shiny and well muscled. Overall, I think it's a great concept and every horse will benefit from more forage, more protein, and less grain. However, I felt that my horses without access to pasture do benefit from additional supplementation. In my opinion, you cannot get away with feeding mediocre hay (which, depending on your location might be all you have) on this diet without good pasture or a supplement.
As an additional note, Doc T mentioned in his forums that horses without pasture may need to be on a protein supplement indefinitely, not just a year. He also shared that PSSM1 horses sometimes experienced negative changes in behavior with soybean meal, so I didn't and won't feed it to my n/P1 filly- she just gets more alfalfa.
