I believe in keeping things as natural as possible providing my horse get's all she needs. Just like us they need a good multivitamin as the forage they get from domestic pasture and hay just doesn't meet all their mineral, vitamins, amino acids, fat, omega oils, etc... My mare was a rescue last year in terrible shape was to be put down when I met her (see comparison photos) and this is all it took to take her from emaciated and week with hurrendous feet, skin, coat, etc... to super shiny, healthy well muscled and strong...
(Amounts) are based on a 15.5 hh, 1,100 mare who is hard to keep weight on 8 flakes hay 2 lbs
"Island Horse" Vitamin Mineral Supplement
specifically designed based on analysis of local forage 2 cups Canola meal
for omegas, lysine and fat 5 lbs Soyhulls soaked
Grazing a few hrs per day in warm months and in cold months a few mins hand grazing or eating on the trails
You might not need the soyhulls which is for extra ruffage and to help keep weight on her. If you guy is an easy keeper you can probably skip that. If this has worked so well for mine who was in really rough shape I'm sure it will work for yours who sounds reasonably healthy to start with. Good luck! And Kudos for trying to keep in natural!
Here are the photos for comparison :)