If you Google "understanding horse nutrition", you will get a lot of hits. Some credible, others not-so-much.
"My Horse University" thru Michigan State Unitiversity, has a great on-line nutrition course but I don't know if one has to pay money to enroll.
I had help from three equine nutritionist when my senior TWH was diagnosed with EMS. From there I just started researching and sorting out what made sense and what didn't; not always easy
None of my horses get grain because two are metabolic and its easier to keep everyone on that same base diet. The two healthy horses don't do anything anyway - lol
The area we live in and my horses' living conditions also play a huge part in what goes into their feed pans
Our soil is very high in iron for example; not good for metabolic horses as iron over-rides copper and zinc. Along with being necessary for healthy immune systems, metabolic horses need copper and zinc to help stabilize their insulin.
Even though I recognize the copper/zinc shortfall, I still asked my vet if the amount in the vit/min supplement I feed would be sufficient; I don't like upsetting the mineral balance because I "think" something
Horses in hard work/training can handle grain because they burn it all up. By hard work, I mean eventers, reiners, ranch horses that are working cattle all day long, etc.
In this day and age of pollution and mineral deficiencies in the soil, every horse can benefit from a quality vitamin/mineral supplement or a quality ration balancer which comes with vitamins/minerals added in.
A bag of vitamins & minerals, and ration balancers are not for horses that eat bagged feed; they are meant for horses that eat only grass and hay as supplements.
If your vet is easy to talk to, I would ask for a recommendation of an equine nutritionist. It has been my experience that nutritionists from feed companies don't listen to everything you say. They are only interested in selling you one of their products, whether it's the right thing or not and that really gets my eye twitching - lol
Hope this helps