I feed my dogs raw, and it's so much easier to get informed with dog nutrition than horse!
If you're a fan of raw, then you need to stop all manufactured horse feeds. They're full of fillers and junk, just like dog food.
I feed my horses a 100% natural diet. I use a custom vit/min blend with added amino acids in a flax base from Uckele Equine. I have my hay analyzed and base my vit/min on that. Uckele can help you build the right vit/min based on your hay, pasture, and water source. (well water often has higher minerals, you'll need to test it if you use a well) horsetech.com also has a great line and will custom blend supplements. Both companies have a good "off the shelf" product if you don't want to analyze your hay. Uckele's is called Equi-Base Grass. Horsetech.com's is called High Point Grass.
I mix my vit/min supplement with 1-4 oz of flax meal, depending on the horse and 1 lb of chopped pre-bloom alfalfa hay. This is the base of my diet.
For my "easy keepers," that's all they get. For my harder working horses or harder keepers, I add 1-3 lbs of Alfalfa pellets and 1-1.5 lbs of plain whole oats. I only have two on oats. Both are hard working horses and a bit too lazy. They need the extra energy and spunk oats has in them. The other 8 horses in my care only get alfalfa products.
This is a very healthy forage based diet that is easily digestible and covers all of my horses' needs. I do not trust what most feed companies put in their feed. "I" don't eat processed food, so I'm not going to feed it to my horses either. My alfalfa pellets have no other ingredients at all. My flax is just plain ground flax. My chopped alfalfa hay has a touch of molasses in it for binding, but very very little. You cannot feel it when you touch it (it feels dry) and you cannot smell it.
Purina is known for having totally different forumals for their feeds depending on where it was milled (they have mills all over the USA) and what ingredients are readily available and cheap (many are imported from overseas!!). There is no fixed formula for all of their feeds (or nearly so). Nutrena isn't any better. I would consider buying Triple Crown's 12% supplement or Progressive's grass supplement, but I don't have any dealers for either company in my area. They use a fixed formula and their supplements are highly concentrated feeds where you need only 1-2 lbs daily for full nutrition.