Most commercial feed products (note I said MOST and not ALL) will provide your mini with the daily requirements for vitamins and just as important, minerals, when fed specifically to manufacturer's recommendations. An all grass and/or hay diet probably will not supply your minis with adequate minerals, depending on where you live and where your forage/hay comes from.
The key phrase here is feeding to MANUFACTURER'S RECOMMENDATIONS according to the instructions on the bag. Many feeds would require us to give our 250 lb (average) minis a pound or more of the product per day to achieve the guaranteed analysis. While this would provide adequate nutrient levels, the calories provided by this amount may be WAY too much for an easy keeping mini. Logic would say, reduce the feed, give only a handful, feed just enough to keep them in good weight. That's fine and good from a calorie standpoint, but then you are short-shifting them on nutrition. It's a catch-22.
You can feed the average, healthy mini most any feed product that is convenient to you, that you can afford and that your horses will eat (NOTE: I said average, health mini with no metabolic issues). However, special attention needs to be paid to the nutrients and calories that feed is providing. If you are feeding LESS THAN what the manufacturer recommends, you will, at the very least, need to supplement minerals on a daily basis (and that means top dressing, not just free choice). The other choice is to choose a more nutrient dense feed that provides all the daily vitamins/minerals/protein at a volume level that does not provide excess calories.
The way to determine that is to spend time reading labels. Most bags are labeled in amounts to be given to the average 1000-1100 lb mature big horse. Reduce that amount by 75% for the average 250 lb mini (with further adjustment needed based on the weight of the horse you are feeding). If the feed requires 6 lbs daily for a big horse, your 250 lb mini will need 1.5 lbs daily -- that's a LOT of feed for an idle, nonworking, pasture pet who probably only needs 1.5% or less of his body weight in feedstuff daily, 50% of which should come from forage. In other words:
a) 250 lb maintenance adult mini needs from 2.5-3.75 lbs of food daily if he is an easy keeper.
b) 50% of that figure should come from forage, so no more than 1.5 lbs of grain should be provided daily -- many will need less than that to maintain condition, but if you provide less than that, your horse may not be getting all the nutrition he needs.
This is why it is so important to WEIGH your feed rather than feeding by volume -- a cup of this, a can of that, a flake of hay. I know this sounds complicated, but it is easy to learn if you take it in small bites (pun intended
).
Robin C