If it is comercial feed (re the purple bag), it is most likely too high in sugar and starch. You can't just trust what the packaging says, you have to read the table of ingredients carefully, and, if the sugar/starch percentage isn't mentioned precisely - you should contact the manufacturer before feeding it to your horses.
If you are feeding oats, you might want to consider a balancer feed to be sure of a better vitamin/mineral balance in your horses' feed.
By the way, a good thing to try out is beet pulp. It's not too high in protein, it's practically clear from sugar and has a 0 percent of starch, and horses digest it very well, just as they digest forage.
I have come a long way, to surrender my shadow to the shadow of my horse.