Okay, here it goes.. ..
I've been around horses for more than 45 years. Back then, horses were fed pasture in summer, hay and grain in winter. According to their work load. Main grain was oats. Whole oats in general.
Up until 5 years ago, I have seen two cases of laminitis/ founder, both ponies.
I fed my herd of 14 good meadow hay, many different plant species and lots of herbs, as a base. Then soaked alfalfa pellets with a little wheat bran to balance Ca:Ph, 50 grams each of soy bean meal and linseed, and oats, barley and corn as needed. Usually by the ounce, not pound. I fed pregnant mares with that, raised babies, fed oldtimers and easy keepers, got my hard keeper OTTB up to weight with it. Two couldn't have oats, would go nuts, others could eat 10 lbs and wouldn't do a step too fast. All my horses were in perfect weight, were shiny and had really good barefoot hooves.
What I've noticed is that somehow somebody made oats the culprit for any diet related problem there is. New fad diseases pop up left and right. New fad feeds also.
I personally think it's not the oats who make horses insulin resistant. It's the " productive" pasture/ hayfield. Too many nutrients, grass seed mixes developed for cows, to produce milk or meat.
I came here, to Cali, from Italy. Hay in Italy and Cali is pretty similar...highly productive. Before Italy I lived in Germany. Bavaria. Hay from mountain meadows. Beautiful hay. Rather meager, protein wise. But all necessary vitamins and minerals. Easy keepers did just fine with that. No need for supplements. Then I went to northern Germany. Dairy land protein rich grass. Again, for cows. Soil depleted of nutrients, supplements needed.
I guess what I'm trying to say is not to blame oats or grain in general for the myriads of diseases our horses are prone to get. Rather the man-made changes in everything.
Back to nature...........