Horses love oats, so they tend to eat it fast and fail to chew it well enough. Whole oat grains are hard enough to resist the equine digestive system. When the oats comes out it is intact and well embedded in fertilizer; perfect conditions for growth, just add water and sunshine and you will have an oat field noone can beat.
The amount of digestible nutrition in the form of proteins, fat and energy contained in oats horses actually do digest, might be overestimated. Meaning; rolled oats is probably far more useful if you want to get the most out of the feed. However, if you want to use oats as a treat for a horse who doesn’t really need the extra nutrition, whole oat grains may not be such a bad idea. Especially as the unchewed grains may be recycled so to speak 😉
It seems that there is a huge difference in digestability between natural grasses and legumes as compared to cultivated and dried oat grains.