Aye, very interesting, as it is true. I know of a herd of horses that winters on 50% grass; pawing through the 3-foot deep snow to get it, and the other 50% of their feed is hay. Not rich hay, either. In fact, the hay is 1/2 quality as all the other hay for sale; for that reason nobody bought it and yet I know someone who feeds it to that herd; in the Spring the herd is fat. So, diet is in order. These horses eat snow, have no blankets, barns, or even a shed. In summer, they drink out of a pond. That's what I call survival!
Definitely not my horse. He's an easy keeper but I'm a mean old grinch. He's always about a 4 on the fatness scale but he quickly gets back up to 6 once the spring grass starts growing.
So many horses/ponies keep there weight because of the round bales of hay, which seem to be the normalway of making/buying hay. Horses do not get enough excerise if you live where you have to keep the horses close to the barn/sheds, because the pastures are snow covered, and electric fencing do not work as well in the winter.