16 years of raising sheep, never had a ewe to prolapse. Flock size has ranged as high as 42 head, so we aren't a major sheep-producer. We've always fed hay free choice, and we have enough pasture stockpiled that they can always root around under any snow and find some grass. I don't understand limiting hay, because how can you limit their grass on pasture? The only grain-based feed they get is before breeding and after lambing. It is a coarse feed, kinda like a horse-feed. It's only fed in the evenings, when they have plenty of grass or hay in their belly. Being ruminants and designed to live by grazing, concentrated starch (corn, wheat, etc) will cause rumen acidosis. This can be a problem in high-producing dairy cows, to the point that some dairies keep a tub of baking soda near their mineral feeder to help counteract the acidity. Did you ever eat too many donuts and get heartburn? Same thing, except it affects cow, sheep, and goat hoofs. Since we cut out most all grain, their is almost no foot trimming. Of the 12 ewes in the field now, 11 have never needed foot care The exception is one sneaky girl who will gobble up the dog's food at any chance.
This was a whole lot to say that most ruminants can't eat enough hay to get obese. Prolapses are due to weak muscles in the perineal area. Back in our market-lamb showing days, we saw a lot of prolapse (mostly rectal) because of the tails being docked all the way up against the rump (not leaving a short stub) and the highly amount of feed they were eating. Since we went to hair sheep and long tails, it has never shown up again. Additionally, the girl usually drop twins or triples, and their choice of doing it inside or on pasture.
This is just what works for us, and hope you find what works for y'all.