When a horse gets a hay belly, they aren't getting the necessary nutrients from the hay, so they eat more of it to get what they need. You can try what they others have said, but I would try giving her some soaked beet pulp. We get it in a 80# bag for $12. Two scoops shredded or one scoop rolled feeds six horses. One of our geldings had such a hay belly in October and November, he looked like he was pregnant with twins! We started our horses in November on beet pulp to help them thru the winter. Our hard keeper put on weight and our 'pregnant' gelding slimmed up. Beet pulp is also great for horses that have ulcers, fyi.