At the large farm that I boarded at for years they had one field that was used for grazing and hay, all the rest of the hay fields were for strickly hay use. The hay fields produced significantly more bales per acre than the pasture/hay field. The farm manager told me that using the field for grazing compacts the soil which reduces the grass growth. I also can tell you that grass stores it's energy in the lower few inches of the stems. Thant's why horses like the short grass better, it really has more sugars in it. Anyway, this energy is what the grass uses to grow. So if you use the field for grazing in the cool months when the grass isn't growing then the horses will graze it short and rob the grass of it's own food for spring growth. This is always true of any pasture of course. It's the reason to not graze a field short. Sucks for us with few acres.
chicamuxen