Regular mowing of fields not in production keeps them free of invasive species, dominant weed populations, wood brush and allows natural grasses and wild flowers to flourish. The mowed material is left in the field to compost. You can mow once a year or more depending upon your objective. If you are trying to get the land back into production for hay without tillage then mow at least twice a year. Once in spring after everything has leafed out and again in late summer or early fall before the frost. Doing this method can return some field to production in under two years providing the soil is fertile and there are no invasive species like Wild Parsnip, Purple Loosestrife or Japanese Knotweed. If those plants are present in great numbers then more frequent mowing and/or tillage is recommended.