My activities on a 120 acre sheep farm here in SE Michigan would indicate that if you do not cut your fields for about 3 years, you will afterwards incur enormous expense to return them to a state suitable for even growing hay, which has to be about the least expensive agricultural land use out there. In most parts that have zoning laws, if you are not engaging in some "legitimate" form of agricultural activity, you will lose your ag status and the increased property taxes will almost certainly force you off your land and destroy the resale value since any new owner would initially have to endure the new high taxes and then over a period of up to 3 years convince the local township office that they were indeed engaging in "real" agricultural activities. Loss of ag status can also have a huge impact on what kind of activities are permitted on the land, the kinds of structures permitted (especially if it is something new).
I have found that hardwood saplings can reach a diameter of close to 2" at the base in 3 years and cutting those and getting rid of the roots if there is any kind of density involved (often they grow 1-2 ft apart and at times even closer) is a major undertaking if this is happening simultaneously on 120 acres. If that 120 acres was divided into multiple 20 acre fields with hedgerows then the hardwood is encroaching relentlessly from both sides of the hedgerow into the fields on either side and it is a constant battle to keep it in check. A 3/4" sapling can do quite a bit of damage to hay equipment, the same applies to sticks falling off trees at the edges of a field.
If you have vulnerable livestock like sheep, the hedgerows provide ideal cover and refuge for predators and deer hunting cannot compensate for the losses you will experience due to the "deer habitat". If you do not have vulnerable livestock (all you do is produce hay for sale), then at least the "deer habitat" does not pose any additional risks to your business venture, beyond keeping it in check.
In my experience you want to mow any fields at least once a year. Fall is often the best time, provided it is dry enough that one is not making any ruts due to the mowing activity. Spring is usually wet enough where one does have to be concerned about ruts, and once the vegetation gets started, it grows high enough in a short time to hide any kind of obstacles, fallen over fence posts etc and of course if growing hay you don't want to brush hog anything that could be productively baled.
If starting from scratch, a drum mower and small round baler for hay production is the least tedious way to go since there is none of the manual handling that one has with square bales and considerably fewer bales to handle. This activity means that brush hogging is primarily confined to the field edges and any trails that you may have and those one may cut a few times a year so they are easy to walk or ride with a UTV.
