Well its a little late now, but Jan and feb. Are great months to ask for permission to look for sheds, many people don't stress as much for that acess, and that to can lead to them seeing your up and up, and its a time of yr most farmers have a lot more free time, to maybe go on a walk with you to show you the property lines and get to know you!
now is prime time planting season, I wouldn't be bothering crop farmers now, even if looking for ground hogs
maybe dairy farmers, as there the one's that have more issues with safety of the cows due them pests!
But something to consider, many people fear guns and new people shooting on farms with live stock, so keep that in mind, can try asking for permission to bow hunt ground hogs, kill a few and it will show your hunting skills with a bow too, over a just a gun!