Tom O., I would be more than happy to let someone photograph my farms, buildings and/or wildlife and I think you will be well received if you do ask permission. I can only speak for my area and point of view but here are some ideas to think about. First of all by asking, the worst that can happen is denial of permission. If you were to trespass without permission, it can be VERY costly here. Landowners are concerned for the liability of people on their land, especially folks they don't know. I would not allow someone around my livestock unless I were with them, strangers aren't always well received by livestock and can become nervous. I do not want anyone on my land just prior to or during hunting season. They may want to know your intentions with the photographs, are they just for your own use or will their barn show up on a calendar next year. We have Amish folks in my area, they have a unique lifestyle, buildings and machinery but DO NOT want to be in any photographs.
Some other options would be shooting from the road, or offering some photos of their homestead to sweeten the deal. If you stop at my house I'll ask if I can go with you, can always learn from others and making new friends is at the top of my list. Good luck with your shooting and post some pics if you take some.