If it is a wood post, hitting the post may cause more damage, but on a barbed wire fence you could have the advantage of knocking the post down and riding over the top, thus preventing the decapitation scenario. An electric fence is often lighter gage wire and will most likely snap and not cause the decapitation.
I believe the shot is real. A good professional photographer would get that shot. (speaking from 20+ years experience) It was probably taken with high quality expensive equipment, ie lens and camera. The photographer probably had multiple chances as well. If he was spraying a field, he was making multiple passes. He would have followed the plane, and just needed to time his shutter. He would also have the ability to take 9 or even more frames per second with some cameras, just by holding down the shutter button. The prop is stopped, so shutter speed is extremely fast. that is why the only real blur is the depth of field.