A scythe can be used to mow along, and under electric fences, to keep the tall grass from shorting out the wires. You have to be very careful not to hit the metal fenceposts, however. Most cracked scythe edges that I hear about, are from people trimming along fence lines and hitting either metal fenceposts, or wire. Keeping your edge very sharp, enables you to mow with much less force, so you can be more slow and precise when nearing a fence post.
If you wish to gather up the fresh cut grass for feeding back at the barn, or if you wish to make hay with it, it's best to mow from right to left along the fence, (standing with right shoulder towards the fence) so that your windrow ends up away from the fence. You can then rake it up and put it in a garden cart, and haul it away. Or, if you want to make hay out of it, spread it out in place, and ted it until dry before hauling it away. For feeding it fresh to the horses, you can mow from the left end of the fence line to the right (standing with left shoulder towards the fence) as in the video below, so that your windrow ends up on the other side of the fence, so the horses can eat it fresh from their side of the fence.