If I do it in the Fall after harvest would it have any effect on crops in the Spring?A heavy rate should kill them. You will want to be careful with the crop, especially soybeans. If you drift onto non-Enlist soybeans you will kill them with the crossbow, and even with Enlist beans your rate would be high enough in a spot-spray situation that you'd likely kill them or injure them. You will be better off with corn, but still make sure not to drift too much.
I like to drive in the beans and spray out towards the ditch when the beans are about 4-6" tall. I'm on 15' rows so I run quite a few over, but at that growth stage they pop right back up. This helps with the drift vs spraying from the road into the field.
If you are the landowner and not the farmer, I would check with your tenant to be sure what you are doing will work with him. He will appreciate you being pro-active with it and will probably help you with all of it.