Where I'm at in East Texas, the Easement is just that. They Utility Company has the right to access your land within the Easement. For gas lines, it's usually 50 feet wide. Power lines seem to vary depending on what's there. They cannot exceed that width. You can do whatever you want on the surface of the land, but if you build anything on the easement, you risk it being destroyed if they ever have to work on the Easement. Fences are very common across Easements, but also roads and even buildings can be built on them, as long as you realize that they can tear them down if they need to do something there.
What's interesting to me is if they want to add to what is already there, they have to pay you for that surface land. You wont have any choice in selling it, but you can fight them for what they pay you for your it. With Gas Lines, they can add pumping stations when they want and you can't stop them. I'm not sure about Power Lines, but my guess is that they can do the same if they want to.
Mowing it is up to you. If it gets really bad, you can complain to them and they will be forced to come clean it up. If you are really good at selling yourself, you can even get them to pay you to clean it up. I know a guy who did that. He didn't make money, but he got it done on his terms and the guys he hired also did some other things for him, so it was a win-win.
When dealing with whoever is in charge of the Easement, remember that they can act like they own it, or you have to do what they tell you to do. That is just bluffing. They have to follow the rules in the Easement, and they are controlled by a Government Agency. Here in Texas, The Railroad Commission is in charge of the Utility Companies. If something is happening, or not happening, that you are not happy with, a phone call to the Government Agency in charge of that Utility will get you almost instant results. Their power is very impressive at getting things done!!!!!
I don't know the rules on spraying chemicals, but I would guess that if you do not want them sprayed on your land, it shouldn't be too hard to make that known. As far as what I've seen, everything is mowed by a Contractor that is hired by the Utility to maintain their Easements. They also fly over the Gas Lines a few times a year, and at least once a year, I see a couple of guys walking the Easements.
In this case, the squeaky wheel gets the grease. The more noise you make, the better they are at taking care of it. Or if you don't want them on your land, the more you do to keep the Easement mowed, the less you will see of them.