Call the power company and find out what chemicals they are spraying. Once you get the name of the chemical ask them what effects it has on your chickens. Then do some independent research to confirm what they say. Don't fall for the tap of believing a site that says all chemicals are bad! They are going to die! They are going to die! It might take a bit of work but try to find out what the real effects are.
It's probably a brush killer herbicide since they don't want trees growing up into their power lines. They want to keep the area open so they have access too. Grass is not a problem for either of these and will help stop erosion, though it may kill grass too.
With EPA regulations, if it harmed wild birds they probably would not be allowed to use it, so it should not affect your chickens either. I'd still want that stuff to dry before I let the chickens or other pets like dogs in there. That's another thing to check though, are the chemicals they are using approved by the EPA for that purpose? They should be following the rules but every now and then you get someone that doesn't. It doesn't hurt for the power company to know that someone is checking up on them. It helps keep them honest. There is bad publicity and some pretty big fines if they don’t follow the rules.
It can be a bit of work and you may have to speak to a few supervisors to get a satisfactory answer, but if you are really concerned this is the way to find out what is really going on instead of just guessing and needlessly panicking.
Hens go broody when you don’t want them to… and won’t go broody when you do. 
