Only problem I have seen associated with power lines is stray voltage.
Place down the road from me was a very well maintained and productive dairy farm. New lines went through and a big substation placed less than a mile from his place.
Stray voltage wiped him out. Went round and round with power company, electric engineers and so forth. Bottom line is he got some money and no longer milks cows on the place cause his barn became a dangerous place for them..
As far as grazing them through some pastures under the wires I wouldn't worry too much about it.
I'm sure that if you look around the internet you will find all sorts of stories designed to scare you though.
Deja Moo; The feeling I've heard this bull before.
Have seen conflicting stories, that's why I am asking. Regarding the grazing, it would be managed grazing so time might be limited in any on place - edge of one corner of one small corner of 120 plot. Thanks