We get it really bad in areas that the ground has been disturbed in, in the feeding areas, wet spots that the cows have tromped into mud, field roads, etc. I have never seen our cows eat it. It will flat out thrive in the super rich soil of a feedlot where nothing else will grow.
The pollen from it this time of year will turn a Case into a John Deere when you drive through it. The stuff will grow 6ft tall if you let it.
Best thing is to spray it early in the summer, glyphosate will kill it, 24D will work as well if you don't want to kill the grass along with it. I have sprayed it later in the year right after mowing, but it is mostly a waste of time since it is nearing dormancy anyway.
I have read that it is actually really high in protein, but because it is so prickly, not many animals will eat it.