An interesting observation regarding pigeon grass

Does grazing have an effect on pigeon grass?  We hay some flood plain pasture, most of which we turn dairy heifers out on to graze for the rest of the summer and early fall.  We rent a few acres of pasture adjacent to our own pasture and that we only hay, we do not graze it.  I noticed recently that the portion of pasture that is not grazed is nearly 100% covered with pigeon grass.  There is very little pigeon grass at all on the acres being grazed.  Other grass fields or pasture that get cut and bale but not grazed are also covered with pigeon grass.  Any thoughts?

An interesting observation regarding pigeon grass