For me, it's Mesquite! We're in one of the worst droughts in decades here in San Antonio. All the grass is dying, but those Mesquites are thriving with no competition from any other vegetation. Mesquite also have tap roots so they can get water from down deep. If it wasn't for the Mesquite, I wouldn't have had to shred the pasture at all this year, but they grow fast and if you let them get too thick, look out!
The thorns on these suckers can be up to 2 inches long, which is why I had the front tires filled with foam as I was constantly getting flats. So far they haven't punctured the back tires, which are not foam filled. Yesterday I went out to shred and the only goal was to hit the Mesquites. This Weekend I will try to finish up only to have to repeat the process in a couple of months to keep it under control.