Take a look at that area and tell me about high grass, weeds, brush and trees. It is a desert for crying out loud. The only resemblance of those items you mentioned grow directly adjacent to the Virgin Creek, they call it a river. Everything else is sand, cactus and scrub, but the cattle have adapted to finding its chlorophyll needs on those native plants.
Our place here in West TX looks like a tropical forest compared to that area around Vegas and we can only run about 15 head a section. In a bad year we drop it to 10.