Overgrazing can be a huge problem. That is why the Taylor Grazing Act was passed, to avoid the issues seen in the Dust Bowl. If grazing herds aren't allowed to move once they have mowed over an area it can have a severely negative affect. That's why the Federal Government hires scientists to monitor and plan these leases. There is plenty of area for herds to move as we're talking about a combined area (630,000 square km) larger than Ukraine.