Having the correct open space in a flight pen is just as important as having cover. The thicker your cover, the more open space you need. Pheasants need a place to sun themselves, dry off or dust. If the entire pen is full of cover the birds will get ornery.
In our flight pens we average 25-30% of open space. The most effective way to create open space is by spraying. A 5-foot boom is attached to the back of our Gators with a spray tank. As soon as cover begins to grow, we drive one pass around the perimeter of the pen, a second pass at the end where the feeders will be located, and one path down the center of the pen, splitting it in half. A mixture of 2-4-D Amine (8 oz for every 25 gal water) will do the job. I like using 2-4-D because it only kills the broadleaves, leaving any grasses. If Round-up is used everything will die, allowing dirt to be exposed leading to tails dragging in the mud. If cover gets ahead of you, mowing with a brushhog can be done, but this is a lot more work and an inconvenience when there are birds in the pen.
Having correct open space allows you to look in the pen and see what is going on and it allows the birds a clear view of where things are, this is especially important the first few days they are placed. You know you have enough open space, when you go past your pens and the birds are lying in the ground dusting and settled in. These are happy birds!