Then mow it as short as possible. Don't be fooled into thinking that cutting low will eliminate the lower grade common because it will not, but it will keep it at minimum. Once a Bermuda grass purity is violated, it is forever soiled until replaced. Proper maintenance will slow the process down. Unless you use a greens reel mower cutting down to 1/4 inch making a putting green, you will never be able to tell which is which. At typical homeowner heights of 1.5 inches with a rotary mower you will never be able to spot much difference.