Corn gluten meal is a very effective preemergent for anything propogated by seed. The key is timly application. Basically you make an application in the early fall for cool weather weeds and another in early spring for summer weeds such as crabgrass and perhaps your bent grass. The application rate is 20lbs. per 1,000 sq. ft. of lawn. In addition, since CGM is 60% protein, it acts as an excellent fertilizer as it contains 9% nitrogen.
However, CGM is ineffective on perennial weeds that are not spread by seeds an in fact fertilizes them.