another picture

Looks like crabgrass to me.

I agree, looks like crabgrass. Best thing to do for now is keep it mowed so it doesn't form seed heads. Otherwise it's going to be dropping next years crop and it will only get worse. Fortunately what you have now should die out after the first frost. A good pre emergent early spring should prevent it from coming back. Also, proper watering and mowing play a big factor in reducing the weed population.

Spot spray 1 teaspoon of "Tenacity" mixed with 3 teaspoons of non ionic surfactant in 2 gallons of water immediately. The crabgrass will turn white in a week and a half and will die. Do not allow seed heads to form. Or it could become a bigger problem next year.

I just had this done on my KBG lawn. This worked great although the price for 8 ounces of Tenacity is about $65.00. Also this has be sprayed using a tank sprayer to get a uniform application. Dial type sprayers cannot be used.