Agressive grass in lawn

If you live in the US, you probably have a County Extension Office, and the County Extension Agent can identify the grass for you. Another option is to find a knowledgeable person at a local farm and garden store, but that can be hit or miss. It is important to have the plant accurately identified, because you won't know what will work on it until you know what it is. Crabgrass preventer (which is what is in your combo packages with fertilizer) is a pre-emergent herbicide that is applied earlier in the growing season before annual grass seeds have begun to sprout. It will then work by killing the seeds as they sprout. If your grassy weed is a perennial, the crabgrass preventer will have exactly zero effect upon it.

Broadly speaking, the only effective control for perennial grassy weeds is a non-specific herbicide such as Roundup, which kills everything it touches. It is difficult to develop an herbicide that can distinguish between good perennial grasses (your lawn grasses: bluegrasses, fine fescues and hybrid perennials ryes) and bad perennial grasses (tall fescues, quackgrass, etc.).

But again, your best bet is to take a complete sample of the weed (stems, blades, and roots) to a local expert for identification and advice as to the best approach for dealing with it.

May also find help by posting a picture of this grass like weed, someone who reads this group may recognize it. Would also help to know which growing zone, and how much lawn area is affected... with relatively small patches it may be able to be dug out. Sometimes the best method for removing weed grass is to plant a more aggressive smothering turf grass like Bermuda or Zoysia.