Before doing a selective herbicide application to the weeds in your grass, let the weeds grow out to achieve the best results at killing the weeds all the way off. The more surface area of the plant you can get the herbicide on, the more the plant will absorb. Cutting the grass right before the application will cut the weeds down, removing the foliage that is needed to get good herbicide contact and absorption. The common thought is to mow right before the application, so the fertilizer doesn't get sucked up by the mower, but this could be making herbicide applications less effective.