April 15-20 usually marks the end of the window for applying traditional preemergence herbicides in central IN, a week earlier in southern IN, and a week later in northern IN. Though most preemergence herbicides do have minor post-emergence properties that will burn back small crabgrass (one or two leaf stage), this is not always reliable. Therefore, consider switching to products containing dithiopyr (Dimension) because it has both pre and post activity. Or you could use pendimethalin or prodiamine tank-mixed with any of the post-emergence products like MSMA (variety of brand names), fenoxaprop (Acclaim), or quinclorac (Drive).