There are some post-emergent herbicides listed for barnyard grass but like crabgrass once we get passed the early growth stage it is harder to manage. The herbicides with fenoxaprop-ethyl and Mesotrione both list barnyard grass but both labels have restrictions on types of lawns or amount of material that can be used. Read labels carefully and follow the directions. The real key with annual grasses is to not provide an opportunity for them to grow. Which means getting the lawn to fill out in areas where barnyard grass or crabgrass tries to take over. These grasses are opportunists and come up in areas where the lawn is thin, too short, or along drives and walk ways where edging exposes soil. Small amounts can be pulled out before going to see since it is very shallow rooted. Raise mowing height to 3 inches will help shade soil preventing germination of crabgrass and barnyard grass seed in spring. Often there is no need for pre-emergent herbicides once a lawn is growing thick and kept at 3 inches. Use of pre-emergent herbicide can be use in spring to stop the germination of this plant. Raising mowing height, fertilizing grass especially in mid spring and late summer are practices that will help fill out the lawn and crowd out unwanted plants like barnyard grass.