Crabgrass comes in two varieties: smooth (Digitaria ischaemum) or hairy (Digitaria sanguinalis). Both are an annual, invasive grass that spreads quickly via seeds. It is often found in soils with poor fertility or during periods of drought. In large quantities, crabgrass will reduce corn yield by robbing the crop of nutrients and water in the soil.
Select a postemergence grass herbicide labeled as safe for the type of corn you are growing. Some herbicides are safe for only sweet corn, while others are safe for all corn varieties.
Mix the herbicide according to the product directions. Follow the specific mixing instructions for either a hand-held sprayer or tank mix.
Perform a patch test. Apply the herbicide to a small area of the crabgrass in the corn field. Wait two to three days and inspect the corn in the area. If the corn does not show signs of damage, you can spray the herbicide in the rest of the field.
Apply the herbicide to all the crabgrass. Spray the herbicide directly onto the crabgrass, as per the herbicide instructions.
Repeat the application process after two weeks if the crabgrass has not been killed with the initial application. Several applications may be necessary to completely eradicate crabgrass.