Crabgrass in cornfields can be extremely difficult to eradicate. The annual crabgrass plant grows well in the heat of the summer. The small seeds can catch on the wind and stick to most mechanical cultivation equipment. This type of seed transfer makes crab grass a prolific plant. While there are some mechanical methods of eradication, perhaps the most effective is the use of a pre-emergent and post-emergent herbicide.
Identify the type of crabgrass. There are two basic species of crabgrass: large and smooth. The large crabgrass will have hairs on the foliage blades that reside both on the upper and lower leafs. Smooth crabgrass will have no hairs on the leaves, but a few will reside on the collar region of the plant--where the leaf rolls around the blade stem. The hairs on the large crabgrass will retain more of the pre-emergent herbicide when applied.
Apply the pre-emergent herbicide to the crabgrass plants when the grass blades are less than 1 inch in height. The corn should not have yet reached an overall height of 12 inches. Consult the manufacturers mixing instructions for application rates. Various herbicides will have different instructions for applications.
Apply a post-emergent spray containing no more than 2 1/2 pounds of atrazine per acre per year. Mix according to labeling instructions. Some post-emergent herbicides may require that the corn, if of a special hybrid type, is resistant to the weed-killing chemical.