What kind of weed is this?
Hello. This plant is a monocot, a type of grass. The reason it is spreading is because every time you work the field and are breaking/cutting the root system new plants are beginning from every little piece of root. Eradicating it is going to mean careful hand pulling for a few years or an herbicide targeted to grass plants. Broadleaf weed killers will not be effective. If you choose an herbicide read and follow the label directions carefully.