North Island dairy farmers are being urged to give yellow bristle grass the boot this month if they want to save money and produce more milk from pasture over summer.
Experts say a sea of noxious, persistent seed head is on its way and December is time to spray the weed before it flowers in existing pasture.
Registered herbicides have a 28-day grazing withholding period after application, so planning is important.
"All the signs are pointing to another bad season for yellow bristle grass this year," says Waikato pasture specialist Will Henson.
"Conditions for germination have been ideal and we know there's a lot of seed in the soil in some areas.