Low soil temperatures could see maize crop growth stunted by grass weeds.
Many Northland crops have more grass weeds germinating than previous years, said Ravensdown Agrochemical technical manager Ryan Glover.
"The difference in maize crops with poor and good weed control practices is massive."
"Grass weeds compete for nutrients, space and light," he said.
"If not controlled they reduce maize yields through further light competition with the lower maize leaves. Weed pressure will also cause maize to hold upper leaves more upright reducing the light intercepted, reducing overall yield.
Weeds can also harbour insect populations and they also will cause the quality of silage to be reduced, said Mr Glover.
"The effects of chemical application of products ... slow, but trials have shown the target weeds' root shoot development was stopped after chemical application ..."