“Pre-emergent herbicides are the foundation for your whole weed management strategy,” says Dawn Refsell, Valent manager of field development for the Midwest. “Your weed management plans for the year can hinge on your success or failure in selecting residual herbicide—it causes a domino effect in many ways.”
But what happens when you see a weed after herbicide application? How do you know if it’s application error, if the residual ran out or if you have a truly resistant weed? If something went wrong with the application, you’ll likely see streaks. “You could have missed rows from clogged nozzles or you didn’t get adequate overlap, for example,” explains Bob Bruss, NuFarm director of technical services.
If the application is the issue you’ll see weeds of many different species. Double check the label in case your soil pH affects the herbicide.
Application can also fail if herbicides require rain for activation.
“If you apply certain herbicides pre-emergent and don’t get activating rainfall in seven days, consider doing something mechanical to activate that herbicide,” says Brent Neuberger, FMC technical service representative. “Using a rotary hoe is one way to activate if you don’t have rain.” Residuals don’t last forever, so it’s important to know how long to expect control.
“You need to check on the average residual after 21 days to see if there are any breaks or weed pressure from resistant weeds the year before,” Refsell says. “Be more proactive as weed resistance becomes more prevalent.”
When scouting, if you see weeds before you expect, check when you applied the herbicide and how long the residual is supposed to last. Early breaks could mean resistance—make a plan to gain control.
Weeds that germinate after the residual runs out will produce several species instead of one. Have a plan in place to control any weeds that emerge after the residual is complete to kill them before they reach 4" tall.
Too much of a good thing isn’t a good thing. Before going hog-wild on herbicide residual, remember to keep in mind what you plan to put in the soil in the not-too-distant future.
“Set expectation based on the kind of residual you want,” Bruss says. “Longer residuals could affect crops next year or cover crops, so the longest residual isn’t always the best choice.”
“Get a handle on it if it’s resistance,” Neuberger advises. “Generally pre-emergent herbicides, at a proper rate, give 85% or greater weed control of labeled weeds. Postemergent application is our chance to correct escapes.”
Work with a local Extension expert or trusted crop adviser to find a multi-pronged postemergent herbicide. Get in fast, too. When weeds are more than 4" tall, they’re harder to kill. Take time this season to plan your pre- and post-emergent herbicide passes for next year because you know what works and what doesn’t in your fields. Don’t let resistant weeds dodge the herbicide bullet again.
Getting ahead of weeds early in the season is critical. Use strategic scouting and deductive reasoning to identify any weed issues and determine the best way to address them.