Herbicide resistance is a growing concern and justifiably so, says Mike Owen, an Iowa State University Extension weed specialist.
Farmers who feel they've done all they can to control weeds, only to have weeds showing up late in the season, are naturally questioning whether weed resistance to herbicides has evolved in their field. "While what they're seeing may actually be evolved resistance to herbicides by weeds, it may not always be the case," notes Owen. "Several factors can contribute to the presence of late season weeds and these factors should be considered when farmers are evaluating their weed control program."
Consider that whatever weed management tactic was used may have run its course too early. In other words, if the herbicide didn't have a residual effect that lasted long enough, weeds may have germinated after the herbicide was no longer working. Waterhemp, for example, germinates all season long and is able to germinate and ultimately break through the soybean canopy after a soil-applied herbicide has dissipated.
How to distinguish herbicide resistance from other weed problems
So how can a farmer know if the weed situation he/she's experiencing is, in fact, the result of evolved herbicide resistance? Owen offers the following advice.
The first question to ask is how thorough was the farmer's weed management strategy? Was a pre-emergent treatment followed by a post-emergent application used? Perhaps the herbicide product or products used weakened the weeds but not enough to kill them and thus didn't complete the job.
Also, what is the field's history? If the farmer has been doing the same thing for years and finds this control tactic is no longer working, it may be a harbinger indicating that herbicide resistance has evolved.
The next step is to look in the field where the weeds in question are surviving. If there are carcasses (dead weeds) next to survivors, that differential response is an indication that evolved herbicide resistance is the issue.
Look at weeds that have escaped, is only one type of weed surviving?
It is also helpful to look at the actual weeds that have escaped. If weeds show signs of herbicide injury, it may be a clue that they were sprayed too late to be killed, or it may show that weed resistance to herbicide does exist. Also, if there's only one type of weed surviving, like waterhemp or ragweed, that, too, signals a greater potential for herbicide resistance.
"Reflecting back on past years, if you realize that one type of weed has survived and it's getting worse, that, too, is an indication of herbicide resistance," says Owen. "Additionally, if a grower has been using one herbicide repeatedly and there's one kind of weed surviving, there is a pretty good chance herbicide resistance has developed."
He adds, "By looking carefully for telltale signs in the field and reflecting on what they've done for weed management, farmers can narrow down the likely reason for weeds that remain late in the season."