Herbicide Resistance vs. Non-Performance: What’s Causing My Weed Escapes

A decision tree to help with weed diagnosis

When you see weed escapes after a herbicide application, you probably wonder “are they resistant?”

Accurate diagnoses of escaped weeds can be the difference between lost yield at harvest and money in your pocket at the end of the year. Weed escapes can result from a number of variables, and discovering the difference between weeds that have developed herbicide resistance versus weeds that escaped due to application errors or herbicide non-performance, is worth the effort.

To correct the mistake and avoid escapes moving forward, it is important to accurately diagnose the situation at hand.

To make this determination easier for farmers this growing season, Aaron Hager, Ph.D., associate professor of weed science at the University of Illinois, suggests farmers put their boots in the field and follow this decision tree for weed escape diagnosis.

Step 1: Confirm weed species that are present

Start by scouting your field to determine what weed species are present. Is it predominately one weed that you notice or do you have a suite of different species in your field?

“If you’re seeing a broad spectrum of different species, there’s a pretty good chance that something went wrong with your herbicide application, something that’s not necessarily related to resistance,” says Hager.

Maybe the herbicide was applied at the wrong rate. Maybe the applicator sprayed the wrong field by mistake. If it was a soil-residual herbicide that you applied, maybe it was sprayed on dry ground and it never rained after the application to incorporate the herbicide.

All of these examples would typically lead to the emergence of many different weed species in the field.

Step 2: Revisit product labels and application records

According to Hager, it is best to revisit your spray record and product labels to confirm if any of the application errors listed above were the cause of the escaped weeds.

Generally speaking, the application rates that are listed on most soil-residual herbicide labels are selected for more than just one weed alone. Rather, they’re selected for multiple weed species to give farmers as broad a spectrum of control over as many weed species for as long as possible.

“Now conversely,” says Hager, “if you have one weed that’s popped up and one weed only, and it’s a species that is listed on the label as being controlled with the product, then you might actually have an instance of resistance.”

Step 3: Test your weeds

Testing your weeds for resistance is always a best practice. This is the only true way to diagnose resistance early and make a proactive plan to get ahead of the problem before your field is overrun with severe weed pressure.

“Sometimes, in the early instances of resistance development, it is not uncommon to see weeds that have survived the herbicide and others of the same species that did not,” says Hager. “It’s rare when resistance evolves to a frequency that farmers notice right off of the bat, so getting weeds tested initially will help you get a jump start on resistance management.”

Farmers can send their suspect waterhemp and Palmer amaranth to the University of Illinois Plant Clinic or other public testing facilities near them for plant diagnosis. Though there is a small testing fee involved, Hager assures it is minimal compared to what the test results can save you on weed control down the road.

Step 4: Tackle Escapes

Whether the escapes resulted from herbicide resistance or just non-performance, the end result is the same: You must attack the escapes that are present.

Review your weed test results and look for any initial misapplications in your spray records. Using those insights, speak with your local crop consultant to determine the best strategy and products to use in your post-emergence herbicide application.

If all else fails, Hager says farmers must get out in the field and hand remove the remaining weeds.

“The one thing that spreads resistance faster than anything else is human apathy,” says Hager. “If you don’t take the time to go pull the escaped weeds, they’ll go through the combine in the fall, weed seeds will spread and you’ll have an even worse problem next year.”

Though walking rows and pulling weeds sounds daunting, Hager reminds farmers that weed management is directly tied to end-of-year revenue. Regardless of the reason behind your weed escapes, if you invest your time and resources now to fix the problem, you’ll be rewarded with fewer weeds and more yield in the future.

Download the Take Action Herbicide Resistance vs. Non-Performance Decision Tree here.

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