Are your weeds declaring “I will survive!”?

I’ve received several phone calls in the last couple weeks regarding waterhemp or other species that survived postemergence herbicide applications (in some cases, more than one!). While most herbicide applications have finished, it isn’t too late to evaluate how well your program worked, whether fields would benefit from a “last ditch effort” to reduce weed seed production, and what changes might be necessary for next year.

Surviving weeds from this year will increase weed pressure in next year’s crop. Recent research compared the effect of poor (30%), good (90%), and perfect (100%) waterhemp control in corn on waterhemp densities the following year in soybean (Yadav et al. 2023). Three weeks after planting soybean, poor waterhemp control in corn meant soybeans contended with approximately 2.5X the number of waterhemp in plots that had ‘good’ control the prior year and over 6X the number of waterhemp in plots that had ‘perfect’ control the prior year (Figure 1). This year’s seed production will become the driver of next year’s weed problems, and the more weed escapes, the more difficult it will be to manage weeds in the future.

While July is usually an unpleasant time to be in crop fields, weeds are easy to detect at this time. While scouting for disease and insect issues prior to any fungicide or insecticide application, check the field for weed pressure. As the summer progresses, weeds will emerge over the soybean canopy and become more obvious as harvest approaches.

Why did the weeds survive?

First, prioritize properly identifying the weed escapes. While many species are easy to identify, some species can look very similar, like waterhemp and Asian copperleaf. Many resources can help with identification, like phone apps, books/guides, or someone who already knows (my favorite).

Weeds may survive a herbicide application for any number of reasons, and determining why they survived is important to plan for 2025. Herbicide rate, soil type, and rainfall are the main factors influencing activity of preemergence herbicides. Was the application rate appropriate for the soil type? Was the rainfall following application adequate to activate the herbicide and make it available in the zone where weeds germinate?

Postemergence herbicides are influenced by many factors, including application rate, weed size, environmental conditions, spray additives, and spray coverage. Spraying weeds that exceed the maximum size specified on the herbicide label is likely the number one cause of postemergence herbicide failures (Figure 2). I have seen some cases this summer where spray coverage seemed to be a problem, including low carrier volumes and cases where weed pressure was so high, smaller weeds escaped herbicide due to being hidden.

The most obvious reason a weed may survive is due to herbicide resistance in the field. When herbicide resistance is developing in a field, escapes are usually found in discrete patches. It is often possible to find surviving plants immediately adjacent to dead individuals of the same species and similar size (Figure 3). Reports continue to trickle in regarding waterhemp populations that survived timely herbicide applications, making resistance to postemergence herbicides a likely issue.

Finally, use this information to make decisions for next year. Surviving weeds may warrant a small change like using a new herbicide or increased application rate, or they could require something more innovative, like changing a crop rotation or planting soybean in narrow rows.

Is it too late for weed management this year?

When soybeans enter reproductive stages, herbicide treatment options are limited. Always check herbicide labels for specific restrictions. A few products, including lactofen (Cobra) and acifluorfen (Ultra Blazer), are labeled for soybeans after they surpass stage R2 (full flower). These products are labeled to control up to 2-3 inch waterhemp. Similarly in corn, most herbicides cannot be applied once corn surpasses the V6-V8 growth stages; if application is allowed, it often requires drop nozzles due to the dense canopy development and may cause concern for carryover to soybean the following year.

Very little information is available on the effect of late treatments on control or reduction in seed production. Carefully evaluate whether the risk and expense of these treatments will justify the benefit. Dr. Hartzler has written articles in the past about these late sprays including “Controlling large weeds – Do you feel lucky?”.

Some farmers may be interested in non-herbicide strategies to kill weeds, reduce seed production, or reduce seed inputs into the soil yet this season. Several tactics are available, though generally used on very limited acreage in Iowa thus far. Hand rogueing, weed electrocution, and harvest weed seed control are all effective non-herbicide options to reduce the impact of weed escapes on the 2025 crop.

Reference:

Yadav R, Jha P, Hartzler R, Liebman M (2023) Multi-tactic strategies to manage herbicide-resistant waterhemp (Amaranthus tuberculatus) in corn–soybean rotations of the U.S. Midwest. Weed Sci. 71: 141–149. doi: 10.1017/wsc.2023.10