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Slideshow: Herbicide resistance and emergence timing create challenges to tackling troublesome weeds.

March 21, 2024

6 Slides

by Ryan Miller

Weeds can be challenging to manage effectively, but they tend to be a little more predictable than insect or disease pests.

From previous experience, we likely know what weed species we will have to manage on each of our farms. Yes, occasionally we will find a new species on a farm or an old species with a new characteristic (e.g., herbicide resistance).

An example of a new species could be Palmer amaranth, a species not known to widely occur in Minnesota, but it could show up on a farm you manage. Maybe you have been managing waterhemp for years, but waterhemp has shown great potential to develop herbicide resistance, and so a new resistance characteristic could certainly complicate management.

Weeds tend to be predictable in terms of what weed species will emerge where and which will emerge first. Weed scientists have been developing complex models for more precisely modelling weed emergence, and we can use this information as well as historical observations to know what weeds will emerge when.

Early emergers

Some of our earliest emerging weeds include woolly cupgrass, giant ragweed and kochia. While kochia has been more of an issue in the cooler and drier parts of Minnesota, it will be one to watch, as recent reports from North Dakota have indicated this species has developed resistance to some of the Group 14 herbicides.

Kochia populations have also been confirmed with resistance to herbicides in the groups 2, 4, 5, 7 and 9.

In recent years, kochia has been more frequently detected in the southern parts of Minnesota, most commonly on farms adjacent to railways.

Because of the widescale adoption of glyphosate-resistant crops, woolly cupgrass has been less of an issue to manage, but it does emerge early at the time of corn planting and has an extended window of emergence, creating challenges for farmers growing conventional crops.

Historically, in southeast Minnesota, giant ragweed has been a major issue. It emerges early, near the time of corn planting, and is a very competitive weed, particularly when it emerges with germinating corn.

Populations of giant ragweed have also been confirmed to be resistant to herbicides in groups 2 and 9. Tillage or burndown herbicides have been important tools for managing giant ragweed, but don’t forget a robust preemergence herbicide program.

Not all preemergence herbicide programs are equal, and products that perform well with small-seeded broadleaf weeds might come up short with the large-seeded giant ragweed.

Mid-emergers

Common lambsquarters and common ragweed tend to start emerging after corn planting has started and near the start of soybean planting.

Tillage can play a major role in managing these weeds.

Starting with clean fields and preemergence herbicide applications, you can help your crop outcompete these two species.

Herbicide-resistant populations are more of an issue with common ragweed (groups 2, 5, 9, 14). While there are some examples of herbicide resistance in common lambsquarters (groups 2, 4, 5), common lambsquarters has physiological characteristics that can make it a challenge to control with herbicides, particularly when conditions are cool and dry.

Late-emergers

Commonly occurring pigweed species are next to start emerging, typically starting near the end of soybean planting. They also have a prolonged emergence period.

Waterhemp peak emergence happens near the end of June and early July. Preemergence herbicides and overlapping residual herbicides are mandatory for the effective management of waterhemp.

Waterhemp has also presented significant challenges for management due to its ability to develop resistance to herbicides.

Minnesota populations of waterhemp have become resistant to herbicides in groups 2, 4, 5, 9, 14 and 27. Multiple herbicide resistance in the same population of waterhemp has become concerning.

In Minnesota, we have examples of populations with two-, three-, four-, five- and even six-way herbicide resistance.

Recent herbicide resistance screening work at the University of Minnesota has found two populations of waterhemp with resistance to imazamox (Group 2), dicamba and 2,4-D (Group 4), atrazine (Group 5), glyphosate (group 9), fomesafen (Group 14) and mesotrione (Group 27).

With six-way herbicide resistance, your postemergence weed control options become very limited.

It will soon be time to get out to the field and start scouting for weeds and other pests. Follow along with us online as we report what we are finding this season.

For more information on herbicide products and corresponding group numbers, check out this United Soybean Board publication.

Miller is a crops educator with the University of Minnesota Extension.

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