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Field Notes session talks weed management and soybean aphid

Angie Peltier, UMN Extension educator, Tom Peters, UMN Extension sugarbeet agronomist and Bob Koch, UMN Extension soybean entomologist

Waterhemp. Photo: Angie Peltier
The following information was provided during a 2023 Strategic Farming: Field Notes session. Read further to learn more about this free program that takes place each Wednesday morning throughout the growing season.

Weed management: a challenge in dry conditions

This growing season began with an abnormally hot mid-May and June, which likely had an effect on weed seed germination, emergence, and management.. We have weed species in Minnesota that are challenging to manage every year, that are all the more challenging to manage in hot and dry conditions.

Kochia emerges first

Kochia is one of the first weeds to germinate and emerge in spring. We control kochia either with preemergence herbicides or postemergence herbicides over very small kochia. Kochia emerged so quickly or before the PREs could be activated by rainfall and in some cases emerged before sugarbeet or wheat. However, one thing that differed in 2023 was that the rest of the kochia seedbank stayed dormant and so we often did not see the extended period of germination and emergence than typically occurs.

Waterhemp: 2023 vs 2022

Waterhemp was less of a problem in 2023 when compared to 2022., Some areas of the state received enough rainfall in 2023 to activate preemergence herbicides immediately after planting, especially fields planted the first week of May, that may not have been activated by extended dry conditions Early herbicide activation combined with May and June 2023’s temperature-driven crop growth provided shading at the soil surface and dry soil conditions meant that waterhemp germination and emergence was disadvantaged.

Evaluating performance of preemergence herbicides

A common question is how did the preemergence herbicides work. Performance of PRE herbicides is related to sufficient and timely rainfall following application since preemergence herbicides need to be carried into the soil by rainfall and be in the soil solution to be passively imbibed through the root or shoot of germinating weed seeds. It is best if rainfall occurs within seven to ten days of application. Weed seeds with sufficient water to germinate and emerge may growth through herbicides on the soil surface if there is no rain after a PRE is applied to carry it into the soil solution. Emerged weeds usually are not controlled when rainfall occurs after weed emergence. This is why learning about weeds and developing knowledge of both germination and duration of emergence patterns is important information to have when evaluating performance of preemergence herbicides.

Be sure to pull and remove waterhemp plants from field

We encourage Minnesota farmers to walk fields and pull waterhemp escapes to reduce 2023’s additions to a field’s weed seedbank. Farmers need to add one more step to the process in addition to pulling waterhemp; they need to carry the plants out of the field. Waterhemp plants make viable seed as early as 2 weeks after the plant flowers. Viable seeds are quite likely to be formed on those plants that are simply pulled and left in the field. Viable seed produced in 2023 can survive to cause headaches for 4 to 6 additional growing seasons and so it is well worth the extra effort to carry pulled plants out of the field.

Weed biology, once again is important as kochia, common ragweed, and common lambsquarters are different than waterhemp and typically require a full season’s growth to produce viable seed.

Post-harvest weed management strategies and lessons learned for 2024

There are two options for waterhemp management late in the growing season with low-growing crops like soybean and sugarbeet: hand-pulling and removing plants from the field or using electrocution like with the ‘WeedZapper’. In addition to involving significantly less physical labor than walking rows, by using electricity to kill weeds that are taller than the cash crop, the WeedZapper stops physiological development in waterhemp, so viable seeds (if not already present) will not be produced.

After harvesting some of the earliest maturing crops like spring wheat, there will still be plenty of prime time left to allow weed seeds to germinate and emerge. First, don’t be in a hurry to till fields. Predation by rodents and other insect pests is an excellent way to eliminate weed seed. Second, using a fall herbicide program or tillage are options for post-harvest weed management.

Herbicide resistance and carryover

Sometimes lack of control is a signal for other challenges like weed resistance. If you applied a postemergence herbicide as directed (at the appropriate weed height, label rate, with required/recommended adjuvants, etc.) and you didn’t achieve desired control or there is a patch of weeds that weren’t controlled by the herbicide, you may have a suspected herbicide resistance problem. Collect weed seeds and send them to Drs. Peters or Debalin Sarangi to be tested for resistance.

We observed herbicide carryover in 2023. The combination of a dry 2022 fall and a snow-covered spring throughout parts of the state resulted in little hydrolysis or microbial breakdown of 2022 herbicide active ingredients, setting up 2023 crops for herbicide carryover injury. With how dry the 2023 growing season has been in some areas, we may be in for herbicide carryover issues in 2024. The only fool-proof way to know would be to do a bioassay in spring 2024.

Check the labels for the herbicides you applied in 2023 for the crop rotation interval for the crop you plan to plant in 2024. Use a bioassay if there are more months that must typically pass between your 2023 herbicide application and when you plan to plant your 2024 crop to be safe. An additional time to use a bioassay is when you think that you have gotten less rain than in a typical growing season and the 2024 crop you have planned for the same field(s) is sensitive to the herbicide active ingredients used in 2023. To run a bioassay, fill several disposable cups with soil from several areas of the field in question in spring 2024, sow the crop seed in the soil, water the soil and wait. If the seed germinates, the seedling emerges and looks fine, your crop is likely safe to plant in that field, if not you might consider selecting a less sensitive crop.

Scout your fields for insect pests!

Threshold-level soybean aphids, spider mites or defoliators in the area does not mean threshold level aphids, spider mites or defoliators in your field.

Areas of the state that have gotten timely rains have had more soybean aphids than in recent years, some at or near the treatment threshold. Areas that haven’t gotten much rain may have spider mite infestations developing. Dry and wet areas of the state alike may have an infestation of green cloverworms or other defoliating insects. However, even when a region of the state has many reports of any important pest, scouting is key! This is because individual fields within a region can have very different pest populations and so the only sure-fire way to know whether your soybeans are infested with an insect (or arachnid) pest is to scout!

Treatment thresholds

Treatment thresholds for soybean insect pests and spider mites have been developed to take emotion and subjectivity out of the decision-process. Treatment thresholds are developed to balance many competing factors, including: not wasting farmers’ resources with unnecessary inputs, reducing the chance that economic crop injury occurs, timing an insecticide/miticide application so that naturally occurring enemies of the pest have a bit of time to work, best timing an application to minimize the chances of having to make another application, providing a bit of time between the treatment threshold and economic injury during which to make a pesticide application.

As a reminder, during soybean reproductive growth stages (flowering through full seed) University of Minnesota Extension’s recommended treatment threshold for defoliating insects such as green cloverworm, grasshoppers and Japanese beetles a field-wide average of 20% defoliation, with the pests still active in the field. We are evaluating, but not yet recommending, the lower thresholds of 10-15% defoliation that some people may be hearing about.

Click here for a more information about managing both soybean aphid and two-spotted spider mites in soybean.

Click here for a more information about managing defoliating caterpillars.

Join us for the final two sessions!

Do you want to attend Field Notes yet this growing season? This program will run from 8:00-8:30 a.m. on Wednesdays through August 23, 2023. Topics will be announced the week of the program and may include issues related to soil fertility, agronomics, pest management, equipment, and more.

Learn more and register

Can’t make the live session? No problem. The discussion-based series will be posted immediately following the webinar to your favorite podcast-streaming service to listen at your convenience. Listen here online.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.

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