By Angie Peltier, UMN Extension crops educator, Thomas Peters, UMN Extension sugarbeet agronomist and Eric Yu, UMN Extension IPM educator
The following information was presented during the inaugural Strategic Farming: Field Notes session of 2026. Use your preferred podcasting platform or listen online to a podcast of this episode.
Budgets are tight in 2026
Producers frequently ask questions about managing inputs to make projected crop budgets in 2026. Managing inputs is a grower-by-grower and field-by-field proposition and can not be answered in a single statement. However, we know that managing weeds must be a priority. Weeds: a) rob yield; b) persist in the soil for up to 20 years; and c) are alternate host plants to soil borne and foliar pathogens. UMN Extension encourages producers to actively manage weeds using integrated weed management strategies.
Crops and weeds are emerging
Weed seedlings have begun to emerge in Minnesota crop fields. With research confirming that some of Minnesota tall waterhemp populations are resistant to most of the post-emergence herbicide groups such as Group 2 (ex. Raptor), Group 4 (ex. XtendiMax), Group 5 (atrazine), Group 9 (ex. RoundUp PowerMax), Group 10 (ex. Liberty Ultra), Group 14 (ex. Flexstar) and Group 27 (ex. Callisto), getting good early-season weed control using preemergence herbicides is all the more crucial.
Each growing season is different, making weed emergence timing quite unpredictable. Preemergence herbicides can help with early season weed control by being present as weed seeds are germinating.
If one has already planted and the crop has not yet emerged, there is still the potential to make an application of a preemergence product. However, if one does not plant into a weed-free seed bed, making an earlier-timed post emergence application is critical.
It pays to use preemergence herbicides
As corn quickly outgrows our ability to easily see weeds challenges without actively scouting for them, the first time some can be alerted to a season-long weed management challenge (to battle in years to come) is from the combine cab. Substandard weed management not only sets up the field for challenging weeds in future years, but could impact your corn crop’s yield potential in 2026.
The critical growth stage for weed control in corn is V6 to V8 or when six to eight leaf collars are present. In addition to a hybrid’s inherent yield potential, the growing environment provides inputs to the developing corn plant about which resources may limit the crop’s ability to set and develop kernels. The number of kernels that a corn plant will form is being set at V5. Good early-season weed control is consequently imperative and preemergence herbicides often provide (in protecting the crop’s yield potential) more than they cost to apply.
The economics of preemergence herbicides in soybeans is a little bit less clear and largely depends upon the weed species present and the growth stage of the crop when weeds emerge. The critical phase to control weeds in soybean is emergence through the V4 stage (when four trifoliate leaves have unfurled) with return on investment varying depending on growth stage at interference and weed species. Research indicates folks either break-even with preemergence herbicides in soybeans or can see up to 300% return on their investment, especially from weed interference in V1 to V3 soybean.
The more indirect impacts of preemergence herbicides are observed throughout the entire crop rotation, and particularly in crops like sugar beets. Sugar beets are at risk of injury caused by their sensitivity to many of the herbicides used in the greater cropping system, leaving much of the sugar beet weed control efforts to take place during the production of the other, less sensitive crops in the rotation. Further, there are fewer weed control options, especially postemergence options for weed control in sugarbeet. Without adequate weed control in less sensitive crops, yield potential of the sensitive crops would be compromised.
Management imperatives for short and long term efficacy of preemergence herbicides
1) Use a preemergence herbicide mixture containing more than one herbicide group
It is more likely that through randomly, naturally occurring mutations a weed plant has genes that allow it to survive a single herbicide group than two herbicide groups simultaneously. Therefore, to slow the speed that resistance to preemergence herbicides develops in Minnesota weeds, the current recommendation is to use a mixture of preemergence herbicides from more than one herbicide group (active ingredients in a given herbicide group (HG) have the same target site in weeds). Tank mixing an active ingredient from HG 14 and HG 15 is often recommended.
2) Use the full label rate of preemergence products
Corn hybrids metabolize the herbicide atrazine rendering it non-toxic to the crop. A similar phenomenon is observed with metabolic resistance in weed populations; through randomly, naturally occurring mutations in a weed plant’s genome, some mutations allow a weed to metabolize herbicide active ingredients from a specific herbicide group. While a mild form of metabolic resistance may be easily managed using the full label rate of an active ingredient, using a lower-than-label-rate is likely to allow weeds with a mild form of metabolic resistance to survive to cause management problems in the future.
Recent research from the University of Wisconsin weed science team showed that using full label rates of a mixture of preemergence herbicides from multiple herbicide groups can result in nearly 100% control of early season weeds.
Weed resistance does not occur in every field. Please contact your County Agent, your Regional Educator or State Specialist if weed escapes are observed from time-proven and effective herbicide programs. Our best opportunity to defeat weed resistance it to identify it early or before it becomes widespread.
Rain is needed to activate preemergence herbicides
Preemergence herbicides are applied to the soil to target weeds at a weak point in their lifecycle – germination. For good weed control activity, preemergence herbicides need to be activated in the soil water solution for germinating weed seeds to passively imbibe. Rain within 2 to 3 weeks after application is needed to ‘activate’ preemergence herbicides. While the amount of rain required for activation varies, a rule of thumb is that 0.5 to 1 inch of rain within a week and 1 to 2 inches total within 2 weeks of the application would be sufficient for activation.
When making a preemergence herbicide application to a field in which weed seedlings have already begun to emerge, be sure that the herbicide mixture also contains chemistries with burndown activity. If you are in a part of Minnesota currently experiencing poor activation conditions for the preemergence herbicides you’ve already applied and weeds have started to emerge, make plans for an earlier than normal post-emergence herbicide application. Once weeds have reached three inches tall, it is time to start spraying!
Dry soils and carryover
If an activating rainfall is not timely and the preemergence herbicide active ingredients become tightly adsorbed to soil particles or organic matter in the field, the conditions might be setting up for establishment issues for a fall 2026 seeded cover crop or carryover injury symptoms on a 2027 cash crop. Herbicide labels indicate rotation intervals for future crops. However, soil moisture and temperature ultimately feed microbes that breakdown herbicides and are dependent on environmental conditions.
Be cautious when conditions favor wind-blown soil
Tuesday, Thursday and Friday, May 12, 14 and 15, respectively, were three red-flag warning days throughout much of western Minnesota due to very dry soil conditions, low relative humidity values and high sustained winds and wind gusts. During conditions like this, when folks need to turn on headlights due to airborne soil particles, know that there are things other than soil particles that one could be at risk of breathing in, such as seed treatment insecticides and fungicides and preemergence herbicides. Work hard to protect yourself and your lungs whenever the air is hazy due to airborne soil particles.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
Disclaimer: Note that inclusion or exclusion of herbicide active ingredients or trade names are for illustration purposes only and neither implies endorsement nor lack of endorsement.
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