The cold, wet spring has impacted not only planting decisions, but also early season weed and pest management. Extension weed scientist Debalin Sarangi and Extension IPM educator Anthony Hanson joined moderator Dave Nicolai, University of Minnesota Extension crops educator, for the June 1 Field Notes session to discuss the issues.
1. Early season weed control
Recently planted corn is emerging in much of Minnesota, and while many fields look clean, but don’t be lulled into thinking that will last. While delayed preplant tillage controlled the earliest emerging weeds, ample soil moisture has allowed some weeds, like foxtail and woolly cupgrass, to get re-established. Emergence for other species has been delayed this season, but warmer temperatures now set the stage for rapid growth.
2. Importance of timely herbicide applications
Preemergence (PRE) herbicides provide a strong foundation ahead of the postemergence applications. Because planting dates were delayed, the postemergence (POST) applications may seem “early” this year, according to Sarangi. Even so, POST herbicides should be applied about 14 days after the PREs or mid-June if fields were planted in late May. Delaying the postemergence application is risky since weeds can quickly grow beyond the stage where they can be controlled.
There is also the possibility that some herbicides could cause carryover injury if they’re applied very late. A mid-July application of Flexstar, for example, increases carryover injury risk to next year’s crop. Check herbicide labels for rotation intervals. This will be particularly important if sugarbeets are in the crop rotation.
3. Layer preemergence herbicides for waterhemp control
Waterhemp is a particularly challenging weed to control in soybean because its emergence pattern extends into August. By layering preemergence herbicides in soybean, weed seedling control can be extended to canopy closure. In this strategy, a Group-15 (ex. Dual, Outlook, etc.) or Group-14 (ex. Valor) herbicide is followed by an additional Group-15 herbicide application about 30 days later to control the July flush of waterhemp. Once the soybean canopy is closed, waterhemp emergence is inhibited.
4. Dicamba label changes
There have been significant changes in Minnesota this year for dicamba product labels for dicamba-tolerant soybean. The cutoff dates for application are now June 12 for areas south of Interstate 94 and June 30 for areas north of I-94. In addition to these cutoff dates, there is a new temperature cutoff. No applications are allowed if the forecasted high or field air temperature exceeds 85 degrees F.
Changes for other herbicides discussed are summarized in a recently published article, Things to consider for successful postemergence applications. Always check the label as it is the final authority.
5. Other weed management tips
To reduce dicamba drift, Sarangi urges applicators to select the right nozzles that deliver coarse droplets. Wind and temperature are factors that need to be considered for not just dicamba, but all plant growth regulator herbicides.
Plant growth regulators (i.e. dicamba, 2,4-D) work best when combined with a glyphosate-type product, because two sites of action are provided instead of just one. Including a Group-15 PRE in the tank mix controls those late-emerging weeds and provides season-long control.
Target weeds when they are no more than 4 inches tall. Once they reach 6 inches, they have developed multiple growing points and will be very difficult to control.
6. Assessing storm damage
Severe storms caused significant damage in several Minnesota communities over the weekend. For the most part, crops seem to have escaped serious injury. Corn is young enough that the growing point is still protected below the soil surface. Soybean planting has been delayed, so few fields were up. If seedlings were still underground, they would have been protected. If, however, soybean crooks were damaged during emergence or plants were damaged below the cotyledons, the stand should be assessed. If soybeans were damaged above the cotyledons, regrowth can occur from axillary buds. For alfalfa, a severely damaged crop may impact first cutting decisions, particularly if it appears that a lot of material might be lost.
7. Scouting tips
Weed, disease and insect pests are rarely evenly distributed throughout a field. As a result, using an “M” or “X” pattern is an attempt to get enough scouting coverage across the field to get a good field average. If it turns out that issues are isolated, then spot-treating instead of a whole field application is an option.
In addition to numbers of pests, pay attention to life stages. That will often impact treatment recommendations or the need to even treat at all. For helpful scouting tips and how-tos, visit Virtual Crop Scout School webinars.
8. Black Cutworm
Black cutworm (BCW) is one of the main insects that the IPM team is keeping an eye on. While this has been a relatively low year for BCW moth flights into Minnesota, there were a few significant captures at the end of April and into early May. For those locations, the larvae should be developed enough to be able to cut corn until about June 20.
Economic thresholds vary depending on the both the amount of plant damage and the BCW larval stage. As the BCW larvae get older, for example, they have less potential to cause economic damage. The threshold is typically 2-3% plants cut, but as larvae approach the adult stage, the threshold increases to 5%.
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