Field Notes: Continued Dry Conditions Affect Weed Control Decisions

Phyllis Bongard, Educational content development and communications specialist
Dry weather and spotty rains across the state continue to impact both herbicide effectiveness and weed control options. Dr. Debalin Sarangi, Extension weed scientist, and Tom Hoverstad, University of Minnesota researcher at the Southern Research and Outreach Center in Waseca, addressed weed management concerns during the June 28 Field Notes session. They were joined by Extension educators Dave Nicolai and Brad Carlson to give updates on crop development and nitrogen concerns in corn. Moderator Ryan Miller, Extension crops educator, steered the wide-ranging discussion.

Crop and soil moisture update

The drought has continued to intensify during the past few weeks in the Upper Midwest region. Spotty rains have given some relief in parts of Minnesota, but precipitation has been inconsistent. In southern Minnesota, for example, 7 to 9 inches of rain fell in early May, leading to ponding and replanting, until it turned hot and dry like much of the rest of the state. As a result, corn and soybean development has been slower than normal and the canopies aren’t closing quickly enough to shade out emerging weeds. Together, these factors are making weed management this season particularly challenging.

Weed control challenges

A tale of two fates for preemergence herbicides

Preemergence (PRE) herbicides are the foundation of a strong weed management program, but they require rainfall to be activated. UMN weed management plots planted in early May got enough precipitation to activate the PREs and those fields look great, according to Sarangi. Other plots planted two weeks later when it was dry didn’t receive the needed rainfall. Consequently, there was limited activation of the PREs and the plots are now full of foot-tall weeds.

The PREs didn’t activate. What now?

While emergence of small-seeded weeds – like waterhemp – is limited in dry soils, they will start emerging again when it rains. With short soybeans and slow canopy development, it could get ugly quickly. What are the best options for controlling this problematic weed in soybean?

Postemergence options

In those fields where PRE activation was limited, Sarangi hopes that a postemergence (POST) herbicide application was made early enough to take care of the weeds the PRE missed. However, if a clean-up option is needed, glufosinate (Liberty) can be applied until the R1 (beginning flower) stage in soybean, while Enlist One (2,4-D choline) can be applied through the R1 stage. If glufosinate is selected, herbicide activity will be better if applied on a sunny, hot day with some humidity. Treating weeds when they’re small becomes even more important when conditions are harsh.

With waterhemp reemerging, adding a residual Group 15 herbicide - like Warrant (acetochlor) - to a tank mix with either of these POST herbicides can help with control. Outlook (dimethenamid-P) may also be an option but it must be applied by V5.

What about the PPO-inhibitor herbicides (Group 14)? Flexstar (fomesafen) has a 10-month rotation restriction to corn, so we’re quickly approaching the application cutoff for this herbicide. However, older chemistry PPOs – such as Ultra Blazer (acifluorfen) and Cobra (lactofen) – may be POST options in soybeans.

Don’t forget the adjuvants!

In dry, hot conditions, some adjuvants can boost herbicide absorption into the plant. Read the herbicide label and if an adjuvant is mentioned, consider adding it.

What about iron?

Row cultivators could be a tool that comes back into play this year, particularly since the soybean canopy is not doing its job.

Carryover concerns

Herbicide carryover can become a serious concern in dry years. After dry conditions last fall and this spring, Sarangi has seen both corn and soybean injury from last year’s herbicide applications. For example, clopyralid (Stinger) and HPPD - likely Callisto (mesotrione) - injury has occurred in this year’s soybean and Flexstar (fomesafen) injury occurred in this year’s corn. Interestingly, the half-life of mesotrione and clopyralid are 15 and 70 days, respectively, which reinforces how drought can impact herbicide longevity in the soil. As we approach the end of June, odds increase that herbicides could carryover to next year’s crop, especially under dry conditions.

Crop development

Even though the number of kernels per ear is starting to be determined now, conditions during pollination play a much more important role. The most critical time is the two-week period from about V12 to pollination when the potential number of kernels per row is affected. The most recent rain will carry parts of southern Minnesota for several days, but more rain will be needed.

Nitrogen concerns

When the top few inches of soil are very dry, corn plants will pick up moisture from bottom of the root zone. Without water, nitrogen at the top of the root zone is not readily available to the crop. Now that the area has received some rain, Carlson doesn’t expect to see nitrogen stress in the immediate future. However, if early season N loss occurred, deficiencies may show up in August.

UMN research has shown that sidedress N applications past V10 or V12 are ineffective. While the trend toward sidedressed N has increased in the past decade, unfavorable weather – too wet or too dry - can put those applications and profits at risk.

Crop and weed management tours

Plan to attend two weed management tours that are scheduled in the next two weeks:

• Rochester Crop Management Tour – Weed management, herbicide resistance screening, corn insects, armyworm & cutworms, cover crops.
• Corn-Soybean Weed Management Field Day - Rosemount – New herbicide technologies, conventional corn, weed control systems in soybean, etc.