Field Notes: Dry spring conditions affect weed management and small grain development

Phyllis Bongard, Educational content development and communications specialist

University herbicide trials illustrate the value of
an effective PRE program.

The late spring and dry conditions have impacted herbicide effectiveness and management decisions. With much of the state abnormally dry, what should producers be considering? Special guest Dr. Rodrigo Werle, University of Wisconsin – Madison Extension cropping systems weed scientist, addressed weed management concerns during the June 14 Field Notes session. He was joined by Dr. Jochum Wiersma, Extension small grains specialist, and moderator Dave Nicolai, Extension crops educator, to discuss how the 2023 spring conditions have affected small grain pests and development.

Weed management considerations

Start clean stay clean

An effective preemergence herbicide program is the foundation of a strong weed management program. PREs provide early-season weed control, reducing weed density and buying time for the postemergence (POST) herbicide applications. In addition, PREs provide flexibility since they’re not dependent on herbicide-tolerant traits. However, what happens to the PREs when there is no activating rainfall?

Fate of PREs under dry conditions

PREs require rainfall to move into the soil and get into soil solution where they can be absorbed by weeds. For ideal PRE performance, Werle likes to see between two- and four-inches of rainfall within 15 days of application. At a minimum, a half-inch rainfall is needed for activation if soils are moist and ¾ of an inch of rain if soils are dry.

Risk of herbicide loss

If herbicides remain on the soil surface, they can be at risk for volatilization or photodegradation losses. In What’s happening to my PRE herbicide on the soil surface?, Dr. Bob Hartzler, retired Iowa State University Extension weed specialist, notes that while several older herbicides (e.g. EPTC, trifluralin, s-metolachlor) are at risk of volatilizing, the risk for most of the commonly used PREs in corn and soybean production is low. Similarly, with a few exceptions – trifluralin, s-metolachlor, and isoxaflutole – most PREs are stable in the environment and at low risk of degrading in sunlight.

For the PREs that have remained on the soil surface, Werle feels that if we catch some rain, they will be activated and provide some control.

PRE selection

In a normal precipitation year, standard 3-way residual herbicide premixes (ex. Resicore, Acuron Flexi) provided good control of giant ragweed in UW corn studies. During the following year (2022) conditions were very dry after PREs had been applied. In this situation, the herbicide that provided the best control was dicamba (Diflexx). While Werle was surprised by the results, the response was consistent over several years. Consequently, if a PRE is going to be applied and the forecast for the following couple of weeks is dry, including dicamba in the PRE program is an option. The dicamba can control weeds as they emerge until the other residual herbicides are activated in the soil.

What if the PREs haven’t activated yet? Be strategic!

Large-seeded weeds – like giant ragweed – are difficult to control because they can emerge from deeper in the soil profile and escape many PREs that may exist in the upper soil layers. In addition to giant ragweed, Werle has also been seeing a lot of grasses – foxtails, barnyardgrass, etc – and while waterhemp has been slow in emerging, he expects it to explode soon.

What does this type of scenario mean for management? If a POST application is triggered now, a residual herbicide should be included. In soybeans, a third pass will also be needed to clean up the late emerging waterhemp. The key is to understand what’s happening on a field-by-field basis so the best POST decisions can be made.

Grass control in conventional corn

In Wisconsin trials, the most effective herbicides for grass control in conventional corn have been nicosulfuron (Accent), tembotrione (Laudis), and topramezone (Armezon).

POST options for giant ragweed and waterhemp control in soybean

While there are several effective options for managing giant ragweed, controlling waterhemp is more complicated. Two herbicides for traited soybeans have provided good waterhemp control in Werle’s studies. The first is 2,4-D choline (Enlist One in Enlist system). Note that performance decreases under high temperatures, so target application days when temperatures are below 85F. The second herbicide is glufonsinate (Liberty in a LibertyLink system). Although overall performance has been good with glufonsinate, combining it with a PPO herbicide (e.g. Cobra, Flexstar, etc.) increased consistency and enhanced waterhemp control. Dicamba (Xtendimax) is no longer an option this year because we’re past cut-off date in Minnesota.

What about waterhemp control in a non-traited or glyphosate-tolerant soybean? In this scenario, PPO herbicides (Cobra, Flexstar) are the only options that provided good control. Does that control come with a crop injury price?

The most soybean injury Werle has seen has been with Cobra (lactofen), regardless of whether glufosinate was applied with it or not. However, injury did not translate into soybean yield reductions. Other PPO herbicides – such as Resource or Flexstar – seem to cause less injury.

Carryover concerns

Herbicide carryover can become a concern in dry years. Keep in mind that the maximum amount of mesotrione (Callisto) applied to corn in one year is 7.7 fluid ounces. Applying amounts above this limit or applying too late in the season can injury soybean the following season.

On the other hand, spraying Flexstar (fomesafen) too late in soybean this year can injure corn the following season.

Small grains update

While the season started a bit later, the rapid transition into warm temperatures translated into fast emergence and good stand establishment. Overall, small grains are on a record pace of development. Some barley, for example, is heading just six weeks after planting – roughly two or three weeks ahead of normal.

The rapid development this season will likely result in a short crop and consequently, lower risk for lodging. Cool temperatures in June have resulted in fewer tillers compared to last year. With the crop in the stem elongation phase, water use goes up to about ¼ inch of water per day. That will increase as temperatures climb. Although rains have been spotty, most of the area could use at least a couple of inches of rain.

Pest report

Due to dry conditions, there have been few reports of small grain disease. While models suggest that conditions have been right for tan spot to develop, crops scouts have not found it. Other than an occasional odd day, the risk for Fusarium head blight has also been very low so far. Wiersma will keep an eye on that as the spring seeded cereals approach heading,

So far, aphid numbers have been low as they migrate north. Grasshoppers, on the other hand, may become an issue again under dry conditions. Armyworm flights have made it as far north as Roseau, where larvae were found in perennial ryegrass. For more information, see the latest Small grains update.

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Products are mentioned for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.

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