Waterhemp in wheat stubble

Editor’s note: The following was written by Aaron Hager, University of Illinois associate professor of weed science, for the university’s crop development Bulletin website Feb. 5.

No sooner have many unpleasant memories of the 2019 growing season begun to fade than we find ourselves looking ahead to potential residual weed management challenges in 2020.

Delays in planting the 2019 crop often accompanied delays in controlling existing stands of winter annual weeds prior to planting. The obvious and unwelcome result was that seed production by winter annual species likely was as high in 2019 as any spring in recent memory.

Tremendous seed production coupled with relatively good conditions for fall seedling establishment and a relatively mild winter thus far combine to set the stage for very dense stands of winter annual weed species this spring.

The composition of winter annual species likely will be similar to what has been common in past years, including horseweed (a.k.a. marestail). Horseweed can be one of the most challenging weeds to control prior to planting no-till soybeans.

Poor control results from several factors, including large plant size and resistance to glyphosate. If horseweed is resistant to glyphosate, a pint of 2,4-D in the spring is generally inconsistent/ineffective when it’s the only product in a tank mix active on the resistant population. It is advisable to control horseweed before plants exceed 4 inches in height.

Adding Sharpen or metribuzin to glyphosate plus 2,4-D can improve horseweed control. Include MSO with Sharpen and be sure to adhere to planting intervals in treated fields where another soil-applied PPO inhibitor is applied.

Glufosinate (Liberty, Interline, etc.) or Gramoxone SL are other options for preplant control. Improved control is common when these products are tankmixed with metribuzin and 2,4-D. Both glufosinate and Gramoxone are contact herbicides, so be sure to adjust application equipment (nozzles, spray volume, etc.) to ensure thorough spray coverage.

Dicamba provides more consistent preplant horseweed control compared with 0.5 lb. acid equivalent 2,4-D. Although dicamba-containing products approved for use in dicamba-resistant soybean varieties allow soybean planting immediately after application, we suggest delaying planting for a few days following application. Be sure to follow any label restrictions and replant intervals for other dicamba-containing products applied before planting any soybean variety.

For example, following the application of Clarity and 1 inch of accumulated precipitation, a waiting interval of 14 days is required for up to 8 ounces of Clarity and 28 days for up to 16 ounces. This use label pattern must be followed regardless of the soybean variety (dicamba-resistant or sensitive) planted.

Tillage is another option to control horseweed. Delay tillage until field conditions are suitable and be sure to till deep enough that all existing vegetation uproots completely.

The United States Department of Agriculture estimates approximately 1.5 million acres of Illinois cropland took the prevented planting option in 2019. Weed populations on many of these acres were reasonably well controlled, but successful weed seed production of numerous summer annual species occurred on many others.

Waterhemp was a common species in prevent plant fields, but other summer annual species (such as velvetleaf, cocklebur, morning glory, etc.) also were common.

It is reasonable to expect dense populations of summer annual weed species in fields where seed production in 2019 was successful, so a season-long weed management approach deserves careful consideration and implementation. This could include tillage to control existing vegetation before planting in combination with applying label-recommended rates of soil-residual herbicides close to planting.

In other words, these fields are poor candidates for post-only weed control programs in 2020. Be sure to scout these fields prior to a post herbicide application to ensure your product(s) of choice will control the weed spectrum present.

Seed of “uncommon” weed species could have migrated into prevent plant fields last season, so do not assume the 2020 weed spectrum will be identical in composition to previous seasons.

Finally, planting these fields last in 2020 could help deplete a portion of the soil seedbank by providing more time for additional weed seeds to germinate before planting. Control all emerged weeds with tillage or burndown herbicides before planting.