By Aaron Hager, University of Illinois Extension

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

Winter Annual Weed Species

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 a 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 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 soybean. 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 tankmix 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 one 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.

2019 Prevented Planting Fields

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 very common species in prevent plant fields, but other summer annual species (such as velvetleaf, cocklebur, morningglory, 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.

Herbicide Carryover

Some may scoff at the notion of herbicide carryover after the often-saturated conditions in 2019, but do not overlook this as a possibility. Delayed planting in 2019 often resulted in delayed application of soil-residual and foliar-applied herbicides. While soil moisture was not terribly limiting after planting, some portion of Illinois (such as the east-central region) experienced near drought-like conditions during July and August. Dry soil conditions can slow the rate of herbicide degradation, especially of those herbicides primarily degraded by soil microorganisms, and increase the potential for damage to rotational crops from herbicide carryover. Herbicide degradation by soil microorganisms usually occurs most rapidly when adequate soil moisture is present. Under extremely dry conditions, the rate of herbicide degradation by soil microorganisms can slow enough to allow the herbicide to persist into the next growing season.

An indication of a herbicide’s soil persistence can sometimes be inferred from the crop rotation intervals listed on the respective product label. Herbicides that tend to persist longer in the soil generally have longer crop rotational intervals compared with herbicides with shorter persistence. For example, labeled crops may be planted anytime following the application of carfentrazone (Aim), while 10 months must elapse between application of fomesafen (Flexstar) and planting corn. Rotational intervals for a particular herbicide sometimes vary by rotational crop, which also provides an indication of which rotational crops are more sensitive to herbicide residues remaining in the soil. Be sure to adhere to all rotation crop intervals for all herbicides applied in 2019.

What can be done to minimize the risk of injury to rotational crops from residues applied during the previous growing season? In the simplest terms, herbicide degradation simply takes time and moisture. Shallow tillage can help distribute herbicide more evenly across a field, and is more likely to help enhance herbicide degradation when soil temperatures are warm and adequate soil moisture is present. Early planting or planting a rotational crop that is very sensitive to the herbicide applied last season might further increase the likelihood of crop injury from herbicide carryover. Ultimately, the susceptibility of the rotational crop determines if persisting herbicide residues will cause any problems. Planting the same crop in 2020 as was planted in 2019 would effectively eliminate the potential for crop injury from herbicide residues. This solution may not be feasible for every situation where herbicide carryover is possible, but it is an option that warrants some consideration. If crop rotation must occur where there is concern for herbicide carryover, delaying planting as long as possible could allow additional herbicide degradation to occur.

Herbicide Resistance

Challenges associated with herbicide-resistant weeds are nothing new to Illinois producers and these challenges will become even more acute in 2020. Waterhemp populations resistant to herbicides from multiple site-of-action groups populate millions of corn and soybean acres. Many Illinois producers have experienced resistance caused by a modification of the herbicide-binding site (known as target-site resistance), whereas fewer producers have dealt with instances where herbicide resistance results from enhanced herbicide metabolism. Many weed scientists consider metabolic herbicide resistance as the next frontier of challenge for successful crop production.

Last spring we announced the discovery of waterhemp populations resistant to Group 15 herbicides, which include acetochlor, dimethenamid, pyroxasulfone and metolachlor. This type of resistance is caused by the waterhemp plant’s ability to rapidly metabolize (i.e., break down) the herbicide before it causes damage. Rapid metabolism is the same mechanism used by corn plants to survive Group 15 herbicides; it would seem waterhemp is mimicking the crop.

Compared with resistance to foliar-applied herbicides, resistance to soil-applied herbicides generally is more difficult to detect in the field. Resistance to foliar-applied herbicides is manifest as treated plants (assuming appropriate application rate and timing) not controlled, whereas resistance to soil-applied herbicides is manifest as a reduced duration of residual control. It is not always possible to predict if residual control is reduced 2 days, 8 days, 14 days, etc., as populations vary in their response to individual Group 15 herbicides. Remember, reduced residual control is not limited Group 15 herbicide resistance; resistance to soil-residual PPO-inhibiting herbicides also results in shortened residual control. These examples do emphasize the necessity of applying full label-recommended rates instead of reduced rates, as reduced rates will further curtail the duration of residual control.

Dicamba in 2020

The Illinois Department of Agriculture (IDOA) announced it will require Special Local Needs labels (referred to as 24(c) labels) during the 2020 growing season for the four commercial dicamba-containing products labeled for use in dicamba-resistant soybean varieties. The Special Local Needs labels include six elements, including a June 20 application deadline and prohibition of applications if the air temperature at the field at the time of application is greater than 85 degrees Fahrenheit OR if the National Weather Service’s forecasted high temperature for the nearest available location for the day of application exceeds 85 degrees Fahrenheit. Local National Weather Service forecasts are available at: https://www.weather.gov.

IDOA recognizes the importance of this technology to Illinois soybean growers and is taking this proactive step to reduce the instances of damage to dicot plant species (including sensitive soybean, many specialty crops and native plants such as trees) in order to preserve the technology for future seasons.

In Illinois, there are five application restrictions for these dicamba-containing products:

• R1 soybean growth stage (from original registration in Fall 2016)
• 45 days after soybean planting (from renewed labels in October 2018)
• V4 soybean growth stage for those using Tavium, a recently labeled dicamba premix
• June 20, 2020 mandated by the Illinois 24(c) label
• Actual OR forecasted air temperature greater than 85 degrees Fahrenheit