Weed resistance to herbicides is a global problem, which usually results from the repeated use of herbicides with the same mode of action. Simply said: “Weeds just got used to that mode of action and cannot be killed with that mode of action anymore.” Similar phenomenon is observed in medicine with disease resistance to antibiotics.

Most importantly, after herbicide-resistance develops in a weed population at any farm, it stays there as long as that resistant seed is present in the soil, which can be a long time (many decades). For example, the triazine-resistant weeds from the 1980s and ALS-resistant weeds from the 1990s are still present at many farms. They were simply forgotten because glyphosate (Roundup) controlled them. However, with the development of glyphosate-resistant weeds, those triazine-resistant weeds aren’t being controlled with atrazine-based products. This suggests that atrazine resistance has lasted in many fields for the last 30 years. The atrazine-resistance gene transfers from generation to generation, which is the reason we still have triazine- and ALS-resistant weeds, despite the fact that they developed 30 and 20 years ago, respectively.

Continual increase in single weed resistance is of great concern; however, what really worries me is the increase in multiple-resistance (multi-stacks) in our major weed species, especially in waterhemp. For example, in other states there are confirmed cases of multiple resistance (three-stack) in waterhemp to ALS+triazine+PPO or ALS+triazine+HPPD inhibiting herbicides. In a few states there is also a four-stack resistance in waterhemp to triazine+ALS+PPO+glyphosate. The most worrisome development is the five-stack resistance in waterhemp, which was confirmed in Illinois and Missouri to ALS+triazine+PPO+HPPD+2,4-D and glyphosate + PPO + ALS + triazine + 2,4-D.

In Nebraska, there are confirmed waterhemp populations with a three-stack resistance (triazine+ALS+ glyphosate or triazine+HPPD+glyphosate). This provides evidence that waterhemp can develop resistance to any herbicide used extensively for its control. Repeated use of the same mode of action can easily result in the evolution of weed resistance, irrespective of the type of herbicide used.

This is a cause of major concern because when weed species start stacking several types of resistance, the number of viable herbicide options becomes greatly reduced. For example, having waterhemp resistant to four or five out of eight primary modes of action leaves only three modes of actions to combat this weed. Further use of the leftover modes of actions will put further pressure on those herbicides, which will result in additional resistance types, thus greatly reducing weed control options.

There is a need to diversify weed control programs, using a variety of non-chemical and chemical tools, including herbicide programs based on different modes of action. This will require the use of both pre-emergence and post-emergence herbicides.

General Guidelines for Resistance Management

Regardless of the type of weed resistance, the following guidelines can help reduce the chance for weed resistance at any farm:

1. Scout fields prior to the application of any herbicide to determine the weed species.
2. Scout your field after herbicide application to look for weed survivors. It takes 10-15 days for glyphosate to kill a weed. It is important to note that many glyphosate-resistant weeds may show initial susceptibility to glyphosate (for example, exhibit the appearance of a “dead weed”). However, the “appeared to be a dead weed” can regrow a week or two later from the top of the plant (meristematic growth) or the side (secondary buds, in the form of branches). A branch will take over as a new stem, producing a new plant with resistant seeds for future infestations.
3. Rotate herbicides, and avoid using the same herbicide mode-of-action in the same field in sequential growing seasons or more than once per year.
4. Limit the number of applications of a glyphosate, or any other single herbicide, in a single growing season.
5. Use mixtures of postemergence herbicides that each control the weeds in question, but have a different site-of-action. Some of the postemergence broadleaf herbicides will also provide additional soil residual activity for prolonged weed control. Utilize residual-based herbicides when possible.
6. Plant into a weed-free field. Use other herbicides alone or with glyphosate as burndown treatments for winter annuals, including horseweed (marestail), either in the fall or spring before crop planting, as it is easier to control those species when small.
7. In glyphosate-resistant crops, use soil-applied herbicides followed by a single application of glyphosate. This will provide additional modes-of-action for weed control, thus reducing a chance for weed resistance. Adding soil-applied herbicides also provides a longer “comfort zone” for weed control early in the season by delaying the critical time for weed removal and reducing the need for multiple glyphosate applications later on in the season.
8. Scout fields after application to detect weed regrowth (glyphosate-resistant waterhemp will regrow within three weeks), or look for escapes or changes in weed species composition (weed shifts). If a potentially resistant weed has been detected, use alternative control methods to prevent the weed from producing seed.
9. Use alternative weed management practices, such as mechanical cultivation, spot spraying with different herbicides, delayed planting, and weed-free crop seeds.
10. Clean equipment before leaving fields infested with or suspected to have resistant weeds.

A great resource for combating weed resistance is the Guide for Weed, Insect, and Disease Management in Nebraska (EC130). Additional information about combating weed resistance is also available at www.takeactiononweeds.com.

Working with Dicamba-Tolerant Soybeans

With increased planting of dicamba-tolerant (DT) soybeans (Roundup-Ready 2 Xtend), the off-target movement of dicamba to non-DT soybeans and other broadleaf crops is of concern. Since the majority of soybean acreage was planted to non-DT varieties in 2017, there were many dicamba drift complaints, some of which led to litigation. For example, the Nebraska State Department of Agriculture received over 90 complaints of dicamba drift onto non-DT soybeans with an estimated impact on 60,000 acres. Across the Midwest there were over a thousand complaints.

It is known that dicamba spray droplets have a tendency not only to drift with any air movement (even very slow wind), but also to move off target when fine aerosol droplets remain suspended during air temperature inversions. Thus, they can move from the target site well after the application up to even 98 hours (four days) after application. This drift can travel two to three miles, or more before being deposited onto nearby fields with various dicamba-sensitive crops including non-DT soybeans.

During the 2016 and 2017 crop seasons at Haskell Ag Lab near Concord, we evaluated the influence of micro-rates of dicamba products (Engenia and XtendiMax) to growth, development, and yield of three sensitive soybean types (Round-up Ready, Liberty-Link and conventional soybeans) at three growth stages (second trifoliate, start of flowering, and full flowering). The dicamba rates included: 0, 1/10, 1/50, 1/100, 1/500, and 1/1000 of product label rates (12.8 oz of Engenia and 22 oz of XtendiMax). To better visualize these micro-rates, consider that 1/10^th of the label rate is equivalent to three tablespoons and 1/100^th is equivalent to one teaspoon applied over the size of a football field (one acre).

Plots had four rows of each soybean type (Roundup Ready, Liberty-Link, conventional and dicamba-tolerant as a check). The three application times were second trifoliate (V2), just before flowering (V7/R1), or at full flowering (R2). The V2 timing was chosen to simulate potential drift at an early stage of soybean growth, which would be the earliest expected time for a dicamba product application. The second and third timings were chosen to simulate potential drift at the later stages of soybean growth, due to potential planting date differences among neighboring fields. For example, some fields might be planted earlier, some later, thus these two timings would capture potential drift among neighboring fields around flowering time. Visual evaluation of injuries was conducted at 7, 14, 21, and 28 days after treatment (DAT). Soybean morphological development included plant height, number of branches, days to canopy closure (for V2 and V7/R1 only), days to flowering (for V2 only), number of flowers (V2 and V7/R1), and days to maturity. Yields of all soybean types were harvested.

Roundup Ready, Liberty Link, and conventional soybeans were equally sensitive to all tested micro-rates of Engenia and XtendiMax. When micro-rates were increased, crop growth parameters were significantly impacted, including: reduction in plant height, alterations in branching pattern, delayed days to canopy closure and delayed date of flowering, reduced flower number, delayed physiological maturity, and most importantly, a reduction in soybean yield. Negative impacts depended on how application date related to soybean growth stage, with the V7/R1 stage being the most dicamba sensitive.  

Engenia and XtendiMax reduced soybean height by as much as 30 inches, depending on the herbicide rate, which also delayed, or completely prevented canopy closure. Almost all rates (1/500 to 1/10) of Engenia and XtendiMax applied during early vegetative stage (V2) delayed soybean flowering by 10 days across all soybean types. Based on ratings conducted at 65 days after planting, an Engenia rate of 1/10 (1.6 oz/ac) applied at V2 stage led to a 56% reduction in flower numbers. When applied at V7/R1, flower number was reduced by as much as 92%.

Both dicamba products delayed soybean maturity by 5-25 days, depending on the growth stages of dicamba application and the dicamba rate. Both Engenia and XtendiMax injured non-DT soybean varieties in a similar fashion. The visual injuries ranged from 20%-80%, depending on the growth stage of application and dicamba rate.

Yields of all non-DT soybeans were significantly reduced by both herbicides irrespective of application time. However, the V7/R1 stage appears to be the most dicamba-sensitive stage, followed by the R2, and then the V2 stages. For example, conventional, Liberty Link and Roundup-Ready soybeans yielded 58, 60, 60 bu/ac respectively in non-sprayed control plots. However, when the same soybeans were sprayed at the V2 stage with 1/10 of the Engenia rate, they yielded considerably less, i.e., 24, 22, and 27 bu/ac, respectively. Yields were further lowered to 18, 15, and 25 bu/ac, respectively, when the spraying occurred at R2. However, extremely low yields of only 3, 2, and 4 bu/ac were measured when the spraying occurred at V7/R1 stage.

Similar yield responses were measured in plots sprayed with XtendiMax. In most cases, the 1/50 and 1/100 of the labels rates reduced soybean yields by 13-16 bu/ac when applied at the V2 stage. Yields were also reduced even with “very low” exposures of 1/500 and 1/1000 of the label rate. For example, the 1/1000 of the label rate of Engenia applied at the V2 stage reduced yields by about 4 bu/ac in conventional, 2 bu/ac in Liberty Link, and 4 bu/ac in Roundup-Ready soybean. The same rates applied at V7/R1 stage reduced yields by 11 bu/ac in conventional, 3 bu/ac in Liberty Link, and 8 bu/ac in Roundup-Ready soybean.

Both Engenia and XtendiMax had similar effects on the growth and development of all non-DT soybeans, clearly showing that non-dicamba tolerant soybeans were sensitive to even very low micro-rates of Engenia and XtendiMax, showing why efforts must be made to avoid drift of dicamba onto sensitive soybeans.

For more information contact Stevan Knezevic at sknezevic2@unl.edu.