Kurt Vollmer, Weed Management Extension Specialist|kvollmer@umd.edu
University of Maryland Extension
Palmer amaranth and other pigweeds have emerged and are thriving. These weeds can quickly become too large and dense to control with many herbicides, so fields should be scouted and control measures implemented immediately. A contact herbicide, such as paraquat, applied at a spray volume of 20 gal/A can be used to control small plants prior to planting.
Soil-applied, residual herbicides may also be included as part of the preplant application. Flumioxazin or sulfentrazone-based products plus metribuzin or pyroxasulfone or S-metolachlor plus metribuzin have proven to provide effective control of Palmer amaranth and other pigweeds. However, planting should not be delayed too long after application, as herbicide efficacy will wane over time. If possible, incorporate with at least 0.5 inches if irrigation to activate the herbicides. Consult the label before mixing any herbicides, and be aware of any rotational restrictions before applying these herbicides, especially to double-crop soybean. Applicators should also be aware of potential drift issues to sensitive areas.
Cultivation can be effective on small seedlings (<3 inches), but larger plants are likely to re-grow if roots or stem remain in contact with the soil. Since Palmer amaranth can emerge throughout the growing season, repeated cultivation is necessary for control. However, local research has shown that spring tillage can result in higher Palmer amaranth densities compared to no-till systems. Other practices such as spring disking, chisel plowing, or vertical tillage will not provide adequate control.
Regardless of the control method used, be sure to scout in a timely manner after each operation to determine if additional control is needed. Tank mixing at least two effective modes of action, such as Enlist One + Liberty, and an additional residual herbicide, is recommended for both postemergence control and herbicide resistance management. Reducing soybean row spacing from 30 inches to 15 inches will also allow for faster canopy development, which will help negate the need for additional weed control.
Kurt Vollmer, Weed Management Specialist, University of Maryland Extension
and Mark VanGessel, Weed Management Specialist, University of Delaware
Palmer amaranth continues to be a major concern for farmers on the Eastern Shore. Many populations are already known to be resistant to glyphosate (Roundup) and Group 2 herbicides (e.g., Sandea, Raptor). Additionally, in North Carolina, some populations have developed resistance not only to glyphosate and Group 2 herbicides but also to Liberty and Callisto.
To investigate if there is a shift in Palmer amaranth response to commonly used herbicides in our region, we collected seed samples from 17 sites across the Eastern Shore. Most of these fields had been planted with soybeans, and seeds were collected in September. After cleaning, the seeds were stored in a refrigerator for three months to improve germination.
We evaluated each population with four herbicides all applied postemergence: Callisto, Reflex, Enlist One (2,4-D), and Liberty. At least five individual plants from each population were treated with each herbicide.
All the populations were susceptible to Enlist One and Liberty at the normal use rate (1 qt/A for each). However, nine of the populations had at least one plant survive when treated with Callisto, and five populations survived a Reflex application.
We are partnering with Penn State University to conduct further analysis to determine if we are seeing Palmer amaranth resistance developing to Callisto or Reflex in our region.
It is important that Palmer amaranth infested fields get treated with two effective modes of action, applied at full rates. Research has demonstrated that tankmixing two effective modes of action is better for managing resistance than using them in sequence. And do not rely on the same modes of action year after year. Scout fields shortly after application to determine if control was adequate, and if it was not, follow up to avoid Palmer amaranth “escapes” from going to seed.
What: Pesticide labeling is changing to incorporate protections for Endangered and Threatened species. Only recently have labeled herbicides addressed ESA, but as additional herbicides are labeled and existing herbicides are re-registered, these changes will be more common. This meeting will address how these changes will impact pesticide applications and discuss what needs to be considered when selecting specific pesticides.
Kurt Vollmer, Extension Weed Management Specialist | kvollmer@umd.edu University of Maryland Extension
Research was conducted at the Wye Research and Education Center to evaluate herbicide tank mixes for postemergence weed control in herbicide-tolerant soybean. As herbicide-resistant weeds continue to drive weed management decisions, options are needed to not only provide effective weed control, but also preserve the value of available herbicides.
Figure 1. Example probabilities of developing resistance when using Herbicide A, Herbicide B, and Herbicides A and B. Using two different herbicide groups decreases the probability that a weed will become resistant to both herbicide groups.
Tank mixing multiple, effective herbicide groups is one tactic that can be used to impede herbicide resistance. By including multiple, effective herbicide groups when making an application, there is a lower probability that a weed species will develop resistance to all herbicides used (Fig. 1). Furthermore, tank-mixing different herbicide groups can have a synergistic effect, where the combined effect of two or more groups is greater than the effects of each herbicide alone. For example, previous research has shown Enlist One + Liberty to be more effective in controlling as common ragweed and Palmer amaranth, compared to each individual herbicide.
Furthermore, including a residual herbicide in the tank when making a postemergence application may be necessary for full-season control of certain weeds, such as Palmer amaranth. Including herbicides with both foliar and residual activity, such as fomesafen (Reflex), in tank with other effective herbicides can help to preserve the utility of these herbicides.
This research examined the effectiveness of tank mixing of herbicides with foliar (2,4-D, fomesafen) and residual (fomesafen, S-metolachlor) for early and late postemergence weed control in soybean. Plots (10 ft. x 25 ft.) were arranged in a randomized complete block design with 4 replicates. Herbicide treatments consisted of applying Reflex (fomesafen), Reflex + Dual Magnum (S-metolachlor), Reflex + Enlist One (2,4-D) or a three way mix of Reflex + Dual + Enlist One (Table 1). The entire study area received 1 pt/A Dual Magnum within 24 hours of soybean planting Enlist E3 soybeans on June 4, 2024. Early postemergence (EPOST) applications were made 2 weeks after planting and late postemergence (LPOST) applications 4 weeks after planting.
Table 1. Postemergence herbicide treatments for resistant weed mitigation and control in soybean.
Herbicide(s)
Rate
Timinga
Reflex
1.5 pt
EPOST or LPOST
Reflex + Dual
1.5 pt + 1.5 pt
EPOST or LPOST
Reflex + Enlist
1.5 pt + 2 pt
EPOST or LPOST
Reflex + Enlist + Dual
1.5 pt + 2 pt + 1.5 pt
EPOST or LPOST
a Herbicide treatments were applied early postemergence (EPOST) 2 weeks after planting or late postemergence (LPOST) 4 weeks after planting soybeans on June 4, 2024.
Broadleaf Weed Control
Application timing did not affect common lambsquarters or morningglory control. Reflex + Enlist or Reflex + Enlist + Dual controlled common lambsquarters better compared to Reflex alone, and morningglory species better than Reflex or Reflex + Dual (Fig. 2).
Figure 2. Control of common lambsquarters and morningglory species 7 weeks after soybean planting. Bars of the same color with the same letter are not significantly different according to Tukey’s HSD (α = 0.05).
Giant Foxtail Control
Applications made EPOST provided better control compared to applications made LPOST (Fig. 2), but giant foxtail control varied from 38% to 78%, with no significant differences among herbicide treatments (Fig. 3). It should be noted that Enlist One does not control grasses. Both Dual Magnum and Reflex can provide some grass activity, but only if applied PRE. Better foxtail control with EPOST treatments could be attributed to overlapping residual control with these treatments. Overlapping herbicides is a tactic that involves sequential applications of herbicides with soil-residual activity to lengthen herbicidal activity before the first herbicide dissipates. As Dual Magnum was included in both PRE and POST applications, the EPOST applications likely provided better overlapping residual control due a shorter application window between the PRE and POST applications (2 weeks for EPOST and 4 weeks for LPOST).
Figure 3. Giant foxtail control following early and late postemergence applications 7 weeks after soybean planting. Bars of the same color with the same letter are not significantly different according to Tukey’s HSD (α = 0.05).
This research highlights the value of adding multiple herbicide groups to the tank at the time of POST applications (Fig. 4). While additional work is needed to confirm the results of this study, the following factors should be considered when deciding which herbicides to include in the tank:
The types of weeds are prevalent in the field. Should the spray program focus primarily on broadleaf weeds, grasses, or both?
The emergence period for the weeds being controlled. Will a single POST application negate the need for additional treatment, or should an herbicide with residual activity be included?
Each herbicide must be effective alone on the target weed. Including multiple herbicides will not be as effective if a weed already has significant resistance to one of the herbicides in the mix.
Figure 4. Weed control with early postemergence applications of a) Reflex + Dual and b) Reflex + Enlist + Dual 7 weeks after planting. Photo credit: Logan Bledsoe.
Acknowledgements
Support for this project was made possible by funding from the Maryland Soybean Board as well as technical support from Jadon Cook, Logan Bledsoe, Sam Denherder, and the University of Maryland Wye Research and Education Center.
Commercial products are mentioned in this article solely for the purpose of providing specific information. Mention of a product does not constitute a guarantee or warranty of products. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by University of Maryland Extension is implied.
Kurt Vollmer, Weed Management Specialist | kvollmer@umd.edu University of Maryland Extension
Pesticide runoff can occur when pesticides are carried by water off an application site. This usually occurs when water is applied to the soil faster than it can be absorbed, resulting in excess water flow across the land. Pesticide runoff into streams can pose risks to aquatic life, fish-eating wildlife, drinking water, etc. Therefore, new ecological mitigation requirements are being added to certain pesticide labels to reduce pesticide runoff.
What is the purpose of these new runoff mitigations?
These measures are designed to reduce pesticide movement out of a treated field. These practices are part of a wider strategy to protect endangered and non-target species, and will be part of future pesticide registration decisions.
Will these new runoff mitigation requirements be applicable to all pesticides?
No, only the application of certain pesticides will require these measures be implemented. Check the pesticide label for a section dedicated to runoff/erosion mitigation.
What do I do if I plan to spray a pesticide with one of these new mitigation requirements?
First, determine whether the application area is in a county designated as having high runoff vulnerability. The EPA has classified all counties on the Eastern Shore of Maryland and Delaware, as well as Anne Arundel, Charles, and St. Mary’s counties as having high runoff vulnerability. Initially, only applications to fields in these counties will require additional mitigation.
Are there exemptions for implementing these mitigations?
Yes, additional mitigation will not be required in counties with high runoff vulnerability if any one of the following criteria are met during an application:
the soil texture is comprised of over 50% sand, a loamy sand, or sandy loam soil;
the application area has a slope of ≤ 3%, or has a perimeter berm system;
the application is a partial field treatment, such as a banded or spot spray application;
the pesticide is incorporated into the soil mechanically or via irrigation; and
the treated field has subsurface or tile drains installed with controlled drainage.
My application area does not meet any of the aforementioned criteria. Now what?
Visit https://www.epa.gov/pesticides/mitigation-menu for a list of options. Even if additional mitigation is required, several EPA accepted practices have already been adopted in the area, such as reduced or no tillage, and the use of cover cropping.
Reports are for crop conditions up to June 6, 2024.
Western Maryland
I have a good friend who says, “I don’t have forty years of experience farming; I have one year’s experience forty times.” This spring has been just like that. We have had more moisture than last year, but then came the hot days, cool weather, and more moisture. Corn is planted, and the first cutting of hay is in the barn. Soybean planting is winding down, and grain harvest will be here before you know it, starting with barley. With the moisture we will see how much disease took its toll. Fungicide applications weren’t always as timely as we would hope because of the frequent showers. Until next month when harvest is underway it is then the story will be told.—Jeff Semler, Washington Co.
Central Maryland
Planting is almost complete. Slugs have been an issue; some soybean fields will need replanting. It was getting a bit dry, which did provide a good hay-making window. However, a thunderstorm is rolling in as I write this, giving us some needed moisture. Small grains are looking good. Summer annual weeds like pigweed, crabgrass, and foxtail are starting to take off with this warmer weather.—Kelly Nichols, Montgomery Co.
Northern Maryland
Spring has been a little challenging in our area. A dry spell in mid-late April made herbicide efficacy less than ideal, followed by several weeks of a lot of rain, which activated herbicides but prevented the timely termination of other cover crops. Early planted corn and beans are up and out of the ground, with earliest planted corn around V6 and getting it’s second shot of nitrogen. Rainy weather in late April early May delayed planting the remainder of the crop by about two weeks. These later planted fields are much further behind than usual but are looking good. There has been some slug damage present, it gets worse the further west you go in the region. Barley nearing harvest and wheat is starting to turn. There are symptoms of head scab but it remains to be seen how severe DON levels are in the grain. The wet weather has made it very difficult to make timely dry hay.—Andy Kness, Harford Co.
Upper and Mid Shore
Corn acreage is looking good, with a nice color and uniform growth. Depending on when it was planted, corn is in the V2 to V5 stages right now. Most of our full-season soybeans are already in the ground. As usual, rain across the region has been hit or miss, with some areas getting a lot more than others. These downpours might not be ideal, but they’re certainly better than no rain at all. Barley harvest is just around the corner. Summer annual weeds are popping up and are ready to impact yield potential. If your residual herbicide program hasn’t kept them in check, now’s the time for a POST application. And remember, using multiple modes of action in your tank-mix slows down the evolution of herbicide resistance in weeds.—Dwayne Joseph, Kent Co.
Lower Shore
has been planted and currently in emergence stage to around V4 stage. Corn is looking good, and being sprayed to combat early season weeds. Soybean planting is currently underway. In a few instances, early planted soybean (early April) had to be replanted due to slug damage. Wheat is looking good and drying down. Farmers are gearing up to cut wheat, which is earlier than anticipated.—Sarah Hirsh, Somerset Co.
Southern Maryland
As I write this, a very welcome rain is falling outside my window. As we move into the hotter days of June, the old adage that we are only a week away from a drought at any time is holding true. Two weeks ago it was too wet to get into fields and this week we have corn fields starting to show curling leaves and drought stress. In general corn is off to a good start with most sidedress N applications completed. There have been a lot of catch up full season beans planted in the last 2 weeks. Slug feeding has been minimal this year. Barley is drying down with harvest expected any day. Wheat will not be far behind. Ryegrass continues to be a challenge for producers in both burndown situations in corn and beans, as well as small grains. Many fields may need a harvest aid to kill the ryegrass and allow for timely harvest of wheat. Given the wet conditions following flowering, we are encouraging producers to get the wheat crop off as quick as possible to maintain grain quality.—Ben Beale, St. Mary’s Co.
Kurt Vollmer, Weed Management Specialist | kvollmer@umd.edu University of Maryland Extension
Italian ryegrass has been giving us trouble the past couple of years. I’ve had several reports of ryegrass control failures following glyphosate applications. Last year, seeds from 49 ryegrass populations from Maryland and Delaware were screened for glyphosate-resistance by Dr. Caio Brunharo’s lab at Penn State. Out of 40 populations screened, all were controlled by glyphosate at 2 lb. ae/A.
This indicates that recent troubles controlling ryegrass may be due to application issues rather than glyphosate-resistance. This species can be particularly tricky to manage this time of the year, so it’s important to remember:
Cold weather affects glyphosate uptake and translocation. Applications should be made when the temperature is greater than 55°F and consistently remain above 45°F for 3 to 5 days to be effective.
Higher rates will be needed to control ryegrass compared to other species (1.25 to 1.5 lb. ae a/A).
Plants should be less than 6” but no more than 8” tall at the time of application.
Other components in the tank can also affect glyphosate performance.
Include a spray grade ammonium sulfate (8.5lb. to 17lb. /100 gal) in the tank to abate water quality issues. UAN and high rates of triazine herbicides (>0.25 lb. ai/A), such as atrazine, that are included in the tank can also reduce glyphosate absorption and translocation.
If glyphosate alone fails, try tank mixing or alternative herbicides. Last year at the Lower Eastern Shore REC, 98% ryegrass control was achieved with glyphosate (1.25 lb. ae/A) + clethodim (0.121 lb. /A) + nonionic surfactant (0.25%v/v) + AMS (8.5lb./100 gal) or sequential applications of paraquat (1 lb/A) + crop oil (1%v/v) + AMS (8.5lb./100 gal) made 14 days apart (Figure 1). In trials conducted in Pennsylvania, glyphosate + 0.02 lb. rimsulfuron/A also controlled ryegrass greater than 95%. Always consult the label for important information such as tank mixing and plant back intervals before applying any pesticide.
Figure 1. Italian ryegrass response 22 days after application to a) non-treated, b) glyphosate + clethodim, c) paraquat fb paraquat plots. Images: K. Vollmer, University of Maryland.
A recent Arizona federal district court ruling has vacated the 2020 registrations for dicamba products used over the top in cotton and soybean production. The ruling is based on violations of federal law requiring the public to have an opportunity to comment on the proposed “new uses” in 2020. This ruling currently means for growers that these products might not be available for the 2024 growing season. Based on the ruling, we are waiting to determine what the defendants will do next. Update: On Feb. 14, the EPA issued an existing stocks order to allow for limited continued usage for the dicambas at issue for stocks no longer in control of the pesticide companies. A table of what is allowed is below; to read that order, click here.
Court’s Decision
This litigation stems from the registrations issued in 2020 and amendments made in 2022 and 2023 by the Environmental Protection Agency (EPA) for the “new use” over-the-top applications of dicamba products. At issue in this is did EPA violate the Federal Insecticide, Fungicide, Rodenticide Act (FIFRA), the Endangered Species Act (ESA), and the Administrative Procedures Act (APA) when approving the “new use” registrations in 2020.
The court agreed with the plaintiffs that the 2020 registration was a “new use” because the 2016 and 2018 registrations had been vacated and canceled by an earlier court ruling. EPA had to treat the 2020 registration for over-the-top applications of dicamba products as a new use. This “new use” registration required the EPA to publish the 2020 registrations and allow the public to comment.
Because of the violations in issuing the 2020 registration for over-the-top use of dicamba products, the court turns to whether an exception should be granted to EPA to remand the registrations without vacatur. Vacatur is a Latin term meaning it is vacated. Based on the record, the court determines that vacatur is warranted and an exemption should not be granted. EPA had failed to consider many potential risks when assessing the new uses, and based on prior court rulings, it had effectively allowed the dicamba products on the market with no registrations since 2016.
What Does All This Mean?
We are currently back in a wait-and-see mode regarding whether dicamba will be available for over-the-top uses during the 2024 growing season. It is unclear at this time if EPA will appeal this ruling and request a stay while the Ninth Circuit is hearing the appeal. If the EPA does appeal and receive a stay, this will allow sales of dicambas used in over-the-top applications to continue. If EPA does not appeal, we could see what we did in 2020 with EPA canceling the registration based on a court ruling but allowing already purchased products to be used during the growing season.
One vital issue to note in all this is that Bayer (the parent company of Monsanto) in 2023 began to bring lawsuits claiming growers saved seeds for replanting the new Xtendimax technology. These are similar to those brought when growers would save Roundup Ready technology before the patent expired, but a few have one difference. Several lawsuits highlight that growers additionally violated patents by spraying dicambas that were unapproved for over-the-top applications. Why is it important to note this here? Many growers may assume that they can spray dicambas unregistered for over-the-top applications, but that can come with stiff penalties from state agencies and EPA and, at the same time, may open growers up to claims of patent violations.
Updated: Table from order highlighting what is allowed.
Kurt Vollmer, Dwayne Joseph, and Alan Leslie University of Maryland
Figure 1. A flame treatment is applied to control emerged weeds in soybean.
Starting clean or weed-free is the key to a good weed control program, especially when noxious weeds, such as Palmer amaranth are present. While conventional growers can use soil-active herbicides to manage these weeds, control is more complicated in organic systems. Flame weeding is a non-chemical tactic that has been shown to control several grass and broadleaf weed species. The majority of flame weeding treatments are applied to emerged weeds; however, studies have also shown flame treatments to have detrimental effects on the seeds of certain weed species post-dispersal. Furthermore, seeds from species such as horseweed (marestail) and Palmer amaranth tend to germinate from shallower depths in the soil profile, and may be more readily controlled by flaming on or near the soil surface. Cultivation/tillage is another tactic that can be used to control weeds in both conventional and organic systems. However, cultivation can lead to additional weed emergence and cannot be used when the soil is wet. Flame weeding may help to supplement weed control when cultivation is not an option.
In 2023, a study funded by the Maryland Soybean Board was conducted at sites in Caroline and Kent county Maryland to evaluate flame-weeding as an integrated tactic for early-season weed control in soybean (Table 1). All plots were flamed immediately after planting followed by 1 or 2 additional flame treatments or flame treatments integrated with a cultivation treatment when weeds reached 3” in height. In addition, different walking speeds (1 and 2 mph) were tested to determine if longer flame exposure improved weed control. All flame treatments were made using a propane-powered Inferno Flame Weeder (Neversink Farms, Figure 1), and cultivation was done using a 25cc 2-cycle gas-powered cultivator (Craftsman).
Results from both studies showed that flame treatments affected overall broadleaf density, but cultivation was needed to attain higher levels of control. Flame treatments alone helped to reduce weed density at the Kent County study relative to the untreated check, with three subsequent flame treatments showing a reduction in broadleaf weed density compared to one or two flame treatments (Figure 2).
Figure 2 (left). Broadleaf weed density at the Kent County site 5 weeks after planting. Figure 3 (right). Palmer amaranth density at the Caroline County site 4 weeks after planting. Values followed by the same letter are not significantly different according to Student’s T-Test (α = 0.05).
While a diversity of species were present at the Kent County site, Palmer amaranth was the dominant species at the Caroline County site. At this site both treatment and walking speed had an effect on Palmer amaranth density 4 weeks after planting. While the majority of flame treatments did not differ from one another, Palmer amaranth density was lower with 3 subsequent flame treatments at 1 mph compared 3 subsequent flame treatments at 2 mph (Figure 3). Similar results were observed with the flame followed by cultivation followed by flame treatment suggesting that longer flame exposure may be needed for effective Palmer amaranth control. It should also be noted that Palmer amaranth varied in height at the time of postemergence applications, with flame treatments having a reduced effect on larger weeds (Figure 4).
Figure 4. Palmer amaranth injury following flame weeding.
While results from both sites showed that flame treatments can reduce weed density, weed control was not maintained at acceptable levels throughout the growing season. In the case of the Caroline County site, the level of the Palmer amaranth infestation was too high to produce a viable crop. These results suggest that preemergence flame treatments are not a viable option for weed management compared to postemergence flame treatments. However, additional research is needed to determine how postemergence flame treatments may be better integrated into a more comprehensive weed control program.
Kurt Vollmer, Weed Management Specialist | kvollmer@umd.edu University of Maryland Extension
Figure 1. Preplant applications of Liberty (left) and Gramoxone (middle) compared to the untreated check (right) 10 days after application at Wye REC. Images: Kurt Vollmer, Univ. of Maryland.
Results from the 2022 Maryland Weed Control Trials are now available. These trials evaluate crop injury (PHYGEN) and herbicide efficacy (CONTRO) for certain weed species. This data can be used to compare herbicide options crops including corn, soybean, and wheat. Each metric is based on a visual assessment on a 0 to 100% scale with 0 being no control/injury and 100 being complete control/plant death. Products with a control rating less than 55% offer poor to no weed control. Growers should use caution when selecting an herbicide program based on one year of data as environmental conditions and weed populations can vary across locations. A copy of this report can be accessed at https://extension.umd.edu/sites/extension.umd.edu/files/2023-07/2022%20Weed%20Control%20Results_FINAL.pdf or by using the QR code to the right, or by calling your local extension office for a copy. For more information on how to interpret this report, contact Dr. Kurt Vollmer (kvollmer@umd.edu).
