Herbicide resistant Palmer amaranth, common ragweed, common waterhemp and marestail continue to pose significant challenges to Maryland producers. In recent years, numerous herbicide tolerant traits have been introduced, including RR2 Xtend (Dicamba, Glyphosate); RR2 Xtendflex (Dicamba, Glyphosate, Glufosinate); LibertyLink (Glufosinate); LibertyLink GT27 (Glufosinate, Glyphosate, HPPD); and Enlist E3 Beans (2,4-D, Glyphosate, Glufosinate). Farmers are adopting this technology as another tool for managing herbicide resistance. This study will continue work started in 2020 examining various combinations of pre-post herbicide using the new herbicide tolerance technology and their effect on weed species commonly encountered. Education regarding weed identification and management strategies continues to be critically important to enable early intervention; thus funding will be requested to carryout field days and winter production meetings.

1) Evaluate efficacy of currently available herbicides used in soybean tolerant trait platforms for
control of Palmer amaranth, fall panicum and other weeds.
2) Evaluate antagonism or synergistic effects of herbicide combinations on Palmer amaranth.
3) Evaluate post-emergence efficacy at multiple growth stages (early, late, rescue).
4) Demonstrate management strategies for herbicide resistant weeds through Extension
outreach efforts such as on-farm demonstrations, regional conferences, published papers,
newsletter articles, field days, and winter meeting updates.

Education regarding weed identification and management strategies continues to be important to enable early intervention; thus funding is also requested to carry out field days and include weed resistance topics in winter production meetings. With many problematic weeds now found throughout the state it is essential that farmers learn the key management strategies to control these weeds. In 2020, due to the COVID-19 pandemic, the workshops were moved to an online Zoom platform. The workshops were offered in collaboration with University of Delaware and Virginia Tech. These workshops were well received, reaching 270 participants over two days. We propose to continue our educational
outreach efforts through another series of all day workshops and field day events focused on proactive and integrated management strategies to avoid future cases of herbicide resistance and current recommendations for current cases of resistance. The workshops would continue to build upon our collaborative approach.

Update:
2021 Progress Report
Efficacy of Herbicides, Timing, and Tank Mixes on Novel Herbicide Tolerant Traits in MD Soybean Systems
Ben Beale, University of Maryland Extension
Alan Leslie, University of Maryland Extension


Herbicide trials evaluating the efficacy of various combinations of post emergent contact and systemic herbicide was established during the 2021 growing season. This is the second year of the research study. The selected site was part of a 8 acre field primarily with Sassafras soils with a heavy infestation of glyphosate and ALS resistant palmer amaranth the preceding crop year. A plot design consisting of both Enlist and Xtendflex soybean varieties was used to evaluate herbicide performance across an early and late treatment. The following 17 treatment protocols are being evaluated:
1 Control
2 Roundup
3 Liberty + AMS
4 Roundup + Liberty + AMS
5 Enlist One
6 Roundup + Enlist One
7 Roundup + Liberty + Enlist One + AMS
8 Liberty + Enlist One + AMS
9 Xtendimax + Drift Guard
10 Roundup + Xtendimax + DRA + VRA
11 Roundup + Xtendimax + Liberty + Drift Guard
12 Xtendimax + Liberty + Drift Guard
13 Select Max + NIS + AMS
14 Select Max + Xtendimax + Drift Guard
15 Enlist One + Select Maxx + NIS
16 Liberty + Select + AMS + NIS
17 Enlist One + Liberty + Select + AMS + NIS

Soybeans were planted on June Data was collected throughout the season including emerged palmer amaranth, percent control of grasses and palmer amaranth 2 weeks and 6 weeks after treatment for both early and late treatments, and percent injury to soybean plants. Response to herbicides has been very good with noted differences between treatments
A separate trial at the same location was conducted to evaluate herbicide treatment protocols for grass control. Stands of fall panicum and barnyard grass were present in the test plots. Herbicide performance was evaluated at three time intervals.
The study is ongoing. Data will be analyzed this fall. Outreach meetings are being planned for this fall and winter.

View uploaded report

Update:

View uploaded report

View uploaded report 2

Herbicide trials evaluating the efficacy of various combinations of post emergent contact and systemic herbicides was conducted during the 2021 growing season to evaluate control of palmer amaranth. This is the second year of the research study. The selected site was part of an 8 acre field primarily with Sassafras soils with a heavy infestation of glyphosate and ALS resistant palmer amaranth the preceding crop year. A plot design consisting of both Enlist and Xtendflex soybean varieties was used to evaluate herbicide performance across an early and late treatment. The following 17 treatment protocols were evaluated:

1 Control
2 Roundup
3 Liberty + AMS
4 Roundup + Liberty + AMS
5 Enlist One
6 Roundup + Enlist One
7 Roundup + Liberty + Enlist One + AMS
8 Liberty + Enlist One + AMS
9 Xtendimax + Drift Guard
10 Roundup + Xtendimax + DRA + VRA
11 Roundup + Xtendimax + Liberty + Drift Guard
12 Xtendimax + Liberty + Drift Guard
13 Select Max + NIS + AMS
14 Select Max + Xtendimax + Drift Guard
15 Enlist One + Select Maxx + NIS
16 Liberty + Select + AMS + NIS
17 Enlist One + Liberty + Select + AMS + NIS

Soybeans were planted on June 24 with Pioneer P45T88 Enlist E3 and Asgrow AG47XFO Xtendflex variety soybeans utilizing a complete randomized block split plot design. All plots received a burn down/residual treatment prior to soybean emergence. Treatment consisted of Dual Magnum at 1 pint per acre; Gramoxone SL 3.0 at 2 pints per acre and Roundup Powermax at 1 quart per acre. Plots were then split, with one half of the plots receiving an application of the post emergence treatments applied 22 days after burndown application (On-time or early treatment) and the other half of the plots receiving a postemergence application 46 days after initial burndown (non-timely or late treatment). Palmer amaranth plants were emerged and present across all plots with an average height of 4 inches at the time of the on-time or early application. Palmer amaranth plants had an average height of 24-30 inches at the time of the late or non-timely applciation. Treatments were rated for percent control of Palmer Amaranth at 23, 32 and 60 days after postemergence application.

Results indicate that most tank-mix combinations tested provided adequate control of emerged Palmer amaranth, with no apparent antagonism. The treatment of Select Max + Enlist One resulted in significantly less control at 22 days after treatment. As expected, treatments with Roundup, Select Max and the control treatments had little to no control of Palmer amaranth. Additionally, the treatments applied late did not perform as well as the early treatment. This study confirms the importance of timely postemergence applications when Palmer amaranth weeds are still less than 4-6 inches in height.

A separate trial at the same location was conducted to evaluate herbicide treatment protocols for grass control. Stands of fall panicum and barnyard grass were present in the test plots. Herbicide performance was evaluated at three time intervals. The plot size was 5 feet wide by 10 feet long. Pioneer P45T88 Enlist E3 were planted on June 24. Postemergence treatments were applied to all plots on July 23. . Barnyardgrass and fall panicum were primary species present. Grass was an average height of 15-20 inches in height. Control percentage was rated from 0-100 on August 4 and August 9 with non-treated control plots and non-treated running checks along each plot being used as a reference point for 0% control.

In the last five years, multiple studies have compared the efficacy of residual herbicide programs for Palmer amaranth control. These studies in conjunction with farmer experiences have provided confidence in developing effective residual herbicide recommendations for control of Palmer amaranth. In addition, recent studies across multiple locations have been conducted to evaluate various complete pre/post herbicide programs across several timing scenarios. There are no new residual herbicides on the market. What has changed is the advent of new herbicide tolerant trait platforms. Current trait platforms available to growers in the 2021 growing season include: Roundup Ready 2 Yield (Glyphosate); RR2 Xtend (Dicamba, Glyphosate); RR2 Xtendflex (Dicamba, Glyphosate, Glufosinate); LibertyLink (Glufosinate); LibertyLink GT27 (Glufosinate, Glyphosate); Enlist E3 Beans (2,4-D, Glyphosate, Glufosinate). These platforms offer greater flexibility and additional management tools that can be incorporated into an integrated weed management program. However, there are several new questions that arise with the availability of this new technology. The most common question is the ability to tank mix various contact and systemic products and the effect on weed control and soybean performance. One limitation of the LibertyLink system is the poor grass control. Tank mixing glyphosate with glufosinate in a LibertyLink GT27 system may improve grass control, but what effect does it have on broadleaf weed control, such as Palmer amaranth?

There are also questions regarding timing of applications, efficacy of new technology on various weeds, and potential antagonism or synergistic effects. Finally, optimal tank-mix partners can become complicated with the new available options. For example, one tank-mix application could include a delayed residual product such as Zidua, Dual or Warrant, multiple contact products such as glufosinate and fomesafen, multiple systemic products such as 2,4-D and glyphosate and several adjuvants or surfactants (Zidua+Liberty+Reflex+Roundup+Enlist+Drift Agent+Crop Oil). This study will help to define best tank mix strategies based on weed pressure present.