Herbicide resistant Palmer amaranth, common ragweed, common waterhemp and marestail continue to pose significant challenges to Maryland producers. In the last five years numerous herbicide tolerant traits have been introduced, including Roundup Ready 2 Yield (Glyphosate); RR2 Xtend (Dicamba, Glyphosate); 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 examine various combinations of pre-post herbicide combinations using the new herbicide tolerance technology and their effects 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 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.

Updated October 28, 2020:
Herbicide trials evaluating efficacy of various combinations of post emergent contact and systemic herbicide were established during the 2020 growing season. The selected site was part of a 52-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 Xtend soybean varieties was used to evaluate herbicide performance across an early and late treatment.

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Updated February 3, 2021:
2020 Final Report
Evaluating Novel Herbicide Tolerant Traits in MD Soybean Systems
Ben Beale, University of Maryland Extension-St. Mary’s County
Alan Leslie, University of Maryland Extension-Charles County

Field Trials:
Herbicide resistant weeds such as Palmer amaranth (Amaranthus palmeri) pose significant challenges to Maryland soybean farmers. Previous studies over the past several years have examined various combinations of PRE and POST herbicides using existing herbicide tolerance technology, and have provided farmers guidance in managing this weed species. In recent years, numerous additional 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 (2,4-D, glyphosate, glufosinate). These platforms offer greater flexibility and additional postemergence options that can be incorporated into an integrated weed management program. However, this gives rise to additional questions about the ability to tank mix various contact and systemic products and the effect on weed control and soybean performance. A field experiment was conducted in St. Mary’s County, MD in a field infested with glyphosate and ALS resistant Palmer amaranth. A randomized complete block, split-plot design was used to test 15 herbicide treatments and two application timings with four replicates (including a non-treated control). Herbicide treatments included each active ingredient targeting Palmer amaranth (glufosinate, dicamba, 2,4-D) alone, and in combination with sethoxydim and glyphosate to control grass weeds. Treatment protocol included a clean burndown program (paraquat) followed by application of a weak residual (S-metolachlor). Soybeans with Enlist and Xtendflex traits were planted, with dicamba and 2,4-D applications separated by an appropriate border. Initial results indicate that all tank-mix combinations tested provided adequate control of emerged Palmer amaranth, with no apparent antagonism, and no effect of application timing. Glufosinate alone provided weaker control of grass weeds than glyphosate or sethoxydim, and there appeared to be an antagonistic interaction between glyphosate and glufosinate in tank-mixes. Future work will repeat the 2020 experiment and include dicamba/glufosinate tank-mixes utilizing newer Xtendflex soybean technology. More information and detailed study results including trial photographs and data charts may be found in the supplemental document titled 2020 Palmer Amaranth Trial.

Outreach Efforts:
A team of Extension faculty from the Delaware, Maryland and Virginia collaborated to offer integrated weed management programming. The team was comprised of Michael Flessner and Vijay Singh from Virginia Tech, Mark VanGessel from the University of Delaware and Kurt Vollmer and Ben Beale from University of Maryland. The original series of seven workshops were postponed in March of 2020 after just one session was completed. The team reformatted the effort as a two-day virtual training in December of 2020. The virtual meeting was complimented with a hard copy packet of program materials including A Practical Guide for Integrated Weed Management in Mid-Atlantic Grain Crops, Program Agenda, IWM Face Covering, and a program booklet with various fact sheets. The virtual sessions were well received with total attendance of 450 participants from the mid-Atlantic region and beyond. A follow-up survey was conducted with 199 total responses. Farmers indicated the program had a value of $4.50 per acre and crop advisors indicted a value of $3.33 per acre. The program was recorded and posted to the GROW website: https://growiwm.org/mid-atlantic-workshop-on-herbicide-resistance-went-virtual-this-year/

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View uploaded report 2

Herbicide resistant weeds such as Palmer amaranth (Amaranthus palmeri) pose significant challenges to Maryland soybean farmers. Previous studies over the past several years have examined various combinations of PRE and POST herbicides using existing herbicide tolerance technology, and have provided farmers guidance in managing this weed species. In recent years, numerous additional 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 (2,4-D, glyphosate, glufosinate). These platforms offer greater flexibility and additional postemergence options that can be incorporated into an integrated weed management program. However, this gives rise to additional questions about the ability to tank mix various contact and systemic products and the effect on weed control and soybean performance. A field experiment was conducted in St. Mary’s County, MD in a field infested with glyphosate and ALS resistant Palmer amaranth. A randomized complete block, split-plot design was used to test 15 herbicide treatments and two application timings with four replicates (including a non-treated control). Herbicide treatments included each active ingredient targeting Palmer amaranth (glufosinate, dicamba, 2,4-D) alone, and in combination with sethoxydim and glyphosate to control grass weeds. Treatment protocol included a clean burndown program (paraquat) followed by application of a weak residual (S-metolachlor). Soybeans with Enlist and Xtendflex traits were planted, with dicamba and 2,4-D applications separated by an appropriate border. Initial results indicate that all tank-mix combinations tested provided adequate control of emerged Palmer amaranth, with no apparent antagonism, and no effect of application timing. Glufosinate alone provided weaker control of grass weeds than glyphosate or sethoxydim, and there appeared to be an antagonistic interaction between glyphosate and glufosinate in tank-mixes. Future work will repeat the 2020 experiment and include dicamba/glufosinate tank-mixes utilizing newer Xtendflex soybean technology.