The first objective will consist of three factors, cover crop, preplant herbicide application, and at-planting herbicide application. Three sites have been identified with sufficient common ragweed populations. A wheat cover crop has been planted at two of the sites; therefore, all 8 treatments will be established. Due to the inability to establish a cover crop in the fall of 2020, only the effects of preplant herbicide and at-planting herbicide will be assessed at
the third site.

The second objective will evaluate sequential herbicide applications on large common ragweed. A broadcast treatment of glyphosate + S-metolachlor will be applied preplant to remove existing vegetation and provide residual control (note: S-metolachlor will not control common ragweed). Herbicide applications will be made when common ragweed height reaches 4 to 12 inches. Treatments will include a group 4 herbicide (Enlist One or Xtendimax), Flexstar, and Liberty
applied as single or as sequential applications made 7 to 12 days apart. The third objective will evaluate herbicide tank-mixes on large weeds at two growth stages. While common ragweed will be the target species for this study, it has the potential to provide data on other problem weeds as well. Herbicide applications will be made when common ragweed height reaches 12 in and 20 in. Treatments will include combinations of Enlist One or Xtendimax, Liberty, and Roundup.

1. Evaluate herbicide application timing for controlling common ragweed with and without a cover crop,
2. Evaluate herbicide options for managing of large common ragweed, and
3. Evaluate herbicide tank-mix efficacy for controlling herbicide resistant weeds at different sizes.

Update:
The following progress was made concerning our project objectives:
Objective 1: Studies were established at sites in Snow Hill, MD and Westover, MD, to assess ragweed control in response to cover crop, preplant herbicide application, and residual herbicides at planting. In addition, a supplemental study was established in a conventionally tilled site in Snow Hill, MD, to assess the effects of preplant herbicide application and at planting residual herbicides only. Soybean was planted between May 4 and May 18 at all three sites. Ragweed counts and height measurements were assessed from April 13 through July 12, every 1 to 2 weeks. Preliminary results have not indicated a clear pattern of differences among treatments in ragweed prevalence one month after planting for the various treatments. However, there does seem to be more ragweed following no cover crop than following a cover crop. The supplemental study preliminary results indicate a burndown herbicide that includes residual herbicides has less ragweed 1 month after planting than a burndown herbicide that does not include a residual herbicide.

Objectives 2 and 3: Studies were established in the previously mentioned conventionally tilled site in Snow Hill, MD to assess control of larger common ragweed. Initial herbicide treatments were applied when common ragweed plants reached 6 to 12 inches tall (June 8, 2021). Sequential applications were made 15 days later on June 23, 2021. Common ragweed in plots not previously treated had reached 14 to 18” at the time of the second herbicide application. Common ragweed control was evaluated every 1 to 2 weeks after application until July 12, 2021. Preliminary results indicate that tank mixing or sequential applications are needed when ragweed is sprayed at 6 to 12" tall. However; control was less when ragweed was 14 to 18", even with tank mixes.

In addition, a field-day was held on July 12 at the Snow Hill research site on herbicide resistant weed identification and management. Soybean board funded research and preliminary results were discussed. This event had 21 participants, and 15 received Maryland pesticide license recertification credits at the event.

Update:
The following progress was made concerning our project objectives:
Objective 1: We completed field research at the study site in Snow Hill, MD to assess common ragweed control in response to cover crop, preplant herbicide application, and residual herbicides at planting. Soybean was planted May 4. Ragweed counts and height measurements were assessed from 13 April through 12 July, every 1 to 2 weeks. On 13 April, there was less ragweed where a cover crop was present than no cover crop. On 10 May, following preplant herbicide, there was more ragweed where no cover crop and no preplant herbicide was applied, as compared to where preplant herbicide was applied or where there was a cover crop with no preplant herbicide applied. On 24 May and on 7 July, there was less ragweed where a residual herbicide had been applied than no residual herbicide.
In addition, we completed field research at the supplemental study site in a conventionally tilled field in Snow Hill, MD, to assess the effects of preplant herbicide application and at-planting residual herbicide. There was less common ragweed 27 days after planting following a burndown + residual herbicide than following just a burndown herbicide or no burndown herbicide.
In conclusion, ragweed primarily emerged in May; however, later emerging ragweed was noticed. Delaying cover crop burdown (“planting green”) plus herbicide application at planting that included residuals provided good control of ragweed. There was no advantage of applying preplant + at planting herbicide, when at-planting herbicide included residuals. Following tillage, applying burndown + residual herbicide at planting resulted in less ragweed 1 month after planting than applying herbicide without residual or no herbicide.

Objectives 2 and 3: Studies were established in the previously mentioned conventionally tilled site in Snow Hill, MD to assess control of larger common ragweed. Initial herbicide treatments were applied when common ragweed plants reached 6 to 12 inches tall (June 8, 2021). Sequential applications were made 15 days later on June 23, 2021. Common ragweed in plots not previously treated had reached 14 to 18” at the time of the second herbicide application. Common ragweed control was evaluated every 1 to 2 weeks after application until July 12, 2021. Results indicate that tank mixing or sequential applications are needed when ragweed is sprayed at 6 to 12" tall. However, control was less when ragweed was 14 to 18", even with tank mixes.

A field-day was held on July 12 at the Snow Hill research site on herbicide resistant weed identification and management. Soybean board funded research and preliminary results were discussed. This event had 21 participants, and 15 received Maryland pesticide license recertification credits at the event. Results from this year's project were published in University of Maryland’s Agronomy News and presented at the Maryland Soybean Board Checkoff research field day. Results are also being presented at winter agronomy meetings. Information from this project has reached over 200 people so far.

Update:
We completed field research at the study site in Snow Hill, MD to assess common ragweed control in response to cover crop, preplant herbicide application, and residual herbicides at planting. Soybean was planted May 4. Ragweed counts and height measurements were assessed from 13 April through 12 July, every 1 to 2 weeks. On 13 April, there was less ragweed where a cover crop was present than no cover crop. On 10 May, following preplant herbicide, there was more ragweed where no cover crop and no preplant herbicide was applied, as compared to where preplant herbicide was applied or where there was a cover crop with no preplant herbicide applied. On 24 May and on 7 July, there was less ragweed where a residual herbicide had been applied than no residual herbicide. In addition, we completed field research at the supplemental study site in a conventionally tilled field in Snow Hill, MD, to assess the effects of preplant herbicide application and at-planting residual herbicide. There was less common ragweed 27 days after planting following a burndown + residual herbicide than following just a burndown herbicide or no burndown herbicide. In conclusion, ragweed primarily emerged in May; however, later emerging ragweed was noticed. Delaying cover crop burdown (“planting green”) plus herbicide application at planting that included residuals provided good control of ragweed. There was no advantage of applying preplant + at planting herbicide, when at-planting herbicide included residuals. Following tillage, applying burndown + residual herbicide at planting resulted in less ragweed 1 month after planting than applying herbicide without residual or no herbicide.

Additional studies were established in the previously mentioned conventionally tilled site in Snow Hill, MD to assess control of larger common ragweed. Initial herbicide treatments were applied when common ragweed plants reached 6 to 12 inches tall (June 8, 2021). Sequential applications were made 15 days later on June 23, 2021. Common ragweed in plots not previously treated had reached 14 to 18” at the time of the second herbicide application. Common ragweed control was evaluated every 1 to 2 weeks after application until July 12, 2021. Results indicate that tank mixing or sequential applications are needed when ragweed is sprayed at 6 to 12" tall. However, control was less when ragweed was 14 to 18", even with tank mixes.

Soybean board funded research and preliminary results were discussed at a field-day on July 12 at the Snow Hill research site. This event had 21 participants, and 15 received Maryland pesticide license recertification credits at the event. Results from this year's project were published in University of Maryland’s Agronomy News and presented at the Maryland Soybean Board Checkoff research field day. Data from this study was used to develop a fact-sheet on the control of large palmer amaranth and common ragweed. An oral presentation discussing research findings was given at the American Society of Agronomy (ASA), Crop Science Society of America (CSSA), Soil Science Society of America (SSSA) Annual International Meetings in Salt Lake City, Utah on 7-10 Nov 2021. Results were also presented at the Wicomico County Farm Bureau meeting on 25 Oct 2021.

Ragweed primarily emerges in May; however, later emerging ragweed has been noticed. Delaying cover crop burdown (“planting green”) plus herbicide application at planting that includes residuals provides good control of ragweed. There is no advantage of applying preplant plus at planting herbicide, when at planting herbicide includes residuals. Following tillage, treatment that used burndown plus residual herbicide at planting resulted in less ragweed 1 month after planting than herbicide without residual or no herbicide. Research continues to support the need to apply postemergence herbicides at the label recommended weed sizes to obtain optimal control. At times when that may not be possible due to weather and other issues, it is important to use multiple, effective herbicide groups in tank mixtures or as sequential applications. Our research showed that at least 90% control was achieved on common ragweed ranging from 6 to 12 inches tall when effective postemergence herbicides such as 2,4-D or glufosinate were applied sequentially or as a single, tank-mix application. However, control with tank-mix applications declined when applied to common ragweed over 14 inches tall.

Farmers have reported substantial losses due to extreme common ragweed pressure and difficulties finding effective management strategies. Populations of common ragweed in Maryland have confirmed resistance to glyphosate (group 9), cloransulam (Firstrate; group 2), and/or fomesafen (Reflex, group 14).

The Maryland Soybean Board has funded common ragweed research trials over the last two years on the Lower Eastern shore. Previous research has shown that, products containing clomazone (Command), linuron (Linex), metribuzin (Dimetric), and flumioxazin (Valor) applied at soybean planting decreased ragweed prevalence (Beale and Morris, 2019). However, there was no clear pattern of a particular herbicide or tank mix being the most effective. Furthermore, our results have indicated that these herbicides can provide 3 to 4 weeks of residual common ragweed control when applied at soybean planting compared to 2 to 4 weeks before planting.

Even with preemergence herbicides, ragweed may still emerge once residual control is depleted, and postemergence control options are limited. Glufosinate (Liberty), 2,4-D (Enlist One), and dicamba (Xtendimax) can control these weeds; however, they are more effective on weeds less than 3 inches tall. If preemergence applications are ineffective and/or postemergence applications delayed, weeds may become too large for effective common ragweed control. As a result, multiple herbicides may need to be included in a single application or multiple applications may be required. Previous research from 2020 demonstrated that sequential applications were required to provide at least 95% control of common ragweed larger than 3 inches tall.

In addition, many commercial soybean varieties now contain stacked traits that allow for the use of at least three different herbicide groups. Among these being glyphosate, glufosinate, and 2,4-D or dicamba. Combining multiple effective postemergence herbicides has the potential to improve control of larger weeds though improved synergism. Preliminary results have shown that tank-mixing glufosinate + 2,4-D can provide effective control of common ragweed and marestail that has reached at least 12 inches in height. Additional, research is needed to evaluate the efficacy of these tank-mixes, as weeds grow larger.

In addition to residual herbicides, alternative weed management tactics need to be investigated for managing herbicide-resistant common ragweed. Previous research has shown cover crops to be effective in reducing common ragweed densities (Beam and Flessner 2019, VanGessel 2017).