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.