Kurt Vollmer, Weed Management Specialist University of Maryland
If you attended the agronomy meetings this past winter, you heard me talk about the importance of using multiple strategies to mitigate herbicide resistance. Rotating and using multiple sites-of-action (SOA), is one strategy that helps prevent weeds and other pests from adapting to a single pesticide group. This can be challenging with so many products on the market. Take Action (https://iwilltakeaction.com) is a farmer-focused education platform designed to help farmers manage herbicide, fungicide and insect resistance. Several tools can be downloaded from this website to aid in your pest control decisions. Among these is an app (https://iwilltakeaction.com/app) that allows the user to
Quickly identify herbicide, fungicide or insecticide brands or active ingredient SOA numbers,
see a list of other SOA numbers to help diversify his or her weed control program,
and search the herbicide, fungicide or insecticide last used to prevent the use of similar SOAs
In addition, the GROW IWM website (https://growiwm.org) provides excellent information on how to use integrated weed management practices for herbicide resistant weeds.
Sarah Hirsh, Agriculture Agent University of Maryland Extension, Somerset County
In the next couple of weeks, common ragweed will begin to emerge (Figure 1). There are herbicide-resistant populations of common ragweed throughout Maryland. In 2019, common ragweed samples from St. Mary’s, Charles, Somerset, Worcester, and Wicomico counties, MD were tested and found to be resistant to glyphosate (group 9) and cloransulam (Firstrate; group 2 “ALS Inhibitors”) herbicides. Furthermore, three-way resistance to glyphosate, Firstrate, and fomesafen (Reflex, group 14 “PPO Inhibitors”) was found for two eastern shore samples from Dorchester Co, MD and Kent Co, DE. Because common ragweed populations can be resistant to glyphosate and other herbicides, pre-planting and residual herbicide control are particularly important.
Figure 1. Common ragweed seedlings 18 April 2019. Early-season control of common ragweed is critical, especially in herbicide-resistant common ragweed populations.
What steps can you take now against common ragweed?
Delay cover crop burn down. In addition to providing other agronomic benefits, cover crops may provide competition with spring weeds, including ragweed, and reduce emergence and growth. Growing conditions have been excellent this spring, so terminating a cover crop would leave an open window for weeds to proliferate.
Plan to apply something other than or in addition to glyphosate at pre-planting burndown. Research trials in 2019 indicated that a herbicide burndown of glyphosate plus Liberty at soybean planting effectively eliminated common ragweed. (Soybean was planted 18 May at one site and 3 June at a second site). The addition of 2,4-D or Banvel is another glyphosate tank mix option, and paraquat or paraquat + metribuzin is another option for smaller ragweed plants.
Apply a residual herbicide at soybean planting. Research trials in 2019 indicated that it was important to apply a residual herbicide at burndown to control post-planting common ragweed escapes. Applying the residual herbicide at soybean planting provided better common ragweed control than applying the residual herbicide at an earlier (4 April) burndown date. A second research trial comparing residual herbicide products found that Command, Linex, Dimetric, or combinations of Command + Dimetric, and Linex + Dimetric all reduced common ragweed prevalence in soybean.
Figure 2. Soybean field with herbicide-resistant common ragweed in Worcester Co, MD. Photo taken 30 Oct 2018.
Common ragweed can quickly get out of hand and dominate a soybean crop (Figure 2). However, with proactive control and scouting, herbicide-resistant common ragweed can be effectively controlled.
