Research HighlightsCover crops help manage Marestail
Herbicide-resistant weeds are an ever-increasing problem for Kentucky soybean farmers, and Dr. Erin Haramoto is on the trail of a possible solution. Unlike many possible solutions, one of Haramoto’s top tips for controlling resistant marestail doesn’t come in a chemical jug.
Building on research done by University of Kentucky’s recently retired weed scientist Dr. Jim Martin, Haramoto and Dr. J.D. Green are working on a project funded by the Kentucky Soybean Board that is evaluating the relative effectiveness of different management tactics for controlling both fall-emerging and spring-emerging marestail, including combinations of fall-applied Sharpen, spring-applied growth regulator herbicides (2,4D or dicamba) and fall-planted winter rye cover crop.
This project was developed in response to questions received by growers who are already utilizing cover crops about the best way to manage surviving marestail with herbicides. More broadly, this research will benefit any soybean growers who have marestail infestations. As marestail has a small, wind-blown seed that can disperse widely between fields and adjacent natural areas, growers without infestations are always under threat of developing problems. This project will generate practical guidance on the interaction between herbicide applications and cover crops – whether they are warranted and necessary or whether they are antagonistic. It will also provide information on the best time to manage fall-emerged marestail.
Marestail has become a major weed for soybean producers in Kentucky and throughout a broader geographic range. Its small seed size makes this weed particularly troublesome for no-till soybean growers; as mentioned above, its prolific, wind-blown seed can disperse far between fields and adjacent natural areas; and its extended emergence period complicates management efforts. In addition to all of these traits, glyphosate resistant biotypes are now widespread throughout the state.
So, management tactics that integrate other chemical options with cultural practices like the use of cover crops are desirable for this species. This project examines the integration of fall- and spring-applied herbicides with fall-planted winter rye as a cover crop to improve management of marestail prior to soybeans.
Cover crops are beneficial in the management of marestail because marestail is highly sensitive to competition and struggles to emerge when competing with other vegetation. As learned in Dr. Martin’s research, vigorous winter wheat stand can suppress early spring emerging marestail by 83 to 99%. However, the success of a cover crop alone in managing fall-emerging marestail will be dependent on the relative emergence times and growth rates of the two species. Marestail that emerges before cover crop planting, or soon after cover crop planting while the cover crop plants are still establishing, may not be successfully out-competed by the cover crop. These marestail plants may overwinter and continue to be a threat to a subsequent soybean planting.
“There are definitely a lot of variables,” Haramoto said. “Where you are, how much rain you get and when you get it, and more.But bottom line, cover crops really do help suppress emergence of marestail. It’s such a tiny seed that it doesn’t take much to stress it and prevent emergence.”
Haramoto added that it is highly preferable to keep a weed from emerging in the first place than to figure out how to kill it. “Very few studies have looked at the integration of herbicides and cover crops together for controlling marestail,” she added. “The cover crops will keep a lot of it from emerging in the first place, then using best management practices and scouting for it – catching it when it’s really small and then spraying – can really make a big difference in the size and amount of marestail during the growing season and at harvest.”
“Cover crops will suppress this weed and keep it from emerging, and then those plants that do sneak through will be killed off in the spring when you burn down the cover crop. If you’re using glyphosate for burndown, add some 2,4D or dicamba to the glyphosate to get the marestail,” Haramoto said.
“For management of all weeds, including marestail, it’s apparent that we should use a residual during planting to prevent emergence. If a grower is using cover crops, their residue can give weed suppression on into planting. How long that suppression will last depends on how much residue is there. My colleagues J.D. Green and Travis Legleiter are working with over-the-top chemistries that should help to control emerging weeds at that later stage of the game.”
“We are always working on ways to help growers effectively manage their weeds,” Haramoto said, “and I will admit that it’s getting tougher and tougher. That’s why we are looking at non-chemical BMPs, or chemistry as part of a comprehensive solution, rather than looking for the next big thing in weed control chemistry. Round-Up was a great magic bullet when it came out, and glyphosate is still an important tool in our toolbox, but it’s not working as it once did and there’s no new magic bullet in the pipeline.”
This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.