Due to a variety of factors, both common lambsquarters and marestail (also called horseweed) are challenging soybean weeds. Both plants have high seed production, increasing resistance to common herbicides, and life cycles that span a significant portion of soybean growth stages.
While these weeds are formidable foes, we’ve compiled some tips and tactics to help you manage lambsquarters and marestail in soybeans.
Marestail and common lambsquarters control in soybeans: Traits that make them tough to manage
Short seed life
Up to 86% of marestail seeds can germinate immediately upon seed shed¹. Similarly, if soil moisture is moderate and temperatures are between 64 F and 77 F, lambsquarters seeds can germinate within a week.²
Resistance to common herbicides
Common lambsquarters has developed resistance to photosystem II inhibitors (triazines) and ALS inhibitors, and researchers have suspected lambsquarters has developed resistance to glyphosate.³
In corn, marestail is relatively easy to control with atrazine and growth-regulating herbicides. However, marestail control is a different story in soybean fields, with populations of marestail showing resistance to ALS-inhibiting herbicides.⁴
Weed life cycle, seed production and dispersal, and germination
Lambsquarters germinates anywhere from mid-April through June and is one of the earliest-emerging summer annuals, typically present before crops are planted. The plant reaches full height and biomass in early July, flowering through July and August. Lambsquarters seeds mature by early August and begin dispersal, which continues through late summer and fall.
Marestail primarily germinates in fall but can also emerge during spring and summer months. Plants begin as a rosette, bolting in April and May and flowering in July. Marestail plants set and disperse seeds from August to October, and up to 91% of fall-emerging marestail seeds can survive the winter, making marestail control in soybeans problematic.⁵ On the flip side, since marestail does not mature until late summer, it is also more competitive with crops late in the season compared to other winter annual weeds.
How to control common lambsquarters in soybeans
There are a greater number of effective pre-emergence soybean herbicides for lambsquarters management than post herbicides. Pre-emergence herbicides reduce early-season competition from lambsquarters and other weeds, resulting in smaller weeds at time of post-emergence treatments.
Early-season control is critical since post-emergence applications of glyphosate provide inconsistent control. The efficacy of glyphosate depends on the plant’s size, maturity and environmental conditions. Once plants reach eight nodes and are more than 6" tall, control becomes even more inconsistent with post applications of glyphosate.⁷
How to control marestail in soybeans
Applying soybean burndown herbicides prior to planting can help control existing marestail plants. Additionally, apply a pre-emergence herbicide for marestail in soybean before planting to prevent seeds from germinating.
Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
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Endnotes
- “Marestail (Horseweed).” Weeds, www.canr.msu.edu/weeds/extension/marestail-horseweed. Accessed 2 Jan. 2025.
- Mohler, Charles L., et al. “Lambsquarters, Common.” SARE, 17 Oct. 2023, www.sare.org/publications/manage-weeds-on-your-farm/common-lambsquarters/. Accessed 2 Jan. 2025.
- Curran, Bill, et al. “Biology and Management of Common Lambsquarters.” Purdue Extension, ag.purdue.edu/btny/purdueweedscience/wp-content/uploads/2021/01/GWC11_Lambsquarters.pdf. Accessed 2 Jan. 2025.
- “Controlling Horseweed (Marestail).” College of Agriculture & Natural Resources, Michigan State University, www.canr.msu.edu/field_crops/uploads/archive/Controlling%20Horseweed%20(Marestail).pdf. Accessed 2 Jan. 2025.
- “Controlling Horseweed (Marestail).”
- Loux, Mark, et al. “Control of Lambsquarters in Corn and Soybeans.” 2014, bpb-us-w2.wpmucdn.com/u.osu.edu/dist/7/3461/files/2014/04/lambsquarter07-1jdcqvi.pdf. Accessed 2 Jan. 2025.
