It’s B-A-C-K! Horseweed, that is

• By Tom Barber •

horseweed
Since the early 2000s, horseweed (mairstail) has become a much bigger problem in the South because it has developed resistance to glyphosate — photo courtesy University of Arkansas

Horseweed (marestail) has always been an issue at spring burndown prior to planting. In the early 2000’s, it became a much bigger problem in the southern United States by developing resistance to glyphosate (Roundup).

Horseweed can germinate and grow in a wide range of temperatures; it commonly emerges in the fall and spring months and depending on environmental conditions could germinate 10 months out of the year.

Optimum high and low temperatures for horseweed germination are 75 degrees Fahrenheit and 52 F, respectively. With that in mind considering the fluctuation of temperatures we have recently experienced in Arkansas, it is understandable why we are seeing another flush of horseweed after the initial burndown applications.

On a side note, this is also the reason that we recommend residuals with the early burndown applications to keep horseweed populations under control until planting. Lots of questions coming in this week about how to control horseweed in emerging corn or what can be sprayed prior to planting with a short plant-back interval.

Horseweed is allelopathic to most crops grown in Arkansas and if left uncontrolled the first eight weeks can cause significant yield losses. Dicamba has traditionally been our go-to herbicide for spring control of horseweed and with the current Plant Board restrictions it is still an option through May 25.

Dicamba: Traditional burndown programs include 0.25 lb ai/A of dicamba, which is equivalent to 6.4 oz Engenia or 11 oz XtendiMax. However, if Xtend crops are going to be planted then higher rates (12.8 and 22 oz/A respectively) of these products can be used and would be the best option for controlling these horseweed populations.

Remember that all plant board restrictions must be followed even for burndown applications. If you have questions about these you should contact the Arkansas State Plant Board, the regulations are posted here:
https://www.agriculture.arkansas.gov/arkansas-dicamba-information-updates

Glufosinate (Liberty, Interline etc.) also provides good control of horseweed populations and can be sprayed prior to planting our major row crops. Control with glufosinate can be variable depending on daytime temperatures, and time of day applied as well as other factors that are important for any application such as coverage, so better coverage generally results in better control, 15-20 GPA is recommended.

In addition to dicamba and glufosinate, Elevore (a new herbicide from Corteva that contains the active ingredient, halauxifen-methyl) at 1 oz/A also provides excellent control of glyphosate-resistant horseweed. Plant-back restrictions following an Elevore application are 14 days to corn, rice, soybean or sorghum, 30 days to cotton and nine months to peanuts.

For soybeans

In soybean, if you are wanting to plant immediately and dicamba is not an option, Gramoxone (paraquat) 48 oz plus 5 oz of Verdict and 6 oz of metribuzin is probably the best chance of taking out a majority of these horseweed escapes. Glufosinate 40 oz will have some activity but will be more consistent when temperatures climb into the 80s. So if temperatures climb to 80s next week then swapping out Gramoxone for Liberty might be the best option.

There are some preemerge products such as Surveil (Valor + Firstrate) that will provide additional control if used at planting. If PPO-resistant pigweed is an issue, residuals such as metribuzin or Zidua should be added to Surveil for additional pigweed control. If horseweed populations persist after soybean emergence, options vary based on technology planted.

Glufosinate can be used in LibertyLink and Enlist traited varieties; Enlist One or Enlist Duo in Enlist varieties; and Engenia, FeXapan, Tavium or XtendiMax in Xtend traited varieties (depending on plant board restrictions). FirstRate at 0.6 oz/A or two shots at 0.3 oz/A will provide additional control in any technology.

Dr. Tom Barber is a University of Arkansas Extension weed scientist. He may be reached at tbarber@uaex.edu

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