By Tom Barber, Extension Weed Scientist
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 US 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 75oF and 52oF, 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 8 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 25th. Traditional burndown programs include 0.25 lb ai/A of dicamba which is equivalent to 6.4oz Engenia or 11oz Xtendimax. However, if Xtend crops are going to be planted then higher rates (12.8 and 22oz/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:
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) at 1oz/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 9 months to peanuts.
Corn: If not emerged, 10-12oz of Verdict plus either Roundup or Gramoxone and 1% MSO is a good option. If corn is up it becomes a little more difficult. This is generally the time where I would recommend 0.25lb ai/A dicamba to the POST mix of atrazine plus Capreno, Halex etc. If this is not an option, adding either 2,4-D (where possible) or glufosinate (Liberty, Interline etc.) to the POST mix is probably the best option. Make sure that you have planted a Liberty Link hybrid before making any glufosinate applications. 2,4-D also has restrictions on where and how it can be used after April 15th. Check the Plant Board website above if you are not familiar with these rules.
Horseweed control in rice is not much different prior to planting, Roundup plus 3oz/A Sharpen is likely the best up-front option if planting immediately. Follow-up applications to kill remaining horseweed can be made after rice emergence with 2,4-D (where possible), Loyant or Regiment.
In soybean, if you are wanting to plant immediately and dicamba is not an option, Gramoxone (paraquat) 48oz plus 5oz of Verdict and 6oz of metribuzin is probably the best chance of taking out a majority of these horseweed escapes. Glufosinate 40oz will have some activity but will be more consistent when temperatures climb into the 80’s. So if temperatures climb to 80’s 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 Liberty Link 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 0.6oz/A or two shots at 0.3oz/A will provide additional control in any technology.
In cotton if you are unable to spray dicamba, 40 oz/A of glufosinate is probably the best option. Gramoxone plus one of the PSII inhibitors such as Caparol, Cotoran or Diuron is also an option, but not as effective on larger horseweed. Coverage is critical with either of these and 15-20 GPA should be used with nozzles that produce medium size droplets. Glufosinate in most varieties will be an option early POST. Approved dicamba formulations (depending on restrictions) can be used in Xtend varieties through May 25th. Once the cotton reaches 5 true leaves, Envoke can be used regardless of variety planted. Envoke has really good activity on horseweed but cotton injury can be more severe if sprayed prior to the 5-leaf stage. If Enlist cotton is planted, Enlist Duo or Enlist One will also provide decent control.