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 months as well as the 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. The majority of calls over this last week have been dealing with recommendations for horseweed control, with the number one question being “what can I spray and plant immediately?”
Unfortunately most of these calls and pictures indicate that the horseweed we are dealing with in southern Arkansas is 6-12 inches tall, similar to the ones on the right. First of all it is very hard to kill larger horseweed with anything short of a disk. So tillage has been my number one recommendation this week. This is not a popular recommendation because most growers want to plant as soon as the fields dry. Usually on large horseweed, we will end up spending a bunch of money for 60% control.
In Soybean: With the current restriction on dicamba applications, 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. Liberty (glufosinate) 40oz will have some activity but will work much better when temperatures climb into the 80’s. So if temperatures climb to 80’s next week (crossing the fingers) then swapping out Gramoxone for Liberty would probably be the best option. I just think it is too cool right now to rely on Liberty for horseweed control. If horseweed populations persist after planting and soybean emergence, Firstrate at 0.6oz/A or two shots at 0.3oz per acre will provide additional control. 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. Once temperatures increase Liberty in LibertyLink soybean is another good option POST.
In Cotton: Again considering the current restrictions on dicamba, Gramoxone plus one of the PSII inhibitors such as Caparol, Cotoran or Diuron is most likely the best option on horseweed with these cooler temperatures other than a disk. Liberty in most varieties will be the only option early POST until the cotton reaches 5 true leaves, then Envoke can be used. 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 good control.
In Corn: If not emerged, 10-12oz of Verdict with either Roundup or Gramoxone is a good option. It works better in corn than soybeans because of the increased rate allowed. If corn is up it becomes a little more difficult. This is generally the time where I would recommend 8-10oz dicamba to the POST mix of atrazine plus Capreno, Halex etc. Since that is no longer an option, adding Liberty (glufosinate) to the POST mix is probably the best option in a LibertyLink hybrid. If hybrid is not Liberty Link, then the options become limited to the standard HPPD plus atrazine mix POST. Another option in corn is the addition of 16 oz/A 2,4-D to the above mentioned products, but the ability to use this herbicide is dependent upon where you farm within the state.
In any crop, it is important to remove these horseweed populations if at all possible. Horseweed is allelopathic to most crops grown in Arkansas and if left uncontrolled the first 8 weeks can cause significant yield losses. So if the field is covered from one end to the other with >6 inch horseweed, it will most likely be cheaper to run the disk and delay planting than to blow the budget for 60% control which could result in lower yields.