By Tom Barber •
Horseweed (marestail) has always been an issue at spring burndown prior to planting. In the early 2000s, it became a much bigger problem in the Southern United States by developing resistance to glyphosate.
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 75 degrees Fahrenheit and 52 degrees 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. The majority of calls over this past 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 pictured.
Horseweed is tough to kill
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 percent control.
In soybeans: With the current restriction on dicamba applications, Gramoxone (paraquat) 48 oz plus 5 oz of Verdict and 6 oz of metribuzin per acre is probably your best chance of taking out a majority of these horseweed escapes.
Liberty (glufosinate) 40 oz will have some activity but will work much better when temperatures climb into the 80s. So if temperatures climb to 80s 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 0.6 oz/A or two shots at 0.3 oz 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 any crop, it is important to remove these horseweed populations if at all possible. Horseweed is allelopathic to most crops grown in Arkansas. If left uncontrolled the first eight weeks, horseweed 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 percent control which could result in lower yields.
Tom Barber is an Extension Weed Scientist with the University of Arkansas in Lonoke. He can be reached at firstname.lastname@example.org.