Managing Late-Emerging Horseweed
DR. LARRY STECKEL
JACKSON, TENN.
In 2017 and now again in 2018, the lack of horseweed (marestail) in many fields for late March is very notable. So what is the reason/s for the lower landscape level horseweed infestations? I think the main cause was that we had very dry fall seasons in 2016 and 2017 which limited fall horseweed germination. Also, applying dicamba or 2,4-D later in the growing season on crops with the new herbicide-trait technologies could also be a contributing factor for the relative scarcity of horseweed. These products provide good control of horseweed thereby reducing the weed seed bank. In addition, the very long cold spell this winter could have contributed to the lack of established horseweed by causing a high mortality rate on the fall germinated population.
So are we out of the woods with respect to horseweed being a problem? If last spring is any indication the answer would be NO! If you will recall we had a huge late spring germination of horseweed in April and May of 2017. It was very wet and warm during that period and many fields had a serious infestation of very large horseweed to control by the time the fields were dry enough to spray and plant.
Based on the 10 day forecast at least the early part of April looks to be a repeat of what occurred in April of 2017. I think we can assume that there will be a good number of fields with a significant infestation of large 12 to 18” horseweed to have to manage by the time some fields are dry enough to spray. So what are some good management options?
In Conventional or Roundup Ready soybeans the most consistent horseweed burn down has been 5 ozs of Verdict mixed with 32 ozs of glyphosate and 16 ozs of 2,4-D. The plant back to soybeans for that mixture is 7 days.
In Xtend soybeans a good tankmix to manage large horseweed would be 5 ozs of Verdict mixed with 32 ozs of glyphosate and 12.8 ozs of Engenia. There is no plant back after that tankmix.
Verdict is not an option in cotton. The best way to go in Xtend cotton is to apply Engenia or XtendiMax mixed with glyphosate. In Enlist cotton go with Enlist Duo. Cotton may be planted any time after those mixtures have been applied. When some of the larger horseweed recover then consider an application of Liberty at the typical thrips application timing to remove any escapes. Remember to apply Liberty in the middle of a warm and sunny day and at a minimum of 15 GPA? to get the best control.
Of course if there is a good bit of Palmer amaranth emerging in fields that have large horseweed another option is 48 ozs of Gramoxone tankmixed with either metribuzin in soybean or Cotoran plus Caparol in cotton. This will take out the Palmer and burn back the horseweed. Most of the horseweed will grow back and can be addressed in-crop with either Liberty, a dicamba product or Enlist Duo depending upon the crop trait package. ∆
DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee