In a March blog I noted the lack of horseweed (marestail) in many fields but thought an April germination event of this weed could come late to the party and pose a problem. On occasion I make a good guess, as that has clearly been the case.
The warm and wet April and May have allowed the weed to grow very quickly. On top of that, more fields than I can recall over past several year are not being burned down until the 11th hour. Quite a few calls have come in over the last week on how to handle these very large 12 to 18” tall horseweed.
Herbicide Resistance Info
When horseweed is taller than 6” let alone 18” traditional burndown herbicides applied alone like dicamba, 2,4-D, Verdict or Sharpen will struggle to provide control. In our research on large horseweed a tankmix of those products provides the best chance for success.
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 restriction 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. There is a good chance that larger horseweed may recover, and if this occurs 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 to get the best control.
If there is a good bit of Palmer amaranth emerging in the same field with the large horseweed another option is 48 ozs of Gramoxone tankmixed with either metribuzin in soybean or Cotoran and 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.