Resistant Weeds Require a More Aggressive Mindset
Published: Friday, March 30, 2018
The following is from Jeff Burbrink, Elkhart County Extension educator.
The mindset for weed control in field crops has certainly changed over the past five years. With herbicide resistance weeds like marestail, Palmer amaranth, waterhemp and giant ragweed, you are asking for trouble if you are not moving away from a glyphosate based spraying program.
Bill Johnson and Joe Ikley, two of Purdue University's weed scientists, have been encouraging growers for several years to be more aggressive with burndown programs. As Joe said recently, we need to be more aggressive than just adding a pint of 2,4-D to something and expecting to get control. He is emphasizing a multi-pass program, spring and fall, and consider hitting the weeds twice in the spring if you have wet weather like we had in 2017.
For those weeds with glyphosate and ALS resistance, the best control is achieved in the years when corn is in the rotation. When soybeans are being planted, consider herbicide resistant traits like Liberty Link, RR2 Xtend or Enlist.
When it comes to marestail, they like to see the field free of the weed at planting. Plan on a two pass burndown program: fall followed by spring, or two passes in the spring. Do not rely on glyphosate or 2,4-D alone. Dicamba and Elevore work slightly better than 2,4-D. Dicamba does not work well on plants above 8 inches tall. They suggest you need a residual marestail control for six to eight weeks after planting. If you have glyphosate and ALS resistant marestail, you really have no good postemergence options in straight RR2Y soybeans right now.
Giant ragweed has become an issue in both corn and soybeans. There is documented resistance in both Group 2 (ALS herbicides) and Group 9 (glyphosate). It also serves as an alternative host to a number of stem boring insects, which in turn can negatively affect how well the herbicides work. It is one of the few weeds that can germinate 5 inches deep in the soil and still make it to the surface.
Giant ragweed has adapted to glyphosate in an unusual way. Some sub-types of giant ragweed have developed an ability to shed leaves when glyphosate is sprayed on the surface. Called "rapid necrosis," the dying leaves keep the glyphosate from circulating into stems and roots. They plants will look sick for a few days while the leaves die back, then new leaves will grow back as if nothing was sprayed in the plant!
For preemergence giant ragweed control in soybeans, they are suggestion glyphosate plus 2,4-D, dicamba or Sharpen for burndown, or Gramoxone plus Sencor plus 2,4-D. In addition, a residual of chlorimuron, chloransulam or imazaquin in ALS (Group2) susceptible populations.
For post-emergence issue with ALS and glyphosate resistant giant ragweed in soybeans, they have four suggested treatments:
• Flexstar followed by Cobra in non-gmo or any traited soybeans
• Two passes of glufosinate in Liberty Link beans
• Dicamba in Extend soybeans
• 2,4-D in Enlist soybeans
Timing of these post-emergence products is critical. Once the weeds are 4 inches or taller, the effectiveness of many products degrades rapidly.
For more herbicide information, go to https://goo.gl/AKMeSi.
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