Source: University of Tennessee Institute of Agriculture

A wet and mostly cold March has us well behind the pace for burndown. As soon as the weather permits corn planting will take priority followed by burndown for grain sorghum and early planted soybean.

What should the burndown strategy be for those crops with planting time upon us? In corn, a burndown at planting of Gramoxone and atrazine can be an effective choice. Another good option would be glyphosate and Verdict or Sharpen, which should be both effective and allow for flexibility if the corn stand is lost and then replanted to soybean. 

The bottom line in corn is that even if some marestail recovers from the burndown application, herbicides like Halex GT or Capreno spiked with either atrazine or Status have been very effective controlling large marestail in our research.

In the case of grain sorghum, please be aware that dicamba and 2,4-D can be used pre-plant for burndown but not pre-emergence. The reason for this is that rain can wash those herbicides into the planting slot and harm stand establishment.

In grain sorghum, a burndown at planting of Gramoxone can be an effective choice. Another good option would be glyphosate and Verdict or Sharpen, which should be both effective and allow for flexibility if the grain sorghum stand is lost and then replanted to soybean. Marestail that survives burndown can be fairly easily controlled with Huskie spiked with atrazine.

Please note that herbicides commonly used in corn, like Princep and Status, are not labeled in grain sorghum due to crop injury potential. Even atrazine should be used with caution pre-emergence as some stand establishment problems have been documented. Post-emergence atrazine applications are fine.

In the case of corn and grain sorghum it will be more critical to control Italian ryegrass than marestail before the crop emerges. In both crops there is no herbicide that can be applied post-emergence that will control ryegrass. Sequential applications of Gramoxone separated by 7-10 days can be an effective ryegrass option pre crop emergence. 

In corn, Select Max can be applied 6 days before planting at 6 ounces per acre, and when tank-mixed with glyphosate has proven to be fairly effective. The plant-back to grain sorghum from a Select Max application is 30 days.

In some cases where the plan is to plant some fields to soybean in early April, dicamba, which has been the “go-to” glyphosate-resistant horseweed herbicide, is no longer an option. Sharpen or Verdict would be a good new “go-to” in these situations for soybean burndown.

Close to Soybean Planting Burndown Options

• Sharpen at 1 ounce per acre or Verdict at 5 ounces per acre, plus Roundup PM at 32 ounces per acre
Management Considerations:

  1. Sharpen at rate of 1 ounce per acre can be applied right up to planting.
  2. For large horseweed (taller than 4 inches), consider increasing Sharpen rate to 1.5 ounces per acre or Verdict to 7.5 ounces per acre and/or 1 pint of 2,4-D. Plant-back to soybean for higher rates of Sharpen or Verdict is 14 days. Plant-back for soybean is 7 days after a pint of 2,4-D.

• Clarity at 8 ounces per acre plus Roundup PM 32 ounces per acre
Management Considerations:

  1. Clarity rates of 8 ounces per acre can be applied up to 14 days before planting and 1 inch of rainfall to avoid crop injury.

• Gramoxone Inteon at 40-48 ounces per acre plus 0.25% NIS plus Sencor at 4 ounces per acre or Canopy at 4-6 ounces per acre or Boundary at 20 ounces per acre.
Management Considerations:

  1. This application can be applied right up to planting.
  2. Gramoxone Inteon rates below 40 ounces per acre will not provide as consistent control of horseweed as higher rates.
  3. For larger horseweed (taller than 6 inches) use 48 ounces per acre of Gramoxone Inteon. 

• FirstRate at 0.3-0.4 ounces per acre plus Roundup PM at 32 ounces per acre
Management Considerations:

  1. This application can be applied right up through soybean emergence.
  2. FirstRate will not provide good control of horseweed under cool conditions. FirstRate works best with highs in the 80s and lows in the 60s.