Controlling Roundup-resistant marestail

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There is not much more frustrating than dousing glyphosate on a weedy field and having some of the weeds laugh at you and continue growing. Weeds certainly have been proven to reduce yields, and the inability to control them with your main herbicide is nerve-wracking. However, many Corn Belt fields are populated with marestail which has become resistant to glyphosate.  And if your cultivation practice is based on no-till, your soybean yields may take a significant yield hit. What do you do?

Marestail is one of those weeds that does not fit the typical schedule of other weeds. It can emerge either in late March through June or in late summer to fall. Those in the spring will form a low growing rosette before growing as tall as 6 feet. And their emerging fall cousins will be growing tall in the spring, and quite tough to control in their advanced stages. Weed specialists Mark Loux of Ohio State and Bill Johnson of Purdue have been working on a strategy for controlling marestail that is resistant to glyphosate. Part of the strategy is built on the use of LibertyLink soybeans and the use of Liberty herbicide. However, there are many alternatives that they suggest which will integrate with your weed control program. One of the benefits is avoiding an average 14 bu. per acre yield loss between a successful program and one which failed to control the marestail.

The specialists say the easiest time to control marestail is when it is in the rosette state, and less than 4 inches tall. Many of the populations have resistance to both glyphosate as well as herbicides that are ALS inhibitors such as Classic, Synchrony, or FirstRate. Their research reports that marestail is not resistant to Liberty, therefore, the use of LibertyLink soybeans are the most effective control strategy. They suggest the use of burndowns and residual herbicides, such as a post emergent application of Liberty at a 29 oz/A rate if the marestail exceeds 6 in.

Fall Application

If you have glyphosate resistant marestail growing this fall, your goal is to control the emerged plants with a burndown, followed by another application in the spring. Fall herbicides should not be expected to carry over to the spring, however if your marestail is not ALS resistant, you could use a product with chlorimuron. Loux and Johnson say Authority, Valor and products with metribuzin should not be used in the fall, but used in the spring. For an economical fall burndown, they suggest: 

  • glyphosate + 2,4-D; or
  • dicamba + 2,4-D; or
  • 2,4-D + a low rate of Canopy/Cloak EX or DF   

Spring Application

If you have a healthy stand of weeds in the spring, do not plant into an existing stand of marestail, but start with a weed-free seedbed say the agronomists. Such a burndown could include:

  • 2,4-D ester plus glyphosate (1.5 lb ae/A)
  • 2,4-D ester plus Gramoxone (3 to 4 pts/A) plus a metribuzin-containing herbicide
  • Liberty (29 to 36 oz/A) or Liberty plus a metribuzin-containing herbicide
  • Saflufenacil product (Sharpen, Optill, or Verdict) plus MSO (1% v/v) plus either glyphosate or Ignite

When applying the burndown treatment, include a residual herbicide to control the marestail until the soybean canopy can shade it out. Such residual products might include:

  • Valor, Valor XLT, Envive, Enlite, or Gangster
  • Authority First, Sonic, Authority XL, Authority Broadleaf, or Spartan
  • Canopy/Cloak DF + metribuzin (total metribuzin rate must be at least 0.38 lb ai/A)
  • metribuzin (Metri DF, Tricor, etc) or Boundary (metribuzin rate must be at least 0.38 lb ai/A)

For post emergent control of marestail, Loux and Johnson say Liberty is the only effective herbicide to do that if the beans are LibertyLink beans. They say if the beans are Roundup Ready and marestail escapes the burndown, the suggested remedy is a combination of glyphosate plus Classic or FirstRate at the highest allowable rates. And they are quick to add that such a combination may provide a variable response but may work on some populations.

Summary:

Marestail is becoming resistant to glyphosate, and may also be resistant to ALS inhibitors, which means control alternatives are becoming more limited, particularly in no-till soybeans, where cultivation is not an option. Because marestail emerges in both the spring and fall, treatment should occur in both times. A fall application can include 2,4-D and other herbicides. Spring applications can also use 2,4-D as a burndown with other herbicides. The only single effective control for marestail is Liberty, which must be used with LibertyLink soybeans.

Source: FarmGate blog


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