Controlling Roundup-resistant marestail
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.
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