Source: Ohio State University Extension
This spring is shaping up to be one where marestail control problems abound, based on the following:
1. Not many fields were treated with herbicide last fall due to wet weather and the late harvest. Fall treatment results in a field free of overwintered marestail in the spring, which takes the pressure off of spring burndown treatments — growers just have to control the newly emerging small marestail. One strategy to compensate for lack of fall treatment is to apply herbicide early in spring when overwintered marestail plants are still small, but….
2. Wet weather so far this spring has largely prevented application of burndown herbicides. And this looks to be the case through sometime next week based on the forecast (not sure this applies to the entire state but much of it for sure), which means that...
3. When spring burndown treatments can be applied, they have to be comprehensive enough to control fairly old, overwintered marestail. We know that the standard-rate glyphosate/2,4-D burndown can struggle in this situation, so some modification/replacement of this may be warranted. Some possible modifications to consider:
• Increase the 2,4-D rate from 1/2-1 pound active ingredient per acre, which can improve control by 10-20% — possibly not enough in late April. Certain 2,4-D ester products are labeled for application at 1 pound per acre when applied at least 15 days before soybean planting.
• Add metribuzin, which has burndown and residual activity on marestail. Works especially well with Gramoxone, Liberty or Sharpen.
• Replace 2,4-D with Sharpen, which still be inconsistent on larger marestail, but adding Sharpen to a glyphosate/2,4-D mixture is a more consistently effective option.
• Replace the glyphosate with Gramoxone or Liberty. Three-way combinations of Gramoxone or Liberty plus metribuzin plus 2,4-D are strongly recommended here. Use high rates of Liberty or Gramoxone.
Ensuring the performance of most of the modifications listed above will require optimization of spray parameters and adjuvants. For example, Gramoxone, Liberty and Sharpen should be applied in a spray volume of at least 15 gpa, and MSO should be included in Sharpen-containing treatments. Keep in mind also that use of Sharpen may necessitate a change in the residual herbicides used this spring.
Current labeling requires mixtures of Sharpen at 1 ounce per acre with products containing flumioxazin, sulfentrazone or fomesafen to be applied at least 14 days before planting on most soils. There are no restrictions of this type for mixtures of Sharpen with any other residual herbicide, including Canopy, metribuzin and Matador among others, which just have to be applied before soybeans emerge.
Growers who at this point know that they will be making a change in burndown programs to apply Sharpen within 14 days of planting may want to discuss residual herbicide options with their supplier before availability of certain products becomes limited.
When recommendations are made for this situation, know that they're based on the average marestail population in the state. This “average” population has a fairly high level of resistance to glyphosate and is also resistant to group 2 herbicides (ALS inhibitors — Classic, FirstRate, etc). In late spring, the 2,4-D will have to carry most of the load in a mixture of glyphosate + 2,4-D, and that 2,4-D only provides about 75% control of marestail that are past the small rosette stage.
It’s possible that in some areas of the state, marestail still responds at least somewhat to glyphosate, with the result that the glyphosate/2,4-D mixture is still sufficiently active even in late spring. A grower's experience can be his guide to some extent here, but know that there will be no later options to control plants that survive an ineffective burndown.
Ohio State University Extension weed scientist Mark Loux talks more about this in the video "Considerations for managing marestail in a wet spring."