As a result of the warm winter and early spring, weed growth in no-till fields is ahead of schedule. Fields not treated with burndown herbicides last fall or during the earlier drier period this spring can have some large weeds at this point. Many fields need time to dry out following the most recent rains before they will tolerate traffic, which will allow weeds to get even larger and more challenging to kill.
Large marestail can be especially problematic due to the combination of glyphosate and ALS resistance in most populations. Cool weather can reduce the activity of the herbicides that have activity on marestail. The standard “glyphosate + 2,4-D” burndown is not likely to control large marestail plants based on recent history, and increasing the 2,4-D rate or adding metribuzin won’t necessarily result in effective control of these plants either.
The issue here is that 2,4-D is carrying the entire load with regard to control of emerged marestail, and it’s not effective enough to control large plants, even with the addition of metribuzin. While Sharpen is more effective than 2,4-D, a mixture of glyphosate plus Sharpen can also become more variable in effectiveness as marestail get larger.
In no-till fields that have not yet been treated with herbicides, consider making changes to burndown programs to ensure complete control of large marestail prior to emergence of nonGMO, Roundup Ready, and LibertyLink soybeans. Postemergence application of glufosinate in LibertyLink soybeans is most effective on marestail plants that emerge after planting, not for control of plants that survived the burndown herbicide treatment.
Burndown treatments should contain at least two active ingredients with substantial activity on marestail. The most cost-effective of these still include 2,4-D, which will require a 7-day wait to plant. Examples: glyphosate + 2,4-D + either Sharpen or Zidua PRO; Gramoxone + 2,4-D; Glufosinate + 2,4-D. The inclusion of metribuzin is recommended for all of these mixtures, and should absolutely not be omitted for the mixtures of 2,4-D with Gramoxone or glufosinate.
Where it’s not possible to wait to plant, choices can be less effective or more expensive. Best approaches here, ranked from most to least effective:
- glufosinate + metribuzin + either Sharpen or Zidua PRO;
- glufosinate + Sharpen or Zidua PRO;
- Gramoxone + metribuzin + Sharpen or Zidua PRO.
Switching to a program that includes Sharpen can result in the need to modify the residual herbicides as well. Mixtures of Sharpen with residual products that contain flumioxazin (Valor etc), sulfentrazone (Authority), or fomesafen (Prefix, etc) must be applied at least 14 days before soybean planting. Waiting this long to plant may not work as we move later into May.
Alternative residuals that can be used in mixtures with Sharpen, and applied anytime prior to soybean emergence include:
- Canopy Blend or Cloak DF + metribuzin;
- Matador + metribuzin;
- metolachlor/metribuzin premix products (Boundary etc).
Total metribuzin rate for all of these should be the equivalent of 8 to 10 oz of metribuzin 75 DF (possibly less on sandy soils low in organic matter).
In fields planted with Xtend soybeans, there is obviously the option to use XtendiMax, Engenia, or FeXapan in the burndown, and then also as a postemergence follow up if necessary. Dicamba is generally more effective then 2,4-D on marestail, especially larger plants, and the combination of glyphosate plus dicamba may be effective enough in the burndown. We have observed reduced control of larger plants in mixtures or dicamba with residual products that contain flumioxazin (Valor etc) or sulfentrazone (Authority). Increasing the dicamba rate may reduce problems in these mixtures.
Be sure to optimize spray parameters and adjuvants. Mixtures containing Sharpen and Zidua PRO should be applied with MSO. Gramoxone should be applied with COC. Gramoxone, glufosinate, and saflufenacil (Sharpen, Zidua) are contact herbicides, which require 15 to 20 gpa spray volumes and nozzles that optimize coverage. Check labels for specifics.