By Mark Loux, Weed Scientist

There have been opportunities to apply burndown herbicides in much of the state over the past several weeks, and some areas have made considerable progress on planting. Other areas have made little progress. We are probably not in a true “late planting situation” yet, but some of the state is now wet and not that warm, with more rain coming midweek.  

The longer-range forecast calls for drier-than-normal conditions and higher-than-normal temperatures. The weeds obviously continue to get bigger under wet conditions, and what is a relatively tame burndown situation in early to mid-April can become pretty hairy by early to mid May.   

In our research plots, we appear to have as good a winter annual population as we’ve ever had, possibly due to a relatively mild winter (weed scientists admittedly probably have more appreciation for a “good” weed population than the rest of the world). There is a substantial difference in weediness between the fields treated with herbicides last fall vs. the lack of a fall treatment. Among other benefits, the fall treatment provides a clean start in the spring that persists for a while and “buys time” in a delayed planting situation. 

Marestail is one of the bigger concerns in a late burndown situation, especially when not initially treated last fall. Many of the other weeds, even if bigger, are still relatively well controlled by minor modifications to standard burndown programs (e.g. higher glyphosate rates, adding another herbicide). Marestail in fields not treated last fall has reached the size and age where a mixture of glyphosate and 2,4-D often won’t work. Substituting Sharpen for the 2,4-D can improve control usually, but even this combination is not infallible as marestail gets larger. Also, we have observed some weakness from the glyphosate/Sharpen combination on dandelion, purple deadnettle and larger giant ragweed.  

The more effective approach is to combine all three herbicides — glyphosate, 2,4-D and Sharpen. The addition of metribuzin can also result in more consistently effective marestail control. And a reminder — deciding to include Sharpen at the last minute can result in a need to alter the residual herbicide program. Labels still allow mixtures of Sharpen with herbicides that contain flumioxazin (Valor), sulfentrazone (Authority), or fomesafen (Reflex) only if applied 2 or more weeks before planting.  

Some things to consider in a delayed burndown situation:

  1. Increase glyphosate rates to at least 1.5 lb ae/A. This will not improve marestail control, but should help with most other weeds. 
  2. Where at all possible, keep 2,4-D ester in the mix, even if it means waiting another 7 days to plant soybeans. Plant the corn acres first and come back to soybeans to allow time for this. Have the burndown custom-applied if labor or time is short. 
  3. To improve control with glyphosate/2,4-D, add Sharpen or another saflufenacil herbicide, as long as the residual herbicides in the mix do include flumioxazin, sulfentrazone or fomesafen if it’s within 14 days of soybean planting. It’s also possible to substitute Sharpen for 2,4-D when it’s not possible to wait 7 days to plant, but this may result in reduced control of dandelion, deadnettle and giant ragweed. Where the residual herbicide in the mix does contain flumioxazin, sulfentrazone or fomesafen, and it’s not possible to change the residual or add Sharpen, adding metribuzin can improve burndown effectiveness somewhat.
  4. Consider substituting Gramoxone or glufosinate for glyphosate. Gramoxone is less effective than glufosinate on marestail, but glufosinate can struggle some in a dense, large no-till burndown situation.  Either one should be applied with metribuzin and 2,4-D ideally. Use the higher labeled rates and a spray volume of 15-20 gpa for best results. A consideration here is that in large no-till weed situations, high rates of glyphosate typically have more value than high rates of Gramoxone or glufosinate, with the exception of glyphosate-resistant weeds.
  5. Among all of the residual herbicides, chlorimuron contributes the most activity on emerged annual weeds and dandelion. This is probably most evident when the chlorimuron is applied as a premix with metribuzin (Canopy/Cloak DF, etc). The chloirmuron may not be much of a help for marestail control, since many populations are ALS-resistant. Cloransulam (FirstRate) has activity primarily on emerged ragweeds and marestail, as long as they are not ALS-resistant. We have on occasion observed a reduction in systemic herbicide activity when mixed with residual herbicides that contain sulfentrazone or flumioxazin.
  6. Late burndown in corn is typically a less dire situation compared with soybeans. Reasons for this include: 1) The activity of some residual corn herbicides (e.g. atrazine, mesotrione) on emerged weeds; 2) The ability to use dicamba around the time of planting; 3) The tolerance of emerged corn to 2,4-D and dicamba; and 4) The overall effectiveness of available post-emergence corn herbicides. 

Overall, while not adequately controlling emerged weeds prior to soybean planting can make for a tough season, there is just more application flexibility and herbicide choice for corn. Having said this, be sure to make adjustments as necessary in rate or herbicide selection in no-till corn fields.