Good news is that cool weather has slowed weed growth, but even so, 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.  One issue with later burndowns certainly is that there can be a need for a more aggressive herbicide mix, but also a need to plant as soon as possible, and these can be conflicting goals.  For example, unless dicamba is an option, we would say keep 2,4-D ester in the mix if at all possible, but this means waiting 7 days to plant. 

Marestail is always one of the bigger concerns in a late burndown situation, especially overwintered plants in fields that were not 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).  Overwintered marestail get tougher to control with increased size and age, to the point that they will reach a 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.  It’s good to have Xtend soybeans and dicamba in the toolbox in this situation.  Glyphosate plus dicamba is more consistently effective than glyphosate plus 2,4-D for control of overwintered marestail.  The addition of metribuzin to any of these can also result in more consistently effective marestail control.  Having said all of this, we find marestail to be mostly absent this year in many of our research fields, so some scouting may help with burndown decisions.

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.  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.  Where these are replaced by a metribuzin-containing herbicide or mix such as Canopy or Zidua Pro/metribuzin, add enough metribuzin to get the rate to the equivalent of 8 to 12 oz/A of metribuzin 75DF.

5.  Consider substituting Gramoxone or glufosinate for glyphosate?  Gramoxone is   less effective than glufosinate on marestail, but the mix of Gramoxone/metribuzin/2,4-D controls marestail well unless they are big.  Omitting 2,4-D from this mix because of planting considerations reduces it from an excellent/good treatment to fair/good.  Glufosinate can struggle some in a dense, large no-till burndown situation, and should also 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.

6.  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.

7.  It is possible to substitute tillage for burndown herbicides.  Make sure that the tillage is deep and thorough enough to completely uproot weeds.  Weeds that regrow after being “beat up” by tillage are often impossible to control for the rest of the season.  Tillage tools that do not uniformly till the upper few inches (e.g. TurboTill) should not be used for this purpose.

8.  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 such as atrazine and mesotrione on emerged weeds – mesotrione has apparently become pretty cheap, and adding a few ounces to an atrazine premix can improve burndown substantially; 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 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.