This is a revision of an article we published in C.O.R.N. two years ago, when rain kept us out of fields until early June in some cases. This year will hopefully not be a repeat of 2011 to that extreme. Wet weather so far this year and the somewhat delayed planting has already had effects on no-till burndown programs, especially in soybeans.

The weeds obviously continue to get bigger under warm, wet conditions, and what is a relatively tame burndown situation in early to mid-April can become pretty hairy by early May. It’s obvious from the calls and emails we have received, along with observations of our research plots, that there is a substantial difference in weediness between the fields treated with herbicides last fall versus the lack of a fall treatment. Among other benefits, the fall treatment does definitely allow a clean start in the spring that persists for a while and ‘buys time’ in a delayed planting situation. The fields that did not receive fall herbicides are much more of a concern as we try to adapt burndown programs to a delayed start that allows the overwintered weeds to create problems.

For many weeds, increasing the glyphosate rate to 1.5 lbs ae/A or higher in mixtures with 2,4-D or Sharpen, will help compensate for larger weed size. This will not help with glyphosate-resistant marestail, and the other issue for marestail is that by the time we can finally plant, we will be unable to use 2,4-D rates higher than 0.5 lb (and this rate still requires a 7 day wait to plant). The mixture of glyphosate plus 2,4-D has become less effective over time in some fields for marestail control. Recommendations to improve control have included application to smaller plants, and increasing the 2,4-D rate to 1.0 lb/A, and so the current situation will probably introduce more variability in marestail control. In fields with larger marestail that did not receive a fall herbicide treatment, control could be improved by supplementing the glyphosate/2,4-D with another herbicide that has activity on emerged marestail, or replacing the glyphosate with another herbicide.

A reminder that there are currently some extenuating circumstances that limit the extent to which we can modify burndown programs. The first of these is the lack of labels that allow the addition of Sharpen to mixtures that contain flumioxazin (Valor), sulfentrazone (Authority), or fomesafen (Reflex). The second is the depleted supply of Liberty, with an emphasis on the use of current stock for POST treatments instead of for burndown. A review of the soybean burndown options for larger weed situations follows, with emphasis on marestail control.

1. 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.

2. 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. 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, and large 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, adding metribuzin can improve burndown effectiveness somewhat.

3. Consider substituting Gramoxone or Liberty for glyphosate? Gramoxone is the less expensive and more available choice here, but generally less effective than Liberty on marestail. Gramoxone should be applied with metribuzin and 2,4-D in a typical no-till situation. Use the higher labeled rates of Gramoxone, and a spray volume of 15 to 20 gpa for best results. A consideration here is that in large no-till weed situations, high rates of glyphosate typically have more value that high rates of Gramoxone or Liberty, with the exception of glyphosate-resistant weeds.

4. 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). This 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 the a reduction in systemic herbicide activity when mixed with residual herbicides that contain sulfentrazone or flumioxazin.

5. 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.

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 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.