Editor’s note: The following was written by Kevin Bradley and Mandy Bish, University of Missouri Division of Plant Sciences, for the university’s Integrated Pest Management website.
Obviously, our season so far has been one for the books. We've been fielding a number of questions over the past several weeks related to the wet weather and how this affects herbicides and weed management practices.
In this article, we provide some answers to the most common questions we've been getting.
Killing failed stands
Many people have floods that killed their corn stands outright; others have stands that were poor and just need to be killed out.
We've written several articles over the years on how to terminate a failed corn stand and plant corn back into it. Not much has changed. One option is Select Max at 6 ounces per acre, but you have to wait six days before you replant the new corn.
Many people don't want to wait to replant, so another option is Gramoxone plus one of the PSII-inhibiting herbicides like atrazine or metribuzin.
It should be noted that just about every pre-emergence corn herbicide that has residual activity does not allow planting of soybean or just about any cover crop species for at least a 4-month interval after the application was made. Most specify soybean cannot be planted until the next season.
The bottom line is, you need to check the replant intervals on the labels of the corn herbicide(s) you have applied to see what options you might have.
Many people are asking if the residual herbicide that was applied is even there anymore. That's a great question and one that is not easily answered. The answer will most likely depend on the method by which the herbicide is degraded in the soil, and the water solubility of the herbicide in question.
Many residual herbicides are microbially degraded in the soil, and having water over the soil for a prolonged period of time (anaerobic conditions) will actually slow herbicide degradation. This could increase the likelihood of the herbicide residue injuring the subsequent crop.
On the other hand, it is certainly possible that the floodwaters could have moved a significant portion of the herbicide you applied away from your field altogether.
Because of all of this uncertainty, if anything is to be planted back into these flooded-out corn fields, it is a good idea to conduct a field bioassay before doing so. This can be accomplished by taking some representative soil samples from the flooded field and also from a non-flooded field where no herbicide has been applied, and then planting some of the crop seed in cups or pots. If the plants in the flooded soils appear normal and grow as well as those in the non-flooded soils, it's a pretty good indication the residual corn herbicide has been degraded to the point that it will no longer cause injury.
If you decide to take the prevent plant option and not plant any other crop, it's a pretty sure bet that it will grow up in weeds this season. Those weeds will likely produce a lot of seeds that will be a problem for you next year.
We would certainly recommend some practice such as tillage and/or a herbicide application at some point in the season to keep those weeds from producing seeds. Even mowing will likely reduce, but not eliminate, annual weed seed production.
Burning down big weeds
Because of all the wet weather, another common question has been about burning down big (2 to 4 feet tall) marestail in fields that intend to be no-till planted to soybean. This is another one where there is no easy option because almost all of our marestail populations in Missouri are resistant to glyphosate, and a good portion of them are also resistant to the Group 2, ALS-inhibiting herbicides.
And these are fields where the marestail is really much bigger than what we would normally be spraying.
Tillage is one of the most effective options in these instances, but if the grower wants to stick with no-till, we have seen more success in these circumstances with more of the contact, burning herbicides. For example, a glyphosate product plus Liberty (1 qt./acre of each) is something we haven't done much of in Missouri in the past, but it has worked well for us in some of our trials and several retailers have been reporting good results with this combination.
Also, Gramoxone in combination with 2,4-D or even Sharpen (or even better all three together) has been a good treatment. If you will be planting Xtend soybean, certainly one of the approved dicamba products (Xtendimax, Engenia, Fexapan) is an option, but our research indicates that on these very tall marestail plants, glyphosate plus dicamba alone is probably not going to give adequate control. Adding Sharpen to that combination will often improve control substantially.
Because of our delays in soybean planting this year, we thought it worth mentioning that for those growers who intend to plant Xtend soybean and then use an approved dicamba product for post-emergence weed control, these applications will now be forced into a timeframe that is likely much later in the season than we have sprayed these products in the past.
We thought it worth mentioning that, based on our experiences in Missouri with the approved dicamba products in the past, the risk of off-target movement and injury to neighboring fields will increase with later-season applications of these herbicides.