• By Larry Steckel •
Rain, rain, rain! We are behind the burndown pace of most Tennessee springs where at least some would have burndown applications applied by this time. This has resulted in a flurry of calls on options.
The trend in recent years with horseweed (marestail) is that it is very hard to find in most fields, and this has continued into spring 2020. As mentioned in an earlier blog, it would appear that the new use pattern for dicamba in-crop has greatly taken down the horseweed seed bank in most fields.
This is particularly true for west Tennessee and less so for middle Tennessee. We likely could expect some spring horseweed germination in some fields, particularly those that were preventive plant last year and let go to weeds or those adjacent to areas that were let go to weeds. Below are some of my thoughts on burndown strategies.
Soybeans and cotton: Emerged grasses and, later in April, emerged Palmer amaranth are a priority to remove prior to the crop emerging.
In 2018 and 2019, herbicide screens on barnyardgrass/junglerice escapes indicated that about 75% of the time, those grasses had some level of glyphosate resistance (2x to 8x). As such, tankmixing clethodim with glyphosate for burndown makes a lot of sense. Adding dicamba in with clethodim and/or glyphosate will reduce the grass control.
Finally, recent screenings of Palmer amaranth tolerance to dicamba collected from field escapes in 2019 showed increased tolerance to dicamba compared to Palmer amaranth collected from fields seven or more years ago. Consequently, it would be best to limit Palmer amaranth exposure to dicamba. Paraquat would be the best fit right behind the press wheel to assure that the cotton or soybean crop starts clean. Save the dicamba for in-crop use.
Corn: Most likely about the time we can get to the field to spray burndowns, it will be near, if not at, planting time. For fields with emerged glyphosate-resistant Johnsongrass, ryegrass, goosegrass or junglerice, a good option would be Leadoff. There is no wait to plant corn after a Leadoff application, and the rimsulfuron in Leadoff should provide effective grass control in most cases.
If the intended crop of corn has to be swapped to cotton or soybeans, then the wait period to plant those crops after a Leadoff application would be 30 days. The exception would be if the soybean variety is STS. Then no wait period is needed.
Another option, particularly if ryegrass is glyphosate- and rimsulfuron-resistant, would be clethodim. We have a 24(c) special local needs label for 6 ounces of Select Max and a six-day wait before corn planting. If higher rates of clethodim are needed as the ryegrass is large, then the plant-back to corn is 30 days.
An additional option if higher clethodim rates are warranted and the 30-day wait is a deal breaker would be to plan sequential applications of paraquat applied about seven days apart.
A good option for fields without emerged glyphosate-resistant grass would be paraquat + atrazine + Dual Magnum or Anthem Maxx/Zidua. This tankmix is a very effective burndown option but does lock the field into corn for the year.
If there is flexibility needed to plant an intended corn field to cotton, then substitute diuron for the atrazine and use some Dual Magnum as well. If the field is flood prone and soybeans would likely be the replant crop later this spring, then glyphosate + Verdict would be a very effective alternative.
Dr. Larry Steckel is a University of Tennessee Extension weed specialist. He may be reached at firstname.lastname@example.org.