When planning a burndown, does a residual make sense? That depends on if a follow-up burndown is planned at planting, the expected planting date and what is the intended crop.
If the expected planting date is three weeks or so off and the plan is to burndown with something at planting, then a residual in with the burndown now is probably not warranted. However, if the intended planting date is more than 30 days away, then a residual can provide the advantage of not having a big, grown up mess to have to plant into.
If corn is the intended crop, then a residual added to an early burndown often will not add much, as most of the popular pre- and post-applied corn herbicides will also control newly emerged horseweed. Many are trying to cut costs in corn and this could be a place to cut back. However, be sure to have a good at-planting or early post-herbicide premix planned if that is the case.
Leadoff is a popular residual option in an early burndown program. Leadoff is a premix of the SU herbicides rimsulfuron and thifensulfuron. It will provide several weeks of residual control of winter annual weeds and will help curtail regrowth of horseweed that has been injured by dicamba that otherwise might re-grow. It will also aid in control of ryegrass and poa over glyphosate alone.
On the management side, do not expect Leadoff to control existing horseweed as it will need to be mixed with dicamba and glyphosate to do a thorough burndown of horseweed. Please note there is a 30-day plant-back to cotton and soybean after a Leadoff application. In cool, wet and higher pH soils, some non-STS soybeans may have their early growth checked up where Leadoff has been used. In corn, it can be applied right up to planting.
Valor, Sharpen and Verdict are the other popular residual herbicides typically used in an early burndown. Of course, Sharpen and Verdict will also do a good job on controlling existing horseweed. The main reason to utilize any residual herbicide in early March is for residual winter annual control. Leadoff, Valor, Sharpen and Verdict will all provide about the same amount of residual control for winter annuals.
Do not expect residual Palmer control from these applications. There is no residual herbicide that will typically provide any measurable Palmer amaranth control in soybeans or cotton when applied during most of the month of March. Also, we need to be more wary of Italian ryegrass this spring.