Source: Dr Mark Loux, OSU
While it’s not terribly late yet, the wet soils and wet forecast could keep most of us out of the fields for a while. The questions about how to deal with burndown herbicide treatments in delayed planting situations are rolling in. One of the most common ones, predictably, is how to kill glyphosate-resistant marestail and giant ragweed and generally big weeds in soybeans when it’s not possible to delay planting long enough to use 2,4-D ester (Enlist soybeans excluded). While we wrote last week about marestail populations being on the decline, this does not mean it’s gone by any means. Overwintered marestail plants become tougher to kill in May, and the fact that fall weather was not conducive for herbicide applications makes the situation worse in some fields. The good news is that we have some additional herbicide/trait options for help with burndown since the last time we wrote an article covering this in 2016, although our experience is that nothing we suggest here is infallible on large marestail.
A burndown of glyphosate and 2,4-D struggles to control marestail in the spring anyway, especially in the absence of fall herbicide treatments. Our standard recommendation, regardless of when spring treatments are applied, is to either replace the 2,4-D with something more effective, or to add another herbicide to supplement the 2,4-D. Sharpen has been the frequent replacement/supplement, and we now have the option to use dicamba in the Xtend soybean system instead of 2,4-D. While it’s possible to use higher 2,4-D rates in the Enlist soybean without waiting to plant, higher rates do not necessarily solve this issue based on our research, although a follow up POST treatment that includes glufosinate or 2,4-D usually finishes off plants that survive burndown. We also would not expect the addition of Elevore to consistently solve this issue either, and it requires a 14 day wait to plant any soybean. There’s a list of suitable soybean burndown treatments in our marestail fact sheet, and also below – these are for fields not treated the prior fall.