One of the questions that has come up repeatedly over the past year or so concerns the appropriate order of vertical tillage versus herbicide application in the spring. Two general principles guide our thinking on this issue: 1) if possible, foliar burndown herbicides should be applied to undisturbed weeds that are not partially or fully covered with soil; and 2) residual herbicides should left on the surface undisturbed by tillage (allowing rain to move herbicide into the soil) following application unless that tillage will uniformly mix herbicide with the upper couple inches of soil.
It can be possible to accomplish the second of these in a no-till field with the appropriate combination of soil tilth, well-distributed crop residue, and an effective tillage tool. This does not describe the situation where vertical tillage occurs, since it fails to uniformly mix soil where the blades/discs run and leaves other areas of the soil surface undisturbed. This is of course why there still needs to be a comprehensive burndown herbicide treatment – the vertical tillage does not effectively remove all of the weeds. One option that avoids the problems with burndown activity versus residual distribution is to make two herbicide applications.
Burndown herbicides can be applied prior to the tillage, followed by residual herbicide application after the tillage. Sounds like extra work and expense for sure. Aside from this, our best assessment where everything needs to happen within a short period of time prior to planting, is that the vertical tillage should occur first, followed by the burndown/residual herbicide application. There may be a benefit to delaying the herbicide application for a while if possible, to allow weeds disturbed by the tillage to recover somewhat.
Where the order of operations is reversed, the best-case situation would probably be to apply residual herbicides far enough in advance of tillage that substantial rain occurs. The rain can move herbicide into the soil profile, which may reduce the negative effect of vertical tillage on herbicide distribution, compared to when all of the herbicide is still sitting on the soil surface. This is an area that we do not have a lot of experience with, so if you have developed a foolproof approach be sure to let us know.