Source: Mark Loux, Ohio State University

Residual soybean herbicides should be included in marestail management programs to control plants that emerge between planting (or burndown herbicide application) and soybean canopy closure. The soybeans can adequately suppress plants that emerge after canopy closure. Even the most effective residual herbicides may not completely control marestail during this period, especially when they are applied several weeks before soybean planting. Some considerations in residual herbicide selection:

  • It is important to know whether the marestail population is ALS-resistant. One indicator of this is whether POST application of FirstRate or Classic retains any activity on marestail in the field. If you are unsure about this, assume that the population is ALS-resistant for the purposes of residual herbicide selection. Reminder: the PRE soybean herbicide effectiveness table in the "Weed Control Guide for Ohio and Indiana" has ratings on both ALS-sensitive and ALS-resistant marestail. 
  • The most effective residual herbicides contain active rates of two components — an ALS inhibitor (chlorimuron or cloransulam) and a non-ALS inhibitor such as Valor, Authority or metribuzin. Premix products included here: Valor XLT, Envive, Sonic, Authority First, Authority XL, Gangster. Canopy/Cloak DF actually also falls into this category, but the rate of metribuzin is fairly low at typical product use rates. 
  • The "Roundup Ready" rates recommended on the labels of these premix products will often be adequate where the population is not ALS-resistant since both components of the premix are contributing to the control. However, in ALS-resistant populations that non-ALS component of the premix is providing all of the residual marestail control, and the rate of that component becomes more important. Another way to look at this — Valor will be just as effective as Valor XLT or Envive for control of ALS-resistant marestail, although control of other tough broadleaf weeds may decrease. 
  • The "Roundup Ready" rates of the premixes can supply a relatively low rate of the non-ALS component (flumioxazin, sulfentrazone or metribuzin), and should be increased or supplemented with additional herbicide to obtain enough residual control of ALS-resistant marestail. We suggest aiming for the following rates: metribuzin — 8 to 10 oz/A of 75DF; flumioxazin  — 2.5 oz of Valor; and sulfentrazone — 0.2 to 0.25 lb ai/A. Some examples based on this: the 4 oz rate of Canopy/Cloak DF contains the equivalent of 3.4 oz of metribuzin 75DF so an additional 5 to 6 oz/A of metribuzin should be added; 4.3 oz of Valor XLT are required to apply the equivalent of 2.5 oz of Valor, or additional Valor can be added to lower rates of Valor XLT. This approach is more problematic with Authority products, where the "Roundup Ready" rates can provide much less than 0.2 to 0.25 lb ai/A, because adding sulfentrazone (Spartan) to get to the higher rate is not necessarily cost-effective. 
  • In our research, residual control from saflufenacil (Sharpen) is often in the range of 40 to 70 percent, which is not usually adequate, and it should be supplemented with additional residual herbicide. Since saflufenacil cannot be applied with products that contain flumioxazin or sulfentrazone, what is the most appropriate partner to mix with Sharpen to improve residual marestail control? Scepter and Pursuit are somewhat variable for residual marestail control, and do not control ALS-resistant marestail (Optill is a premix of saflufenacil and Pursuit). We suggest that Canopy/Cloak DF and/or metribuzin are probably the most appropriate tank-mix partners where glyphosate + Sharpen is used as the burndown. As indicated above, whether using metribuzin alone or mixing with Canopy/Cloak DF, aim for a total of 8 to 10 oz/A of metribuzin 75DF. The other approach here is to use Ignite for burndown of marestail instead of glyphosate + Sharpen (the cost is about the same), which allows use of any residual herbicide.