Effective marestail management programs include appropriate burndown herbicides and also residual herbicides, to control marestail that emerge between soybean planting and early to mid-June.

Some assumptions we make about marestail at this point based on our knowledge of resistance and herbicide activity:

• The marestail populations in many Ohio soybean fields are resistant to glyphosate. Do not plan on trying to control emerged plants with glyphosate alone in burndown or POST applications. If you are fortunate enough not to have glyphosate-resistant marestail yet, following the guidelines here will reduce the risk of developing resistance.
• Many populations are resistant to both glyphosate and ALS inhibitors (chlorimuron. cloransulam, etc).

The ramifications of this include the following:  POST application of Classic, Synchrony, or FirstRate will not control marestail; in burndown applications, the chlorimuron and cloransulam components of premix products (Canopy, Valor XLT, Authority First, etc) will not help control emerged marestail, and will not provide any residual control.

So the other component of the premix (Valor, Authority, or metribuzin) has to carry the load for residual control.

With the exception of very small marestail plants, it’s not usually possible to adequately control emerged plants with 2,4-D alone or combinations of 2,4-D with residual herbicides such as Canopy, Sonic, Envive, Authority First, etc.

Even in glyphosate-resistant populations, the addition of glyphosate to 2,4-D is essential to obtain effective control.

While Valor and Authority cause contact herbicide symptoms on marestail leaves, these herbicides do not actually provide any control of emerged plants. Because of the contact burn on leaves, some antagonism between Valor and Authority and glyphosate/2,4-D is possible.

Effective burndown options are shown in the list that follows. These should all be effective for control of plants up to 4 to 6 inches tall, although we have controlled larger plants.

Where it’s not possible to use 2,4-D ester due to lack of time between application and soybean planting, the cost of an alternative burndown may be twice that of glyphosate+2,4-D.

This is money well spent, since marestail plants not controlled by burndown herbicides are not likely to be controlled this season.

1. Glyphosate + 2,4-D + AMS. Use glyphosate product rates that provide the equivalent to 1.1 to 1.5 lbs ae/A. Increasing the 2,4-D rate from 0.5 to 1.0 lb ae/A may improve control, but also increases the delay between application and soybean planting from 7 days to 15 to 30 days, depending upon the product. Some 2,4-D ester products, such as Weedone 650, E-99, Salvo, or Rage D-Tech (a premix of 2,4-D ester plus carfentrazone) can be applied 15 days before planting at the rate of 0.5 to 1 lb ae/A.

Weather conditions need to be favorable for plant growth. Cold temperatures before, during, or after application may reduce control. Some grower/dealers have reported increasing variability with the effectiveness of this treatment, especially on larger plants. Apply to marestail plants that are less than 4 inches tall for optimum results.

2. Safulfenacil + glyphosate + MSO + AMS. Saflufenacil product/rates for soybeans: Sharpen (1 oz — before soybean emergence; 1.5 oz — at least 14 days before planting); Verdict (5 oz); Optill (2 oz). Sharpen is the least expensive option among these. An MSO-based adjuvant is a required additive, and it must contain at least 60% methylated seed oil. Do not substitute nonionic surfactant or petroleum oil concentrate for MSO.

Thorough spray coverage is important. A spray volume of 15 to 20 GPA is normally recommended, especially for such situations as dense stands of weeds and variable plant sizes, as well as plants that emerged in the fall and overwintered. Do not use saflufenacil as a tank mix partner or sequential application within 30 days of other PPO (Protoporpyrinogen Oxidase) inhibitor herbicides such as Spartan (sulfentrazone), Valor (flumioxazin), etc.

3.  Ignite 280 (29 to 36 oz/A) + AMS (3 lb /A).  Ignite 280 (glufosinate) may be applied as a broadcast burndown treatment before emergence of any conventional or transgenic variety of soybean.

Thorough spray coverage is important. Apply in a minimum of 15 gpa, and increase to 20 gpa in a dense canopy of weeds. Avoid nozzles that produce primarily coarse droplets. Weather conditions need to be favorable for plant growth. Warm temperatures, high humidity, and bright sunlight enhance the performance of Ignite 280. Weed control may be reduced when weeds under stress due to cool temperatures, drought, or extended periods of cloudiness. The addition of metribuzin (4 to 6 oz of 75DF) can improve control and compensate somewhat for large plant size or adverse environmental conditions.  Unlike saflufenacil, Ignite can be mixed with any PRE soybean herbicide, making it a good option where the intention is to use a PRE product that contains Valor or Authority.

Residual Control Of Marestail In No-Till Soybeans

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.

• 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%, 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.