Multiple years of research on fall and spring herbicide treatments have consistently shown that the value and effectiveness of residual herbicides for soybeans is maximized when applied in the spring, says Mark Loux, weed control specialist at Ohio State University.

"The bottom line is that is can be a big mistake to apply all of the residual herbicide in the fall, with the intent to omit the spring pre-plant treatment and apply just post-emergence herbicides in next year's soybeans," Loux says.

The researcher says fall application of chlorimuron products, such as Canopy EX/DF, can control many summer annual weeds through early June the following year, but the residual control of ragweeds and marestail is often reduced greatly with fall applications, compared with application of residual herbicide in the spring.

He says this is partly due to the prevalence of ALS resistance in those weeds, but a lot of the problem also lies with their biology and ability to emerge well into the growing season.

"Post-emergence control of many marestail populations is close to impossible due to herbicide resistance," Loux says. "The goal of a marestail management program is to ensure that the combination of fall and spring burndown and residual herbicides results in a weed-free seedbed at the time of soybean emergence, and little to no emergence of marestail between soybean emergence and crop canopy closure.

"Even the most effective marestail management programs can fail to completely achieve this, but they often keep the populations low enough in the soybeans that they are not problematic."

Marestail plants that emerge in late summer or fall are easily controlled with a fall herbicide treatment, Loux adds. However, he says it's essential to realize that a fall herbicide treatment is not likely to accomplish everything that’s needed in an effective marestail management program.

"An effective program does not necessarily involve application of herbicide in the fall," Loux says. "A combination of the appropriate burndown and residual herbicides applied in April can adequately control marestail, even those that emerged the previous fall."

One option in fields where other winter weeds are not a problem, Loux says, is to skip the fall herbicide treatment and apply a combination of burndown and residual herbicides in April when marestail are still small.

In marestail-infested fields requiring a fall herbicide treatment for management of other winter annual annual weeds or dandelion, he says it's essential not to apply all of the residual herbicide in the fall. This also applies to those fields that are typically so wet that soybeans cannot be planted until mid- to late May, loux adds.

"In this situation, the goal of a fall residual herbicide treatment might be just to ensure that marestail are not too large when burndown herbicides are finally applied in May," he says.

Regardless of the type of herbicides applied in fall, an effective rate of a residual herbicide should still be applied in the spring to maximize control of marestail that emerges in May and June, Loux says. The most effective residual herbicides for spring application include two modes of action, to ensure effectiveness on ALS-resistant marestail.

Some examples, Loux says, include: Envive, Valor XLT, Gangster, Sonic, Authority First and Canopy DF + metribuzin. However, he says it should not be necessary to apply something this broad-spectrum or costly in the fall.

He suggests one of the following approaches:

1. Apply a combination of glyphosate and 2,4-D in the fall, followed by application of residual herbicide in the spring prior to soybean emergence. At the time of soybean planting, the field is likely to be infested with marestail that emerged earlier in spring, so include effective burndown herbicides (2,4-D, Gramoxone, glyphosate or Ignite, or some combination as appropriate based on herbicide resistance, plant size and time until soybean planting) to control emerged plants.

2. Apply 2,4-D with Canopy DF or EX at fairly low rates (e.g.. 1 ounce of EX or 2 ounces of DF) in the fall, followed by application of residual herbicide in the spring (with burndown herbicides if the residual from fall does not hold marestail through planting). It's possible to follow the fall Canopy application with a spring application of a chlorimuron-containing herbicide, as long as the total does not exceed the maximum labeled rate of chlorimuron for the soil type.

3. In ALS-resistant populations where Canopy will fail to provide any residual control of marestail, it may be possible to substitute a combination of 2,4-D with metribuzin in the fall. This combination should control most emerged winter annuals, but can be weak on dandelion. Follow with application of residual herbicide in the spring (with burndown herbicides if the residual from fall does not hold marestail through planting).

"The idea here is to apply a herbicide treatment in the fall that adequately controls emerged weeds, provides some residual if desired, but does not break the bank and allows use of the majority of the residual herbicide in the spring," Loux says.

He says options 2 and 3 would be most suitable for fields that are wet well into spring, where the goal is to control at least some of the marestail that emerge in early spring. Loux adds that Canopy certainly provides substantially longer residual control than metribuzin, but use of metribuzin preserves the option to plant corn the following spring.