Farmers who have existing infestations of winter annuals or marestail, or need to control biennials (wild carrot, poison hemlock) or perennials (dandelion, quackgrass, Canada thistle) should apply herbicides in the fall.
The primary value of fall herbicide treatments is to prevent these weeds from getting too tall and hard to control in the spring. Fall emerged marestail tends to be the highest weed seed producer the following year.
Here is a core group of fall applied herbicides that will control emerged weeds: For soybeans planted in the spring: Canopy EX or DF + 2,4-D, Glyphosate + 2,4-D, or Metribuzin + 2,4-D (excluding dandelions). For corn planted next spring: Basis + 2,4-D, Glyphosate + 2,4-D, or Simazine + 2,4-D (except dandelion).
It's possible to use other treatments in the fall, such as dicamba + 2,4-D, Autumn + glyphosate or 2,4-D, or Express + 2,4-D, but these are not more effective than the treatments listed above (and can be less effective on dandelion). Glyphosate + 2,4-D is the most effective for control of most perennial and biennial weeds, and glyphosate can be added to the other treatments to accomplish this. Combinations of 2,4-D plus Canopy or Basis have been the most consistently effective on dandelion.
Only Canopy products provide substantial residual control of annual weeds that emerge in spring or early summer. Our research shows that it is largely a waste of money to apply residual soybean herbicides other than Canopy in the fall with the goal of controlling weeds that emerge after soybean planting the following spring, because they all largely dissipate during winter to the point of providing little to no activity the following spring.
In most soybean fields it is a big mistake to apply all of the residual herbicide in the fall, with the goal of making only post emergence glyphosate applications the following year. It is possible to do this with Canopy products in fields with low populations and no glyphosate resistance issues, but most fields benefit from use of residual herbicides in the spring, along with some additional burndown herbicide if needed. This program (fall residual followed by POST glyphosate next year) is frequently doomed to fail in fields with marestail problems. Most of the marestail in the state is now glyphosate-resistant, and many populations are resistant to ALS inhibitors also. This means that it is impossible to control marestail after soybeans have emerged, unless the field is planted with LibertyLink soybeans.
In those fields requiring a fall herbicide treatment for management of other winter annual weeds, where marestail also occurs, it is essential not to apply all of the residual herbicide in the fall. The majority of the residual herbicide should be applied in the spring, to maximize control of marestail that emerges in May and June. The most effective residual herbicides include two modes of action, to ensure effectiveness on ALS-resistant marestail. Examples: Envive, Valor XLT, Gangster, Sonic, Authority First, Canopy DF + metribuzin.
Where the marestail is resistant to ALS inhibitors as well as glyphosate, fall application of Canopy will fail to provide control of spring-emerging marestail. This will usually not be improved with application of a premix such as Valor XLT/Envive, Authority First/Sonic, etc. Our research shows that the best approach where marestail are resistant to both glyphosate and ALS inhibitors: Apply glyphosate + 2,4-D in the fall and follow with broad-spectrum residual herbicides in spring combined with enough burndown herbicide to ensure complete control of marestail. This can be a challenge in the parts of the state where wet soils in spring can result in late planting and big marestail. But applying more residual herbicide in the fall won't solve this problem, and we have burndown options that can work on big marestail (Ignite + Sharpen, Ignite + metribuzin, glyphosate + Sharpen, etc.).
Some farmers wonder whether they should spray their fields before or after planting cover crops. If you are using cereal rye as a cover crop, drop out the glyphosate and apply only the 2-4D to take out the marestail. Since cereal rye is a grass, the 2-4D has only a negative effect on broadleaf weeds. You can also apply the 2-4D in the early spring if you want the cover crop to continue growing OR use 2-4D with glyphosate to kill off the cover crop and weeds in the spring.