As we progress through the harvest season, many are beginning to consider fall herbicide applications for the winter annual weeds that are now emerging in corn and soybean fields around the state. In Missouri, some of our most common winter annual weed problems are henbit, purple deadnettle, chickweed, shepherd's-purse, field pennycress, fleabane, and horseweed (also known as marestail). Dandelion, a perennial that can be present in the fall and early spring, is also becoming more common in some areas.
Some producers like fall herbicide applications because it enables them to reduce spring workloads and control winter annual weeds in the fall rather than using a burndown herbicide prior to planting in the spring. Due to the extremely wet spring and flooding we experienced in some areas this year, those producers who made fall herbicide applications last year seemed to benefit because they did not have dense mats of weeds present at the time of corn or soybean planting. Because of this, I have heard from many producers that have a "renewed interest" in making a fall herbicide application this year.
My opinion of the utility of fall herbicide applications has always centered around one issue; the ability of the program to replace the need for a spring burndown application prior to corn or soybean planting. If you choose to go with a fall herbicide application it must replace the need for the spring burndown. Otherwise, it seems to me that a fall herbicide application is just an added cost in the overall weed management program.
If you choose to make a fall herbicide application, you should choose a program with enough residual activity to control winter annual weeds that may come up later this fall as well as any that may emerge next spring. From what I have seen, weeds like henbit and purple deadnettle germinate predominantly in the fall while weeds like horseweed, field pennycress, and common chickweed are more variable and may germinate throughout the fall or spring. In our research, the control of some winter annual weeds with a non-residual treatment like glyphosate plus 2, 4-D in the fall has been extremely variable compared to residual herbicide programs. This is why I generally do not recommend these kind of non-residual fall herbicide programs.
For the most part, the herbicide options available in the corn market have not changed dramatically over the past several seasons. Some of the most common fall herbicide programs used in fields planted to corn include Princep, Basis, Valor, and Autumn. Authority MTZ (Authority + Sencor) is a new option that is now available for fall application to fields that will be planted to either corn or soybean in the spring.
For fields that will be planted to soybean, Canopy EX and Valor have been available for several years and both have residual activity on a variety of winter annual weeds. In 2007, Autumn was also labeled for fall application prior to soybean planting. This year, several new products are available for use in the fall market. Envive and Enlite, which are prepackaged mixes of Classic, Valor, and Harmony at different ratios, can now be applied in the fall prior to soybean planting. Similarly, Valor XLT, a prepackaged mix of Valor and Classic, is another new addition to this market. FMC also has Authority First (Authority plus Firstrate) and and Authority MTZ (Authority plus Sencor) available for application in the fall prior to soybean planting.
With all of these fall herbicide programs, 2, 4-D ester should be added to enhance the control of broadleaf weeds present at the time of the application. If winter annual grasses are present you should also consider applying glyphosate with your residual herbicide treatment, as very few of these products provide any control of winter annual grass weeds.
Although fall herbicide programs can provide good control of winter annual weeds up to corn or soybean planting, our research has shown that applications of these same residual herbicides made 30- to 45- days prior to planting in the spring can provide similar levels of winter annual weed control. Additionally, our data indicate that early spring applications of these herbicides will provide provide better control of early-emerging summer annual weeds than applications of these same herbicides in the fall.
So far, this has not been a very popular message or commonly adopted practice in Missouri. And probably for good reason. First, applying these residual herbicides in the early spring rather than the fall is adding to the spring workload, not reducing it, which is one of the main reasons producers have adopted fall herbicide applications in the first place. Second, we often experience very wet springs and it is sometimes difficult to even get across fields to make a herbicide application at this time of the year. Regardless, our results still indicate that with our current herbicide arsenal, the early spring timing is better if your objective is to achieve excellent winter annual and some summer annual weed control. If your primarily goal is just to eliminate your winter annual weed populations, then our research indicates that there are a variety of fall herbicide programs that will perform similarly.
REVISED: July 15, 2015