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Should I Apply Fall Herbicides? Consider These Factors

September 25, 2013
Horseweed
Figure 3. Inconsistent control of herbicide-resistant horseweed populations like this in the spring may be one reason to consider a fall herbicide application.  

(Shared by Kevin Bradley, Integrated Pest & Crop Management Program, University of Missouri. Click here to view the source website.)


As harvest season begins to get underway, some calls are coming in and a number of people are starting to ask about fall herbicide applications. There are a number of factors to consider when deciding whether or not a fall herbicide application might fit your corn or soybean production system, and some of the more important of these are discussed below.

#1. Spring Weather Uncertainty

One of the reasons that this whole concept of fall herbicide applications first came about was because of the desire of some producers and retailers to spread out their workloads and remove at least one of the tasks that we would normally do in the spring back to the fall. As illustrated in Figure 1, there are usually less suitable field workdays in Missouri during the months of March and April when early spring preplant herbicide applications are typically made than in the months of October and November when fall herbicide applications could be made.

Figure 1 University of Missouri IPM
Figure 1. Number of suitable field workdays in Missouri (30-year average).

This is largely due to the excessive rainfall that we usually receive in the spring versus the fall, which often makes timely applications of preplant burndown herbicides very challenging during this time of year.

#2. Impact on Soil Conditions

The removal of winter annual weeds with fall herbicide applications can have a significant impact on the soil conditions experienced at planting. Obviously, dense mats of winter annual weeds can make planting difficult, but the results from our research and from others shows that winter annual weeds can increase soil temperatures, can "wick" significant amounts of moisture from the soil, and can take up available soil nutrients intended for the developing crop.

As illustrated in Figure 2, we’ve found that the removal of winter annual weeds with fall herbicide applications resulted in higher soil temperatures when compared to areas with dense infestations of winter annual weeds. In corn, these differences were especially pronounced once soil temperatures reached 50°F (Figure 2).

Figure 2 University of Missouri IPM
Figure 2. Influence of winter annual weed removal with a residual fall herbicide application on soil temperature prior to corn planting as compared to non-treated plots with a dense cover of winter annual weeds.

Overall, in our experiments winter annual weed removal achieved through residual fall herbicide applications increased soil temperatures by as much as 5° in corn and by as much as 8° in soybean.

The presence of winter annual weeds also leads to reductions in soil moisture content at the time of planting. In our research, soil moisture content at planting was as much as 13% higher in corn and 6% higher in soybean where winter annual weeds were removed with a fall or early spring preplant herbicide application compared to locations with a dense cover of winter annual weed species.

Lastly, some recent research published by weed scientists at Kansas State University has shown that winter annual weeds are also likely to remove available nitrogen (N) from the soil. When averaged across 14 sites in Kansas, the average N uptake from winter annual weeds was approximately 16 lbs of N per acre. The authors also reported that waiting to remove winter annual weed infestations until spring reduced N uptake in developing corn plants.

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