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As summer turns to fall most producers across the state have their minds focused on the upcoming harvest but it's not too early to be thinking about fall herbicide applications. Some producers like fall herbicide applications because it enables them to reduce spring workloads by controlling winter annual weeds in the fall rather than using a burndown prior to planting in the spring. Many have also speculated that fall herbicide applications have a beneficial impact on the insect populations and soil conditions experienced at planting. In an effort to learn more about the impact of fall and spring herbicide applications on winter and summer annual weed populations, soil conditions, and insect populations, we have conducted a number of experiments in no-till corn and soybean fields over the past several years. Here we provide a brief summary of the results.

First, we have found in both our corn and soybean experiments that fall applications of non-residual herbicides like glyphosate plus 2,4-D are often ineffective because additional germination of winter annual weeds usually occurs in the spring. This usually translates into poor winter annual weed control at planting, thereby negating the whole purpose of the fall herbicide application. For this reason, for fall applications we generally recommend a residual herbicide like Canopy EX or Valor in soybean or Princep, Basis or Valor in corn for winter annual weed control.

Our results also indicate that applications of residual herbicides made in early March can provide the same level of winter annual weed control as applications of these herbicides in the fall. Furthermore, our data indicate that early spring applications of residual herbicides provide better control of emerging summer annual weed seedlings than fall herbicide applications. This is especially the case with the Canopy EX treatment in our soybean experiments. Although we realize that this timing--early March--defeats some of the purposes of the fall application timing, our results 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 experiments show that fall and early spring applications of residual herbicides will provide similar levels of control.

Dense stands of winter annual weeds such as henbit are the reason that some producers choose to make a fall herbicide application. Photo by Kevin Bradley.

In our evaluations of the effects of fall herbicide applications on soil temperature, we found that the removal of winter annual weeds with fall herbicide applications did increase soil temperatures compared to untreated plots, but only when air temperatures reached 70 degrees. Therefore, based on our research, the notion that fall herbicide applications will always increase soil temperatures is somewhat misleading.

Insect populations in our experiments were highly dependent on the presence of winter annual weeds and it was difficult to see any specific effects of individual herbicides or application timings. In our soybean studies, several weeks after planting insect pest populations in the untreated plots that contained winter annual weeds were 10-fold higher than those in plots treated with a herbicide to remove these weeds. The predominant insect that we encountered in these studies was the negro bug, which appears to be an emerging pest of no-till soybean in Missouri. Although we saw little to no differences in the incidence and severity of insects in our corn experiments, the impact of fall herbicide applications on black cutworm populations should not be overlooked. It is known that black cutworm moths are attracted to fields with dense populations of winter annual weeds, and that black cutworm larvae can cause severe injury in corn.

It is clear from the results of our experiments that there are many other factors, other than weed control, that you should consider when deciding whether or not to make a fall herbicide application. Probably the most important one for a grower that hasn't been mentioned thus far is cost. Make sure that the fall herbicide program matches your needs and won't just be an added cost to your operation. As mentioned previously it is of little value to apply a herbicide in the fall to control winter annual weeds and have to come back with a burndown in the spring anyway. Due to the wet weather usually experienced in Missouri in the early spring, we realize that this timing may not be as convenient or feasible as the fall application timing. However, our results clearly show that with our current herbicide options, you will achieve similar levels of winter annual weed control and even better levels of summer annual weed control with residual herbicide applications in early March compared to the fall. This does not mean that you won't need to apply another herbicide in-crop for summer annual weed control--you will--but the need for your burndown should be eliminated.

Kevin Bradley
Nick Monnig
573-882-4039