Winter annual weeds like henbit, purple deadnettle, and chickweed are already emerging in many corn and soybean fields throughout Missouri, and as we progress with our corn and soybean harvest many growers are once again asking about the utility of fall herbicide applications.
As I have said in many previous newsletter articles and talks on this issue, our research indicates that applications of residual herbicides made in early March (or sometime similar in the early spring) can provide the same level of winter annual weed control as applications of these same herbicides in the fall. In addition, 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 our current herbicidal options available in soybeans.
Although we realize that this early spring timing defeats some of the purposes of the fall herbicide strategy, 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 perform similarly. Although I usually write each year about specific products for use in the fall herbicide market, I'm not going to do that this year. And that's because fall herbicide applications are about more than just the weeds. This is something that often gets lost in the decision-making process surrounding fall herbicide applications. So, this article is going to be about some of the other factors that you should consider when deciding whether or not to make a fall herbicide application.
First, one significant issue to consider is that many winter annual weeds like henbit and purple deadnettle can serve as alternate hosts for soybean cyst nematode. Researchers at Purdue University have recently done a great deal of work on this interaction and have a website and publication you can reference to learn more about this issue: http://www.btny.purdue.edu/weedscience/SCN/index.html.
Second, dense fields of henbit and other winter annual weeds that are flowering in the early spring are attractive sites for black cutworm moths to lay their eggs in which can then hatch and feed on the developing corn crop. In our research we have also seen that winter annual weeds can act as an alternative host for corn flea beetle and some Lepidopteran insects in corn, and can act as an alternate host for Negro bugs in soybean.
Lastly, another factor to consider is that 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 to say the least, but the results from our research also show that winter annual weeds can "wick" significant amounts of moisture from the soil. In both our corn and soybean experiments conducted across 6 site-years, we observed that the untreated plots that contained winter annual weeds had a significantly lower soil moisture content than plots that were treated with herbicides and contained no winter annual weeds. If we're experiencing a wet spring, then from that standpoint having winter annual weeds present may actually be a good thing. However if it's dry, we certainly don't want the winter annual weeds to take the available soil moisture from the emerging corn or soybean crop.
In addition to soil moisture, in our research we've found that the removal of winter annual weeds with fall herbicide applications does increase soil temperatures compared to areas with dense infestations of winter annual weeds (Figures 1 and 2). In our experiments this difference was especially pronounced once temperatures reached 50°F in corn and 68°F in soybeans.
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. Another one that I didn't mention 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.
To see a more complete summary of the results from some of our fall herbicide trials in corn and soybeans, you can go to our website and view a more detailed slideshow at: http://weedscience.missouri.edu/extension/extension.htm.
REVISED: October 2, 2015