What’s all the purple stuff growing out in these fields? That’s the question I’ve been getting from both farmers and nonfarmers alike over the last couple of weeks. Well, the short answer is – it’s henbit (mostly). But the fact that more and more people are asking is, to me, a sign of a worsening issue facing Illinois farmers: winter annual weeds. 

Henbit (Photo provided by Torey Colburn)

In Illinois corn and soybean production, we tend to have laser focus on our summer annual weeds – waterhemp, giant ragweed, lambsquarters, velvetleaf and others. And with good reason, because they can be troublesome throughout our growing season.  Over the course of my 16-year career, I have battled year in and year out with these species. But over the last several years, I’ve really taken notice of the increasing amount of winter annual weed pressure in the spring. 

Historically, winter annual weeds like henbit, purple deadnettle, field pennycress, shepherd’s purse, butterweed (ragwort) and even marestail (horseweed) didn’t typically produce significant pressure until mid-to-late May – and by then, we had already gotten a handle on most of them. So, what’s changed? Numerous factors have played into creating the scenario we see today.  Reducing fall tillage reduces the potential for erosion and improves water quality, but it also favors the life cycle of these winter annual species that germinate in the fall. Much of the state is also experiencing warmer winters, which is allowing some of these weeds to get an earlier start in the spring. 

So, what can we do? 

In most cases, these winter annuals are controlled with spring tillage if they are small enough or burned down with herbicide applications in the spring ahead of planting. Some of those burndown applications can be expensive if those weeds start to get some size to them. Every time we increase the rate of a product or add another product to the spray tank, we add cost to that application. A typical soybean burndown application contains a combination of glyphosate and a plant growth regulator component – usually 2,4-D or dicamba. Be sure to look at the label, both carry planting restrictions. 

But what if we didn’t have to try to control so many different species all at once, or what if we could limit the amount of weed pressure the burndown needs to remove? An integrated approach to weed management is critical to keeping weeds at bay and preserving our effective herbicide chemistries. Producers should consider the use of fall-applied herbicide programs, particularly in fields that have a history of winter annual weeds or fields where practices are being used that favor winter annuals.  

Additionally, increasing numbers of producers are finding that there can be noteworthy advantages provided to a weed management system by integrating new practices such as cover crops as another tool in the toolbox. Cover crops – especially cereal rye before soybeans – are known to be effective at suppressing both winter and summer annual weeds. While it is true that maximizing weed suppression using cereal rye before soybeans means letting it grow to 2-2.5 tons of biomass per acre, it is also true that significant weed suppression (especially of the winter annuals) can be achieved with less. This is where I think farmers can find some immediate benefit in using the cereal rye. 

Cereal Rye Henbit Suppression (Photo provided by Torey Colburn)

Driving around the countryside this spring, I’ve taken notice of the difference in winter annual weed pressures in cover-cropped fields vs non-cover-cropped fields (even where a winter-kill cover crop species was used). We still need to terminate the overwintering cover crop though.. But suppose the cover crop has done a good job suppressing winter annuals; in that case, we can likely terminate it with just a simple glyphosate application. This can offer cost savings and additional flexibility for the timing of the application of residual products targeting the summer annuals. If there are some winter annuals present and additional burndown help is required, adding saflufenacil (Sharpen) can be a great way to get a clean field and avoid the planting restrictions of 2,4-D and dicamba.  

The bottom line: The more we understand the biology of the target pest – in this case, winter annual weeds – the better we can develop effective control strategies. Since winter annuals germinate in the fall, why not plant a cover crop with a similar life cycle to compete with them, while also gaining the added benefits of erosion control and improved soil health? An integrated systems approach can help address agronomic concerns on multiple fronts.  

Oat-Radish winter kill suppressing winter annual weeds (Provided by Torey Colburn)