Delayed Planting and Soybean Weed Control

Every year growers battle weeds in soybeans. We spend most of the winter making crop plans, including weed control, then Mother Nature decides to give us a wet spring. Here are some thoughts around controlling weeds in soybeans in a delayed spring:

Winter Annuals – Any field that did not receive a tillage pass in the fall will have some pressure from winter annual weeds like henbit, chickweed and marestail. These weeds germinate in the fall and then finish their life cycle in early spring. The best means of control for winter annuals is a planned herbicide program in the fall. These fall herbicide programs have shown to be very successful in managing winter annual pressure.

However, the weather was not cooperative last fall, so many of these fall-applied herbicide applications weren’t made. The alternative when not being able to make fall applications is to control these same weeds very early in the spring, like March for instance. Well guess what, the weather hasn’t allowed these applications either. So now as we sit in mid-May with heavy winter annual weed pressure, what are our options?

In these situations, the key herbicide site of action for controlling winter annuals is the group 4 growth regulator herbicides like dicamba and 2,4-D. Remember, these weeds are finishing their life cycle anyway, so this herbicide site of action just speeds them along on this process. It is also a good plan, in my opinion, to add residual products to this application as well. Residuals are key in a compacted spring as we will discuss later in this article. There has been some concern in the past that the residual products may get “tied up” in the mass of dying winter annual weed tissue, but I have not found that to be a problem. However, before no-till soybean planting, make sure that soil conditions are correct where those patches of winter annuals were controlled, as the soil in those patches tends to hold moisture.

One last point on the group 4 growth regulator herbicides is that, depending on the trait package in your soybean variety, there may be a length of time required before you can plant. Make sure you discuss this with your seed provider and your local crop protection retailer. Some traits, like Roundup Ready 2 Xtend® soybeans, allow immediate planting after certain dicamba containing herbicides.

Value of residual weed control – In a delayed spring, some might think that there is less value to residual weed control, especially when tillage is utilized before planting. I would suggest that it is just the opposite. Because the growing season is shorter, it means we will have less time once the crop has emerged to get to all the early season work that needs to be done. Therefore, I would suggest not only utilizing residual herbicides pre-emergence, but I would think about raising the rates of those herbicides.

Why increase the rate? With late planting, there will be a shorter length of time until soybeans “shade the row.” We need to take advantage of using overlapping residuals to stop weeds from emerging rather than allowing the pre-emergence pass to “break” and having to control emerged weeds that are harder to manage. This is especially nice for waterhemp and Palmer amaranth since they are so tough to control after emergence.

What about delayed planting and dicamba? First and foremost, don’t ever plant into actively growing weeds, especially waterhemp and Palmer amaranth, with any soybean trait system.  It is imperative that you start clean with tillage, or with an effective burndown herbicide program.

If Mother Nature allows us to plant Roundup Ready 2 Xtend soybeans before June 5, utilize your planned pre-emergence soybean residual program and follow up in mid-June (Before R1 and June 30) with a Roundup® + labelled soybean dicamba + residual product application. This still gives time to apply a post-emergence application of a labelled soybean dicamba 20-30 days after the pre-emergence trip. If we make our pre-emergence trip in early June, this would put our post-emergence application at about June 25-30th. What If weather or crop growth stage does not allow us to apply the labelled soybean dicamba in the post-emergence trip? If this happens, it would be necessary to change our post application to just Roundup + a residual product and there are many of them available.

If Mother Nature forces us to plant our Roundup Ready 2 Xtend soybeans after June 5, my recommendation would be to continue to use the planned residual pre-emergence program. In fact, I would use the highest labeled rate of these products. Why increase the rate? Because these pre-emergence herbicides may be your resistant waterhemp control if the weather (or label restrictions) don’t allow us to utilize a labelled soybean dicamba herbicide post-emergence as discussed above.

Your local retailer is your best source of information for rates and adjuvants, or any other questions you might have. I know the weather we have had is frustrating, but we will get through this!