The value of a residual in a weed management plan

By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and Soybean Check-off

The working definition of a weed is any plant growing in a place where it is not wanted. That means that in a monoculture field, such as soybeans, anything other than the desired soybean plant is considered a weed. More is being learned about the yield reduction caused by competition from weeds, as well as other biological impacts weeds can have on growing crops. Having a weed management plan before the season starts is the first step in reducing the negative impacts of weeds on crops. The Ohio Field Leader had the opportunity to visit with Chad Threewits, agronomic service representative at Syngenta, to discuss weed management strategies and the evolving challenges of herbicide-tolerant weed pressure.

OFL: There have been a lot of challenges with weeds and the effectiveness of herbicides the last several years. It seems that almost every other year, there is another new herbicide-resistant weed that rears its ugly head. Can you share some advice about scouting at this point in the growing season and about weed management in general?

Threewits: That is true for many growers and fields I have visited across the state. The water hemp has just continued to drive what we do differently. The problem continues to grow and continues to be harder to manage. I think in Illinois, we have water hemp populations that have developed seven-way resistance to herbicides. Unfortunately, we don’t really have seven different modes of actions readily at our disposal to control a lot of this. Another way to look at it is that the tools are getting fewer in the toolbox. With that thought in mind, it will take a whole system approach to regain control of the weed situation.

OFL: It is mid-June. What should soybean growers be doing at this point in the growing season?

Threewits: This is a great time of year to look at what weed pressure we have out in the fields and consider managing the weed seed bank. If you can manage that seed bank and try to control that, whether it is in corn or soybeans, that is where we have really seen success. If you can manage that seed bank and keep weeds down, that is the first step.

Farmers can try to manage it in a couple of different ways. By using the stronger residual products in the herbicide plan, products like Tendovo® herbicide, we are getting more active ingredients on the front side. By applying residual products, it gives us a longer window, and it is also pretty safe on young soybeans too.

OFL: When does a residual herbicide best fit in the weed management strategy?

Threewits: The farmer’s goal is to maximize yield. Controlling weeds with more herbicide is one way, but if we start increasing the rates of certain herbicides, it can be a little tough on the soybeans as well. One nice thing about Tendovo® is that the control comes from that increased rate that we have incorporated in the product already.

Another part of a weed management strategy is coming back with overlapping residuals. Start by putting down a strong residual, then come back with another residual at post-application. The days of driving down the road in the pickup truck and rolling the window down to see if the weeds are big enough to spray are over, especially with water hemp in the picture. Once you can see it from the road, it is probably already too late.

If growers are now just getting out to make the post-application because they got rained out and could not get back in, that is the value of what an early residual product does. We have had two opposite years, the last two, and both have been really hard to manage weeds. Now with the rainfall, and if we stay wet, the water hemp is going to take off. It loves heat and moisture, just like the crop does. As you know, it gets much harder to control weeds the later we get in the season, especially once they get to a certain point, there is not a lot we can do about them.