Tips for early herbicide application

A primary concern of many recent callers about our current situation is whether it is too early to apply residual herbicides. They are undoubtedly aware that applying residual herbicides too early can reduce their effectiveness in late May and early June when the later flushes of marestail and waterhemp are emerging. Applying residual herbicides this early does introduce an element of variability in their activity later in spring, but this depends somewhat on the progress of soybean planting and crop development. The ideal scenario would be that soybeans are planted within the next couple of weeks and conditions are favorable for rapid soybean development and formation of a crop canopy. As a result, the crop is capable of providing control when the residual herbicide activity runs out. The risk of applying this early is that subsequent wet and/or cold weather will prevent timely soybean planting, which ultimately stretches out the time until crop canopy development. In this situation, we are likely to see breaks in residual herbicide activity prior to crop canopy, but the control that does occur is certainly much better than not using residual herbicides at all. It’s a little difficult to predict late-season control of marestail, and we sometimes observe effective residual marestail control regardless of the timing of the residual herbicide application. Here are a couple of suggestions relative to the early application of residual herbicides:

1. Increase residual herbicide rates. We have been suggesting increasing residual herbicide rates above the fairly low “Roundup Ready” rates to improve residual marestail control, and this is especially relevant for early application. It’s probably feasible to increase rates by 20 to 30 percent above the low rates for many residual products, or use the higher rates within the rate range for a given soil type (either way, do not exceed the maximum rate for the soil type). 

2. Split the residual herbicide. Apply some residual herbicide now with the burndown treatment and the rest when the crop is finally planted. The advantage of this approach is that it places some of the residual closer to the time of late-spring weed emergence, and also compensates for late soybean planting. Where soybean planting is delayed, application of the second half of the residual herbicide can also be delayed, and applied when it is most needed. 

There are various ways to split the residual herbicide, but something as basic as half the total amount applied early followed by the rest at planting would work. Or mix up the residual herbicides, applying one type of residual early and switching to another later. For products that are relatively short-lived in soil, such as Valor, Authority and metribuzin, the majority of the residual is best applied close to the time of planting.