Big weeds, waiting to spray means more yield loss
The point of this data is simple; it illustrates that in many Missouri soybean fields we are losing yield as a result of early-season weed competition and waiting too long to make our post-emergence herbicide applications. One of the fundamental principles of weed science is that the more weeds there are and the bigger these weeds get, the more yield loss we will experience. To some extent, this principle is also illustrated in the results from last year's survey contained in Figure 1. Although this figure does not show the weed density at each location, it does show the time that the post-emergence herbicide applications were made. For the most part, herbicide applications made to soybeans in the later stages of growth resulted in higher yield loss than those made at earlier stages of growth.
So, as you consider the timing of your post-emergence herbicide application in soybean this season, consider the price of a bushel of soybeans and what a 2 or even 3 Bu/Acre yield loss means to you. There are plenty of companies promoting various practices that will "enhance" yield by 2 or 3 Bu/Acre; our data indicate that in many Missouri soybean fields, we may be losing this much yield to weed competition.