Yesterday at the UW Pest Management Field Day, the benefits of preemergence herbicides in managing early-season weed competition and herbicide application timing in soybean were clearly visible. In two field trials, we had applied an array of preemergence broadleaf herbicides, which will be followed by postemergence applications of glyphosate.  The intent of these preemergence herbicides was to provide initial suppression of broadleaf weeds so that glyphosate applications could be delayed. The delayed glyphosate applications would then be closer to soybean canopy closure and we hope this will avoid the need for a second glyphosate application. Several of the preemergence herbicides were at reduced rates to keep the treatment costs affordable. 

 

In one trial, the dominant weed is common ragweed with lighter densities of common lambsquarters, giant foxtail, and velvetleaf (Fig. 1). The plots scheduled to be treated with only glyphosate will need to be sprayed soon to avoid potential yield loss from weed competition and so that the weeds are small enough that glyphosate is still highly effective. 

 

 

Figure 1. Soybeans awaiting a postemergence glyphosate application on July 1.

 

However, the plots treated with preemergence herbicides had greatly reduced weed densities and the remaining weeds were generally smaller (Figure 2). The degree of weed control or suppression varied among the herbicides in this trial as expected because some herbicides were inherently more effective on common ragweed. Still, the tested herbicides such as Authority First or Sonic, Authority MTZ, Authority Assist, Boundary, Gangster, Valor, etc. all provided significant weed suppression and will allow glyphosate applications to be delayed. I expect the overall level of weed management to be greater than a single application of glyphosate. In your fields, you should be able fine tune management systems by selecting herbicides to match the weed species in the field. The benefits of this approach may be greater for soybeans planted in rows as compare to drilled soybeans, which canopy sooner.

 

 

Figure 2. Soybeans treated with a preemergence herbicide on July 1. 

 

Beyond the benefit of these two pass programs for improved weed management for this year, this approach should also hold longer-term benefits. Dr. Dave Stoltenberg also spoke at the field day about the current status of glyphosate-resistant weeds and his research assessing the risk of selecting a glyphosate-resistant weed. Dr. Stoltenberg referred to the risk of resistance as a “numbers game”, which means that the greater number of weeds sprayed with glyphosate increases the probability that a resistant weed might be found. In terms of our soybean field trials, it is clear that a greater number of common ragweed will be sprayed with glyphosate in Figure 1 than in Figure 2. Therefore, the risk of selecting a glyphosate-resistant common ragweed (examples of glyphosate-resistant common ragweed exist in other states) is greatly reduced where the preemergence herbicides were used. 

 

Although the option to use preemergence herbicides in soybeans is long gone for this season, it may be worth considering if or where it may have a fit in your management for next season as glyphosate applications are completed this month.