Recent discussions of weed management in soybeans are often centered around glyphosate, rates, weed heights, and application timing. However, canopy development and weed shading are also important, especially when a herbicide without residual activity like glyphosate is used. If the initial glyphosate application controls all of the emerged weeds, it would be ideal if the soybean canopy was developed sufficiently to shade out weeds that emerge later. Thus, we should consider the light conditions that a young weed seedling experiences. Using a simple approach, I recently assessed the canopy development of row and drilled soybeans at the Arlington Ag Research Station. I wanted to “see” what a weed would see within a soybean canopy. Although one approach would be to lie down on the ground and look up, I grabbed a digital camera and snapped several images with the camera facing straight up from the ground. 

 

The view from down under is interesting. Although the weather has been somewhat dry at Arlington, the soybeans are growing well. The row soybeans seem to be nearing canopy closure (Figure 1), but the view up through the canopy reveals ample sunlight for weed growth if they were present.  The drilled soybeans had a canopy that appeared more complete when viewed from above (Figure 2). Below the drilled soybean canopy, the amount of light was greatly reduced, but not completely eliminated. While the amount of shading in both of these situations will continue to increase during the summer, the drilled soybeans clearly have the early advantage in canopy development. 

 

What does this mean in terms of weed management? One answer may be that row soybeans may have a more frequent need for a second application of glyphosate compared to drilled soybeans in a similar situation. A second answer may be that a residual herbicide may have more benefit in row soybeans than drilled soybeans. Of course, there are good reasons for planting soybeans in 30-inch rows such as lower seeding rates, better soybean placement and emergence, and greater air flow for white mold management. However, it is also important to consider the consequences of wider soybean row spacings – the soybean canopy may not develop as rapidly as with drilled soybean, which can affect weed management. 

                                    

Figure 1. 30-inch row soybean in mid-July (left) and corresponding view through the canopy (right).

 

  

 

 

Figure 2. Drilled soybean in mid-July (left) and corresponding view through the canopy (right).