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Mishap with corn weeds? Coverage, timing could explain

Brad Haire brad-haire-farm-press-corn-GA-2018-a.jpg
I struggle with herbicide recommendations when field corn is knee-high.

How many times have you heard me or other ma” weed scientists say, "Timing is everything, especially when it pertains to the application of postemergence (POST) herbicides?" More than a few I hope with good reason! 

Simply put, smaller weeds have always been and always will be easier to control.  Part of the reason why is that it is much easier to get maximum herbicide coverage on a smaller weed when compared to a larger weed. 

In my 28 years of Extension work, it has been my observation that many field corn growers tend to delay the timing of their POST herbicides. Usually this decision is influenced by many factors including inclement weather or equipment breakdowns.  However, it can also be by choice. 

I often struggle with herbicide recommendations when field corn plants are knee-high, which is an all too frequent request.  Weed control results with applications made this late are usually less than desirable due to a reduction in spray coverage.  With that thought in mind, I recently conducted some very practical and applied field research to prove this point.   

In 2020, I conducted a timing/coverage trial in a Miller County, Ga., corn grower’s field using a commercial sprayer, a JD-4630, 10 GPA, 90’ boom, 20” nozzle spacing, 32 PSI, 11 MPH, 8003-E nozzles, to evaluate the effects of delayed timings on spray coverage.  In this trial, I placed 2” X 2” and 6” X 6” wood blocks in the field with water-sensitive spray cards at different times.  I then asked the grower to apply his standard herbicide program.  After application, the Miller Co.  Extension Agent Cody Powell and I analyzed the spray cards promptly in the field with an app designed for use on a smartphone. The University of Western Australia and the Department of Agriculture and Food - Western Australia developed the SnapCard app.   

Figure 2 illustrates the differences in spray coverage that occurred between two application timings, either 20 DAP/V4 stage or 30 DAP/V6 stage.  It is obvious to me (and I hope to you) that spray coverage was significantly reduced when applications were delayed until 30 DAP/V6 stage of growth when compared to the 20 DAP/V4 stage of growth.  After those same water-sensitive spray cards were analyzed with the SnapCard smartphone app, it was determined that spray coverage was reduced by around 40% when herbicide applications were delayed from the V4 stage until the V6 stage, just 10 days. 

I think that everyone can agree that less coverage is not desirable with POST herbicides.  The reductions in spray coverage associated with delayed timings could easily explain previous weed whoopsies that you might have observed.   

As you begin preparing for the upcoming field corn growing season, remember that any delays in the timing of POST herbicides will result in reduced spray coverage that could ultimately influence performance.  POST applications around the V3-V4 stage of corn growth and before the tallest weed in the field exceeds 3” in height give you the best chance to get optimum control.  Make it a priority to be there on time. 

As always, good weed hunting!

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