Corn hybrids that sport glyphosate-resistance have become the norm in Wisconsin, which creates different opportunities for weed management. Weeds in glyphosate-resistant hybrids can be sprayed with glyphosate in one or two passes postemergence, with a program of glyphosate premixed or tank mixed with another herbicide, or with a preemergence or residual herbicide, which is followed by glyphosate. Each program can work extremely well to control weeds in the right situation.

 

Remember, the goal of weed management is to maximize corn yield potential. Killing weeds is just a necessary step to achieve that goal.  Of course, the critical part in reaching the goal of yield protection is killing the weeds on time, which leads to two interesting observations about weed management in Wisconsin.

 

First, we have learned that many corn fields in Wisconsin that are sprayed postemergence may not be sprayed on time. Numerous research studies have shown that corn yield is lost when weeds are sprayed after they exceed 4-inches. If killing weeds before 4-inches is the target, are we hitting this target? The answer doesn’t look good. In 30 surveyed corn fields that were sprayed postemergence in 2008, over 75% of the fields had an average weed height of 4 inches or greater when they were sprayed. The average weed height was 6 inches tall, which means some were even worse. This all points to the potential for yield loss. The yield loss predictions, based on the heights of the common weeds in the fields (lambsquarters, grasses, and velvetleaf), suggests an average yield loss of 6.5%. With 150 bu/a and $4 corn, this would equal a $39 per acre profit loss. 

 

Second, Carrie Laboski and I conducted weed competition studies that show weeds can be incredibly competitive with corn for nitrogen. We measured nitrogen uptake by weeds and corn yields when weeds were controlled at a 4-inch height and at a 12-inch height. These two timings were separated by only 8 days, basically an inch of weed growth a day. In the different 2 years of the study, the 12-inch tall weeds had pulled 25 and 67 lb of nitrogen per acre from the soil by the time they were controlled.  This nitrogen was no longer available for corn growth. As a consequence, corn yield was cut an average of 12 and 18 bu/a in the 2 years when weeds were killed at 12 inches tall compared to when weeds were controlled at 4 inches. The most interesting part of the study was that we could regain this yield when high rates of nitrogen were applied. However, with the high cost of nitrogen, effective weed control is certainly more cost effective than over applying nitrogen just to feed the weeds. 

 

Both of these studies highlight the need for timely weed management to protect corn yields. There are several options for achieving this goal and defeating weeds in glyphosate-resistant corn. A preemergence or residual herbicide greatly increases the window for postemergence applications of glyphosate. These preemergence herbicides can often be applied at half rates when followed by glyphosate. We have demonstrated this with several different herbicide programs in field experiments. Herbicides that fit this program are SureStart or reduced rates of Lumax, Camix, and the atrazine premixtures (e.g. Bicep Lite, Harness Xtra, Keystone, G-Max Lite, etc.).

 

Glyphosate can also be applied early before weed competition occurs. Corn growers just need to understand the risk of not getting all of their corn fields sprayed on time if spraying total postemergence programs. Early applications increase the chance that later flushes of weeds might need a second application. To avoid retreating with glyphosate or the temptation to spray later (to get by with one application), residual herbicides (mentioned above) can be tank mixed with glyphosate or a glyphosate premix that includes a residual component like Halex can be used. 

 

In either case, weeds can be effectively managed in glyphosate-resistant corn. We just need a plan a program that controls weeds from the start or gets it done on time. Corn is too valuable to lose 5 or 10 bu/a to weed competition. 

 

 

 

Larger weeds in corn (top photo) were killed with glyphosate and provided excellent weed control and an excellent “looking” corn crop (same plot in right photo). However, corn yield was lost. Is this satisfactory?