Regardless of whether you intend to plant Roundup Ready©, Liberty Link©, or conventional corn varieties this year, it's always important to plan ahead and think about what type of weed management program you will utilize. Over the past six years, I've had the opportunity to evaluate a lot of different herbicides and herbicide programs as part of our general herbicide evaluation research at the University of Missouri. Recently, I've compiled much of this data in order to better understand which herbicide programs are most likely to provide highest corn yields. Understand that the objective of this "data mining" exercise was not to compare any one specific herbicide treatment to another, but rather to compare the different type of herbicide program approaches one might utilize in a corn production system.
After examining the data, I found that I was able to make a fair comparison of three different program approaches that have been evaluated in 55 trials in Missouri over the past six years. I have to admit that I really wasn't all that surprised by what I found. The results showed that in 67% of the trials examined, highest corn yields were obtained with a two-pass program consisting of a preemergence herbicide application at or near planting followed by an in-crop postemergence herbicide application. A one-pass postemergence program that also contained a residual herbicide provided highest corn yields in 29% of the trials, while a one-pass preemergence herbicide program at or near planting provided the highest corn yields in only 4% of the trials.
Collectively, what all of this indicates to me is that depending on the year, environment, soil type, and weed spectrum, either of these program approaches might work for you, but year-in and year-out, the two-pass preemergence followed by postemergence herbicide program should provide the highest levels of weed control and corn yield in either conventional, Roundup Ready©, or Liberty Link© corn. I want to emphasize that the response to these different programs is likely to vary from one location to another, and will be highly dependent on the weed spectrum that you have in your fields. So, if you have been using a onepass preemergence herbicide program for years and have no complaints, chances are you probably have a pretty low weed density in your fields and/or you do not have very many weed species that germinate later in the season. In these situations, I agree that there's certainly no reason for you to switch herbicide programs at this time. However, with the typical spectrum of weeds that we have in Missouri like cocklebur, waterhemp, common and giant ragweed, sunflower, foxtail, fall panicum, and others, our data show that a postemergence application is usually required for weeds that have escaped or germinated since the initial preemergence herbicide application.
Just to reiterate and so you don't misunderstand my next statement, I certainly do believe that for the typical Missouri corn grower, the two-pass preemergence followed by postemergence herbicide program is the option that offers the least risk and best chance at obtaining optimum weed control and corn yield. However, what these percentages do not reveal is the changes that have occurred in these program approaches in recent years.
Without question, the one-pass postemergence plus residual herbicide program has been "gaining a lot of ground" over the past several years. No doubt this is due to Roundup Ready© corn adoption and the ability to get good control of small emerged weeds with a glyphosate product, and then get residual control of later season weed flushes with a residual herbicide. Some companies have recognized this and prepackaged their own products like Halex GT from Syngenta which contains glyphosate, Dual II Magnum, and Callisto all in one jug. I think we will probably see more of these kinds of product concepts in the future.
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REVISED: February 29, 2012