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Regardless of whether you intend to plant a Roundup Ready, Liberty Link, or conventional corn variety this year, it is important to think about the type of weed management program you will utilize. Since we have evaluated a lot of different herbicide programs over the years, I decided to “mine” our database of research results (www.weedscience.missouri.edu/weedtrials/index.cfm) in order to understand which program approach is 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 for weed management in corn.
I was able to make a fair comparison of three different program approaches that have been evaluated in 61 trials in Missouri over the past 11 years. What I found was that in 41 of 61, or 67% of the trials, highest corn yields were obtained with a two-pass program that consisted of a pre-emergence herbicide followed by a post-emergence herbicide. A one-pass post-emergence program that also contained a residual herbicide provided highest corn yields in 28% of the trials, whereas in 5% of the trials a one-pass pre-emergence herbicide program provided highest corn yields.
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, but year in and year out, the two-pass herbicide program is most likely to provide the highest levels of weed control and corn yield. 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 one-pass pre-emergence 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. However, with the typical spectrum of weeds that we have in Missouri (i.e., cocklebur, waterhemp, ragweeds, sunflower, foxtails, fall panicum, etc.), our data show that a post-emergence application is usually required for weeds that have escaped or germinated since the initial pre-emergence herbicide application.
REVISED: October 1, 2015