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If you have heard anything over the past several months, you've heard that we are going to be planting more corn this year in Missouri and throughout the U. S. And not only more corn, but according to all the predictions I've heard, more Roundup Ready^© corn. According to USDA statistics, last year we planted 21 percent of our corn acreage in Missouri with herbicide tolerant varieties. This does not mean that all of this 21 percent was Roundup Ready^©, but certainly the vast majority of acres were. This figure, 21 percent, is for some reason much lower in our state compared to any state that borders us. In some of our neighboring states, there was twice this level of adoption of herbicide tolerant (and presumable Roundup Ready^©) corn last year. To be honest, I'm not sure why our numbers have stayed this low. Regardless, just about every seed company that I have listened to over the winter meeting season has predicted that our Roundup Ready^© corn acres are going to increase as they have each year over the past several years. Five years from now, I've heard some predictions that we will be planting 75 percent or more of the corn acreage in the United States with Roundup Ready^© or other glyphosate-tolerant varieties. I don't know if these predictions will come true or not, but it is clear that each year we have been planting more Roundup Ready^© corn in Missouri. So, the following are some general weed management considerations for you to think about as you head into the planting season and consider what type of corn variety and herbicide program you are going to choose.

First, if you are moving towards more Roundup Ready^© corn, you have to consider what weed management program will fit you the best in this system, just as you would if you were planting a conventional corn variety. I don't think it will be wise for growers to conduct weed management practices in Roundup Ready^© corn as it has been and is currently being done in Roundup Ready^© soybeans. Why? Because the research clearly indicates that corn is much more susceptible to early season yield loss than soybeans, and that we can't stand to wait to remove the weeds until they are 6-inches or more tall. By this time, significant corn yield losses will already have occurred. There's no question that glyphosate will be a good postemergence herbicide option for use in Roundup Ready^© corn, but our research has shown that the "safest" programs are two-pass programs that consist of a preemergence followed by a postemergence herbicide.

Before I came to Missouri, Dr. Bill Johnson had looked over many years of research and over 2,000 treatment observations that had been conducted here in this state and found that in 12 out of 13 years, a two-pass program consisting of a preemergence herbicide followed by a postemergence herbicide provided the best overall weed control in July. In only one out of 13 years did a one-pass total postemergence program provide better weed control than the other programs, and a one-pass preemergence program never provided better weed control than the other programs evaluated.

This kind of historical analysis paints a very clear picture for me. In fact, I found this data so interesting that I decided to conduct a similar exercise and look at what the response to corn yield would be under these different kinds of programs. Understand that these are not comparisons of specific herbicide treatments and trade names, but rather are comparisons of different program approaches. I was able to make a fair comparison of three different program approaches in 29 trials that have been conducted in Missouri over the past five years. What I found was that in 19 out of the 29 trials where these comparisons could be made (65 percent of the time), highest corn yields were obtained with a two-pass program consisting of a preemergence herbicide followed by a postemergence herbicide. In 8 out of the 29 trials (28 percent of the time), a one-pass postemergence program that also contained a residual herbicide provided highest corn yields, whereas in 2 out of the 29 trials (7 percent of the time) a one-pass preemergence 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 that you have, 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 highest corn yields. In other words, the two-pass program is "safer" than the other programs. In some ways, it becomes an issue of how much risk you are willing to take and these results indicate that the least risk will occur with a two-pass program.

Another important thing to consider at this time of the year is what your crop rotation and herbicide programs are going to be not only this year but over the next several years. From the perspective of preventing and/or managing resistant weeds, we have got to do more of this than has been done in the past. More than likely, if you are planting soybeans, you are planting Roundup Ready^© soybeans and treating with glyphosate at least once per season. And why wouldn't you? It is a very effective, economical program. If we start planting more Roundup Ready^© corn, then many fields will be in a Roundup Ready^© corn- Roundup Ready^© soybean rotation where glyphosate will be utilized as the primary, if not only, postemergence herbicide. This will greatly increase the likelihood of selecting for a glyphosateresistant weed, even in a corn-soybean rotation, because we are still spraying the same herbicide (glyphosate) in the same place over time. The best way to decrease the likelihood of resistant weed development under these scenarios is to break up the cycle with a herbicide that acts at an alternate site of action.

One way to accomplish this is to stay with conventional herbicides in a conventional corn-Roundup Ready^© soybean rotation. To my knowledge, no glyphosate-resistant weeds have been selected for in this kind of crop rotation and herbicide system. Another way to reduce the likelihood of selecting for glyphosate-resistant weeds is to apply preemergence herbicides in Roundup Ready^© corn. As discussed previously, I consider this almost a necessity in order to protect corn yields and hope we will not move away from this practice as we plant more and more Roundup Ready^© corn. Alternative postemergence herbicides is another consideration, but for the most part if a grower has paid the tech fee associated with the Roundup Ready^© technology, then more than likely the grower is going to use glyphosate as the postemergence herbicide of choice. One last way to decrease the likelihood of glyphosate-resistant weed development is to apply herbicides other than glyphosate in Roundup Ready^© soybeans, which will be the focus of another article in the next few weeks.

Kevin Bradley
BradleyKe@missouri.edu