For 2007 your decision is more corn. That is a given. But for 2007 is your decision more Roundup Ready corn? Chances are, the percent of acres immune to glyphosate will edge upward. Some farmers will go full bore with it because of the need to increase efficiency and produce more corn acres in the same amount of time as last year. Other farmers may ease back, concerned about the increasing number of weeds becoming immune to glyphosate, and they will try an herbicide program designed to avoid future problems. Where do you line up, and if undecided, what should you prepare for?

Instead of sampling the Eastern and Western ends of the Cornbelt, let’s travel north and south along the Avenue of the Saints (Louis & Paul). (Alright, from Missouri to Minnesota, for those of you who don’t get out much.) University of Missouri weed specialist Kevin Bradley reported in a winter newsletter, “Missouri is the first and only state where three glyphosate-resistant weeds— tall water hemp, common ragweed, and horseweed, have been identified. Bradley said Missouri holds this dubious honor because of the large number of acres planted to continuous soybean, 99% of which use a glyphosate system for weed control.” With some of those acres being switched to corn this year, there will be an instant problem with controlling weeds in Roundup Ready corn.

In the March 16 Missouri Integrated Pest Management newsletter, Bradley suggests 2006 corn in Missouri was only 21% Roundup Ready, much less than neighboring states where as much as half of the crop in glyphosate resistant, “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.” But for 2007, Bradley says glyphosate practices cannot continue as they have, if you are embarking on continuous corn. “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.” He is pushing a pre-emergent herbicide program followed by a post emergent program, which can be glyphosate.

Bradley’s recommendation comes from 29 different tests last year with a variety of herbicides. “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.” Bradley says Roundup Ready beans followed by Roundup Ready corn is asking for problems with weeds becoming glyphosate resistant. “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.”

Due north, weed specialist Jeffrey Gunsolus at the University of Minnesota has similar concerns, expressed in his research, Ten Years of Roundup Ready - where are we now? He says in the 10 years before 2005, the amount of corn acres with a pre-emergent herbicide application declined from 73% to 49%, while glyphosate acreage climbed from 0% to 49% of corn acres. And Gonsolus adds, “One recent trend in soil residual herbicide use in corn is a decrease in acetochlor use rate by 26% over the 2003 to 2005 time period. In both corn and soybean, as glyphosate use increases the use of ALS herbicides dramatically decreases. Glufosinate, though often linked to a Bt-trait has had limited market penetration in the corn market.” Gunsolus joins Bradley and other Cornbelt weed specialists with pleas for diversity in chemical use for corn weed control, “Also, can some chemical diversity (i.e. different modes of action) be retained on the corn acres? Currently (2005 data), 41% corn ground is treated with atrazine (0.5 lbs a.i./A), 24% is treated with a plant growth regulator and 17% is treated with mesotrione (Callisto).

In addition to the diversity of the chemical program, Gunsolus also recommends a pre-emergent and post-emergent system, “Our research has shown that using one-half of the labeled use rate of acetocholor followed by a V2-V5 application of glyphosate is as effective as a two-pass postemergence treatment of glyphosate with less risk of yield loss due to a late initial application of glyphosate.” Just like Missouri, a fair piece to the south, the most difficult Minnesota weeds to control are waterhemp, lambsquarters, and ragweed.

Summary:
Regardless of where you farm in the Cornbelt, weeds are becoming more and more resistant to glyphosate, but that has been our chemical of choice, and in 2007 Roundup Ready corn will be planted on 2006 Roundup Ready soybean stubble. Weed specialists are increasingly recommending a pre-emergent herbicide program that will help interrupt the life cycle of weeds that are becoming glyphosate resistant.