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Even as recent as a year or two ago even the mention of a preemergence herbicide application in Roundup Ready soybeans at a grower meeting or field day would have brought a few laughs from the audience. This past season, however, I noticed a little more acceptance of this practice and from what I can tell, applying residual herbicides in soybean, even Roundup Ready soybeans, appears to be gaining a little ground in Missouri. Don't misunderstand me, the efficacy and economics of a properly timed two-pass glyphosate program in Roundup Ready soybeans is hard to beat. However, there are still some reasons you may want to consider the use of a residual herbicide in your soybean weed management program.

First, residual herbicides may be a good option for you simply from an "insurance" or yield protection standpoint. Often, a properly timed early postemergence glyphosate application to weeds that are 4- to 6-inches tall somehow mysteriously turns into an application where the weeds have reached 10- or 12-inches tall and the soybeans are barely noticeable. Without fail, I see this in some Missouri fields each year. Last year it seemed to be particularly bad. The reasons are many; it may have been too wet and you couldn't get in the field to spray, there is just too much to do and too many acres to cover, etc., etc. Regardless of the reason, when this type of situation occurs some yield has already been lost. To see the effects of increasing weed sizes and weed competition on soybean yield loss for yourself, go to http://weedsoft.unl.edu/weedsoftApps.htm and use the WeedSOFT yield loss calculator. This is a tool developed by a number of university weed scientists that enables you to estimate the season-long yield loss, as well as the yield loss that has already occurred, for soybeans that are at a particular stage of growth and infested with specific weed species. It will also estimate the additional yield loss that may occur if you delay treatment.

Another reason you might want to consider the use of a residual herbicide in Roundup Ready soybeans is due to the increasing number of glyphosate-resistant weeds that are being identified in Missouri and throughout the U.S. Almost exclusively, these weeds have been discovered in continuous Roundup Ready cropping systems where glyphosate has been used as the sole active ingredient for weed control. In Missouri, perhaps the most concerning development is the confirmation of glyphosate-resistant common waterhemp that occurred in northwest Missouri in 2006. Since this initial confirmation, we have conducted greenhouse research on waterhemp populations sent in last year from other locations around the state and have found glyphosate-resistant waterhemp in at least 5 other counties.

One of the best ways to prevent these kinds of situations from developing is to rotate to herbicides other than glyphosate that act at an alternate site of action, such as with preemergence residual herbicides. In our research, some of the more effective residual herbicides used in soybean for common waterhemp include S-metolachlor (Dual II Magnum, Cinch, etc.) and premixes containing S-metolachlor (Boundary, Prefix CP), sulfentrazone (Spartan) or premixes containing sulfentrazone (Sonic, Authority First), flumioxazin (Valor) or flumioxazin premixes (Gangster, Valor XLT), and alachlor (Intrro).

Kevin Bradley
882-4039