Not long ago, I wrote an article in this newsletter about the increasing cost of glyphosate and how this situation makes preemergence herbicide applications in soybean more feasible. Even if this weren't the case, I think there are still some reasons to consider the use of preemergence residual herbicides.
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 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. 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 increasing number of acres that have glyphosate-resistant waterhemp. Results from surveys we conducted last year indicate that glyphosate-resistant waterhemp now occurs on approximately 200,000 acres of soybean in this state. 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.
If you have decided to apply a preemergence residual herbicide in soybean, I think there are two primary factors you should consider in order to choose which product will work best for you. The first is obviously price. The products listed in Table 1 range in price from about $8 or $9 to about $15 or $16 per acre, so the product you choose can have a big impact on net income. The second consideration is whether the preemergence residual herbicide you choose will match the spectrum of weeds that you have in your field. As illustrated in the results from an experiment we conducted last year (Table 1), many of the preemergence residual herbicides available provide good suppression of broadleaf weeds, but little control of grass weeds. If you have heavy grass weed pressure, you may need a product that provides suppression of both grass and broadleaf weeds prior to the planned postemergence glyphosate application.
It is important to note that the results in Table 1 are only from one year of research and that we will be conducting similar research over the next few seasons to understand this issue better. However, another thing that I think the results in Table 1 illustrate is that few, if any, preemergence residual herbicides provide season-long control of the common weeds we encounter in soybean production in Missouri. These programs are designed to buy you time and eliminate the need for the first pass of glyphosate from the traditional 2-pass glyphosate program. Our research indicates that following these preemergence residual herbicide treatments with a postemergence glyphosate treatment when needed will usually provide excellent season-long weed control and optimize soybean yields.
|Table 1. Influence of preemergence herbicide treatments on grass and broadleaf weed density at the time of the postemergence glyphosate treatment (Columbia, MO 2007).|
|Treatments b||Rate||Weed Density at
|product / A||-------------------- #/m2 --------------------|
|Prowl H2O*||2 pts||22||20|
|Valor XLT*||3 ozs||58||18|
|Authority First**||3.22 ozs||52||10|
|Gangster FR+V**||0.3 + 1.5 ozs||46||8|
|ª Grass weeds included foxtails and large crabgrass. Broadleaf weeds included waterhemp, morningglories, cocklebur, and prickly sida.
b Single asterisk (*) indicates treatments that required a glyphosate treatment on June 26th. Double asterisks (**) indicates treatments that required a glyphosate treatment on July 3rd.
REVISED: April 5, 2012