Maximize glyphosate activity with a few simple tips
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Postemergence herbicide applications for weed control in soybean are quickly approaching. There are several things that should be considered to maximize weed control with glyphosate from these applications in Roundup Ready soybean. Following these guidelines can help reduce the risk of glyphosate failures, as well as reduce the chances of lower soybean yields due to weed competition.
There are over 30 different glyphosate
products that growers in Michigan can choose from for weed control in
glyphosate-resistant soybean. With all of these choices, it is important
to keep in mind that not all glyphosate formulations are created equal.
Knowing your glyphosate product is essential to achieving optimum weed
control. There are two main differences in the many available glyphosate
products. Glyphosate products can differ in the concentration of
glyphosate acid in the formulation (glyphosate acid is what kills the
weed). This concentration is expressed as pounds acid equivalent per
gallon (lb a.e./gal). Different glyphosate concentrations will change
the amount of product used for the various formulations. Table 10 in the MSU Weed Control Guide for Field Crops (E-434) is a compiled list of several glyphosate products and the product use rates for equivalent amounts of glyphosate acid per acre.
Another difference in glyphosate products is whether a surfactant needs to be added to the spray solution or if the formulated glyphosate product has a built-in adjuvant package. Products like Roundup WeatherMax, Touchdown Total, and several others have built-in adjuvant systems. Even though all of these products have a built-in adjuvant system there can and many times are differences in the type of surfactant formulated in the product. These differences may equate to differences in weed control under extreme conditions. However, under most conditions there are no differences between these products especially when they are used at the correct rates, at the appropriate application timings. For products where the addition of a surfactant is recommended add a high quality non-ionic surfactant at 0.25 to 1.0 percent v/v. Table 10 also includes information on whether the addition of a surfactant is recommended for a particular glyphosate product.
Addition of ammonium sulfate (AMS)
Ammonium sulfate (AMS) should always be added to all glyphosate products. We recommend adding dry spray grade AMS at 17 lbs/100 gal or the equivalent of 17 lbs/100 gal of liquid AMS products. The addition of AMS minimizes the negative effect of hard water on glyphosate activity and is important for velvetleaf control, regardless of water quality.
Applying the appropriate glyphosate rate
in glyphosate-resistant soybean is important for consistent weed
control. Proper glyphosate rates should be based on weed type, weed size
and spray volume. In most cases the appropriate rate to use for weed
control in glyphosate-resistant soybean is 0.75 lbs a.e./A of glyphosate.
This rate will effectively control several annual weed species between
two and eight inches tall. However, if weeds become larger or if harder
to control species such as common lambsquarters or giant ragweed are
present, increase the glyphosate rate to 1.1 lbs a.e./A or 1.5 lbs a.e./A
to adequately control these weed species. In addition, by matching the
appropriate glyphosate rate to the correct weed size you reduce your
chances of weed control failures under extreme conditions.
Glyphosate application timing is everything. It is important to make timely glyphosate applications to minimize the chances of soybean yield loss due to early-season weed competition and to maximize weed control. The optimum time for glyphosate applications is when weeds are four inches tall in narrow-row (7.5 and 15 inches) soybean and six inches tall in wide-row (30 inches) soybean. Controlling weeds at these times reduces the chances for soybean yield loss, as well as reduces the risk of weed control failures of larger weed that may be under stressful conditions (drought, stem-boring insects, coverage issue, etc.).
Windy conditions in the narrow application
windows that we see this time of year make the use of drift reducing
nozzles or drift reducing agents almost a necessity to reduce or prevent
off-site particle drift. When using these technologies, it is important
to know their limitations. Using these technologies without the proper
spray pressure and spray volumes can lead to reduced weed control with
glyphosate. To maintain effectiveness with these technologies make sure
to follow manufacturer’s recommendations on pressure and volume, apply
the appropriate glyphosate rate for the target species, and always check for a uniform spray pattern.
Combinations of some of these technologies can reduce spray coverage
that may result in reduced weed control. Following these guidelines,
particularly checking the spray pattern, can help determine if the weeds
are receiving an adequate dose of the herbicide for weed control.
Following these guidelines will help maximize glyphosate performance and reduce the risk of soybean yield loss due to weeds in the 2008 season.