MSU Extension

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 early planted crops are quickly approaching. There are several things that should be considered to maximize weed control with glyphosate from these applications. Following these guidelines can help reduce the risk of glyphosate failures, as well as reduce the chances of lower yields due to weed competition.

Glyphosate formulation

There are over 30 different glyphosate products that growers in Michigan can choose from for weed control in glyphosate-resistant (Roundup Ready) crops. 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 PowerMax, Touchdown Total, and several others have built-in adjuvant systems. Even though all of these products have a built-in adjuvant system, there can be, 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.

Application rate

Applying the appropriate glyphosate rate in glyphosate-resistant crops 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 crops is 0.75 lbs a.e./A of glyphosate. This rate will effectively control several annual weed species between two to 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 1.13 lbs a.e./A to adequately control these weed species. The appropriate rate of individual formulated glyphosate products to match the equivalent rates of pounds of acid equivalent can be found in Table 10 of the MSU Weed Control Guide for Field Crops or on the herbicide label. In addition, by matching the appropriate glyphosate rate to the correct weed size, you reduce your chances of weed control failures under extreme conditions.

Application timing

Glyphosate application timing is everything. It is important to make timely glyphosate applications to minimize the chances of yield loss due to early-season weed competition and to maximize weed control. The optimum time for glyphosate applications in corn is before weeds are four inches tall, and when weeds are four inches tall in narrow-row (7.5 and 15 inches) soybeans and six inches tall in wide-row (30 inches) soybeans. Controlling weeds at these times reduces the chances for 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.).

Application technologies

Windy conditions in the narrow application windows that we see this time of year make the use of drift reducing nozzles and/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 2010 season.