If managed correctly, soybeans can be a fairly easy crop to grow. Weeds, on the other hand, usually don’t need any coaxing at all to take off like a rocket and lead long healthy lives at the expense of your beans unless you know how to control the pests.

None of the Southern soybean-producing states is immune to them and Georgia is no exception. Recent surveys of county Extension agents in Georgia indicated that the top 10 most troublesome weeds in soybeans are sicklepod, morningglory, pigweed, nutsedge, Texas panicum, cocklebur, Florida beggarweed, bristly starbur, bermudagrass and coffee senna.

According to Dr. Eric Prostko, Extension weed specialist with the University of Georgia, it has been estimated that 86 percent of the soybeans planted in the United States during 2004 were herbicide resistant varieties. Sicklepod, the number one weed problem in Georgia soybeans, is very susceptible to glyphosate and can be managed using the Roundup Ready production system. However, two applications of glyphosate may be required to provide season-long control.

In conventional soybeans, Prostko says, the best method to control sicklepod is to use a systems approach that includes a preplant incorporated or preemergence application of metribuzin (Canopy SP or Sencor) followed by a postemergence application of Classic. Caution is advised when using metribuzin because several restrictions on soil type, organic matter, pH and variety exist. Refer to the specific herbicide label for these restrictions.

Python (flumetsulam) can be substituted for metribuzin in those situations where metribuzin use would be prohibited or not preferred.

Tropical spiderwort on the rise
UGA Extension weed control recommendations note that tropical spiderwort, also known as hairy wandering jew or Bengal dayflower, has become an increasing problem in many of Georgia’s soybean fields. However, only a few fields trials have been conducted in Georgia regarding the control of tropical spiderwort in beans, so much of the information is based upon results from control studies in other crops.

For example, planting in narrower rows and increasing soybean plant populations will help improve the control of tropical spiderwort through competition and shading.

The most effective herbicide control strategies for tropical spiderwort, according to UGA, include using a combination of both preemergence and postemergence herbicides. Preemergence herbicides with residual activity on tropical spiderwort include Axiom, Dual Magnum, Canopy SP, Canopy XL and Sencor.

Postemergence herbicides that have fair to good activity on tropical spiderwort include Basagran, Classic and Pursuit.

Gramoxone Max or Aim, applied post-directed, can provide effective burndown control of tropical spiderwort. When using Gramoxone Max post-directed, the soybeans must be at least 8 inches in height and the herbicide should not be sprayed higher than 3 inches on the soybean plant.

In Roundup Ready soybean systems, glyphosate can provide fair to good control of tropical spiderwort if it is applied to plants that are three inches tall or less and under ideal growing conditions. However, Pursuit or Classic can be tankmixed with glyphosate to improve the control of this weed.

Combination strategies work
UGA Extension weed control recommendations point out that the most effective weed management programs in soybeans use a combination of cultural, mechanical and chemical control strategies. Cultural practices, such as planting date, planting rate and row spacing improve weed control by enhancing the soybean’s competitive ability.

Cultivation, a non-chemical approach, controls weeds between rows. And a multitude of herbicides are labeled for use in soybeans and can be applied at many different application timings.

Because of the extensive number of herbicides labeled for use in soybeans, just about any weed problem that arises can be controlled. It is just a matter of how much money can be economically justified for weed control in soybeans.

To view the complete UGA-CAES weed control recommendations and an update on recommended soybean herbicides, visit www.cropsoil.uga.edu and click on Extension.

Information for this article is based on the UGA-CAES 2005 Soybean Production Guide.