Daniel O. Stephenson IV, James L. Griffin and Donnie K. Miller

Herbicide resistance is defined by the Weed Science Society of America as the "inherited ability of a weed biotype to survive and reproduce despite exposure to a dose of herbicide that was previously effective on the population."
 
Herbicide resistance within weed populations has become a tremendous problem in both crop and livestock production systems. Worldwide, 357 herbicide-resistant biotypes have been identified. In the United States, 62 weedy plant species have been documented as herbicide-resistant. In Louisiana in the 1990s, weed resistance was documented in barnyardgrass, common cocklebur, itchgrass and johnsongrass. In 2010, Palmer amaranth and johnsongrass populations were confirmed as resistant to the herbicide glyphosate. Glyphosate is commonly known as Roundup but is sold under various trade names.

The first glyphosate-resistant Palmer amaranth population in Louisiana was found in Concordia Parish. To control 50 percent of the Concordia Parish population required a rate of glyphosate 3.7 times the normal use rate of 32 fluid ounces per acre. In 2011, Palmer amaranth populations in Madison and Tensas parishes were confirmed as glyphosate-resistant. Glyphosate-resistant johnsongrass has been documented in Point Coupee and Rapides parishes. To control 50 percent of the Point Coupee johnsongrass population required a glyphosate rate 10.2 times normal, and the Rapides population required a rate 7.2 times normal.

Development of glyphosate-resistant weeds is due primarily to the use of glyphosate as the sole weed management tool. Applying multiple applications of glyphosate during the crop cycle and over successive years essentially selects for individual plants within a population that are inherently resistant to the herbicide. Over time the weed population shifts to the resistant population, resulting in weed control failures.

In Arkansas, Georgia, Mississippi, Tennessee and other states, crop producers are spending between $75 and $150 per acre for removal (herbicides and hand-hoeing) of glyphosate-resistant Palmer amaranth. Prior to glyphosate-resistant weeds, those producers typically spent $20 to $50 per acre for weed control. Palmer amaranth has become the primary target when budgeting for weed control in a crop. Fortunately, Louisiana crop producers have not encountered the problems with weed resistance that other states are facing. However, unless proper weed management programs are implemented to help mitigate herbicide resistance, LSU AgCenter scientists fear that Louisiana could face a similar fate.

To help mitigate herbicide resistance problems, soil-residual herbicides should be included in a weed-management program. It is critical that fields be weed-free at planting and for at least six weeks after soybean emergence. Not only does this prevent possible germination and emergence of potentially herbicide-resistant weeds, it will lessen physical competition between the growing weeds and the soybean. Various soil-applied herbicides can be applied pre-plant, preemergence and in-crop. Table 1 provides examples of soil-residual herbicides labeled in Louisiana for application prior to or at soybean planting. All of the herbicides when properly activated via rainfall or irrigation have the potential to provide excellent control of grass and broadleaf weeds.

Following the application of a preplant or at-planting, soil-residual herbicide, it would also be important in herbicide-resistance management to co-apply a herbicide with glyphosate that provides foliar or residual control of weeds. Table 2 provides examples of foliar and soil-residual herbicides labeled in Louisiana for use after soybeans have emerged. Even in the presence of a glyphosate-resistant weed population, glyphosate is still effective on many weeds that impair soybean production. These include barnyardgrass, broadleaf signalgrass, browntop millet, hophornbeam copperleaf, morningglory, red rice, sprangletop, sicklepod and velvetleaf. When applying a herbicide prior to glyphosate or when another herbicide is applied with glyphosate, the weed population in the field is exposed to a herbicide mode of action (the specific mechanism and growth process affected by the herbicide) that is different from glyphosate and that is critical to management of herbicide-resistant weeds.

Another option for managing herbicide-resistant weeds is Liberty Link soybeans. Soybean varieties have been developed that are resistant to postemergence applications of the herbicide Ignite, which has a different mode of action than glyphosate. Producers must not apply glyphosate to Liberty Link soybean, or Ignite to Roundup Ready soybeans, or soybean death will occur. All herbicides listed in Tables 1 and 2, except for glyphosate, can be applied alone or in combination with Ignite for weed management in Liberty Link soybeans. Research by LSU AgCenter weed scientists has shown that soil-applied herbicides should be used in Liberty Link systems for maximum weed control. In addition, Ignite should be applied to 2- to 4-inch weeds for maximum utility, indicating the importance of timing in weed management.

When planning a soybean weed management program either in the absence or presence of herbicide-resistant weeds, it is imperative that multiple herbicides with different modes of action be used. Research by LSU AgCenter weed scientists has shown that season-long weed control can be obtained when a soil-residual herbicide is applied at planting and followed by a timely postemergence application of glyphosate along with other herbicides.

Effective weed management programs will also help prevent weeds from producing seed, which could result in a shift over time to weeds more difficult to control. At present, herbicide-resistant weed problems in Louisiana are in the infancy stage compared with neighboring states. With proper planning of weed control programs and herbicide selection, herbicide resistance can be managed, and Louisiana may be able to avoid the serious problems encountered in other states.

Daniel O. Stephenson IV, Assistant Professor, Dean Lee Research and Extension Center, Alexandria, La.; James L. Griffin, Lee F. Mason LSU Alumni Association Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; Donnie K. Miller, John Baker Professor for Excellence in Weed Science, Northeast Research Station, St. Joseph, La.

(This articles was published in the spring 2011 of the Louisiana Agriculture Magazine.)