Weed Management Made Easier with Herbicide-resistant Crops

James L. Griffin

In traveling through the major crop-producing areas of Louisiana in the late 1980s and early 1990s, it was common to see fields infested with many grass and broadleaf weeds. In some cases, it was difficult even to distinguish the crop. At that time, particularly in soybeans, the herbicides were so narrow in their weed control spectrum that two or three herbicides applied together might be needed to control all weeds in fields. But because commodity prices were low and herbicide costs were high, it was just not economical to control all weeds present in fields. Consequently, weeds thrived and caused significant yield loss.

Traveling through those same areas of the state in 2003 presents a starkly different picture. Fields are cleaner than they have ever been. This positive change can be attributed directly to advancements in weed management technologies through the development of herbicide-resistant crops. These new crops, even though developed for the most part by private companies, were evaluated extensively by LSU AgCenter weed scientists. These weed control technologies include the following.

Roundup Ready Soybeans, Corn, Cotton

Roundup and other glyphosate-containing products are nonselective, foliar-applied herbicides that control many annual and perennial weeds. Roundup was initially evaluated in the South for preplant weed control in reduced tillage systems, but the role expanded with the development of herbicide-resistant crops. The glyphosate-resistance (Roundup Ready) gene from Monsanto was introduced in the United States in soybeans in 1996 and in cotton in 1997. In 2003, about 90 percent of the soybean, 40 percent of the corn and 85 percent of the cotton acreage in Louisiana was planted with Roundup Ready varieties.

Liberty Link Corn

Glufosinate (trade name Liberty) is a nonselective, foliar-applied herbicide not only effective on grasses but also on many hard-to-control broadleaf weeds. In general, Liberty is less active on grasses when compared with glyphosate but is more effective than glyphosate on some broadleaf weeds. As with glypho-sate, Liberty was initially evaluated as a preplant herbicide in reduced tillage systems, and both herbicides have little soil residual activity. Glufosinate-resistant (Liberty Link) corn was marketed in the South in 2003, but in Louisiana less than 5 percent of the corn acreage was planted with Liberty Link varieties. Rice lines have been developed in Louisiana with resistance to glufosinate, and research is underway to evaluate weed control programs. However, the technology is still in the early stage of development.

BXN Cotton

Weed control technology using the BXN system in cotton was first evalu-ated in 1990. Cotton varieties with the BXN trait, unlike susceptible weeds, are able to tolerate Buctril herbicide through production of nitrilase enzyme, which metabolizes bromoxynil, the active ingredient in Buctril, to an inactive form. This technology offers cotton growers the flexibility to control many broadleaf weeds as a foliar application without fear of crop injury. In 2003, less than 5 percent of the cotton acreage in Louisiana was planted with BXN varieties.

Clearfield Corn, Rice

In Clearfield corn, Lightning herbicide (a premix of imazethapyr, the active ingredient in NewPath and Pursuit, and imazapyr, the active ingredient in Arsenal) can be used to manage weeds. Lightning’s weed spectrum is not as broad as Roundup or Liberty, but it does provide some soil residual weed control. In 2003, less than 5 percent of the corn acreage in Louisiana was planted with Clearfield varieties.

By exposing rice seed to chemical mutating agents and treating seedlings with herbicide, researchers at the Rice Research Station in Crowley, La., have developed rice varieties tolerant to NewPath herbicide, which contains the active ingredient imazethapyr. See the articles about Clearfield rice on pages 23 through 26. The Clearfield tech-nology from BASF has provided growers with the ability to selectively control red rice in domestic rice—a breakthrough in the management of this major pest problem. In 2003, around 10 percent of the rice acreage in Louisiana was planted with Clearfield varieties. Unlike Roundup Ready, Liberty Link and BXN technologies, the Clearfield technology is not considered a GMO (genetically modified organism).

Herbicide Resistance: Advantages, Concerns

Development of herbicide-resistant crops has offered weed management options with economical advantages to Louisiana growers. Availability of these new technologies has enabled soybean, corn, cotton and rice growers to manage problem weeds that have limited production more effectively. Weeds may no longer be the most limiting factor to crop yield potential in Louisiana.

These technologies have resulted in a shift toward reduced tillage systems. Of the soybean, corn, cotton and rice acreage in the state, around 50 percent is managed using some form of reduced tillage or minimum tillage. Research in the state has clearly shown a reduction in soil and herbicide loss from fields where reduced tillage programs have been implemented. Additionally, the availability of herbicide-resistant technologies has resulted in a shift toward dependence on use of post-emergence foliar-applied herbicides that allow growers to customize weed control programs on a field-by-field basis. This integrated approach to weed management has reduced the cost per acre and the potential for carryover problems associated with use of some soil-applied herbicides.

Along with the benefits of the technologies, however, come potential problems. Availability of Roundup Ready technology in many crops grown in rotation with one another and the continued use of glyphosate can increase the potential for development of weeds resistant to the herbicide. In Tennessee and Arkansas, a glyphosate-resistant marestail has been identified, and, in other states, ryegrass resistant to glyphosate has been identified. In time, weeds will develop resistance to a herb-icide when used year after year. In Louisiana there are no documented cases of weeds resistant to glyphosate so far, but researchers have observed a shift in weed populations toward those less sensitive to glyphosate where the herbicide has been used for several years.

The most effective means to help avoid development of herbicide resistant-weeds is to alternate herbicide use. For example, a grower might use a Roundup Ready variety for a couple of years and then change to a Liberty Link or Clearfield variety. This can be difficult to do when considering all factors, but using herbicides with different modes of action in a rotation program will help prevent or slow the development of herbicide resistance.

James L. Griffin, Professor, Department of Agronomy and Environmental Management

10/26/2004 1:43:52 AM
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