Problems with Roundup Ready weed control

On July 14, I attended the Integrated Pest Management Field Day at the University of Missouri's Bradford Research and Extension Center located near Columbia. As one might expect, the primary theme was dealing with weeds that are resistant to the Roundup Ready system in a number of crops. The trails that we saw dealt with soybeans, corn, and grain sorghum. Southeast Missouri is the only area with cotton and rice, so there was no discussion about these crops.

Roundup is the brand name of a broad spectrum herbicide that was developed by Monsanto. Its chemical name is glyphosate. Roundup has been around since the 1970's and since then there have been a number of crop species that were genetically engineered so that Roundup could be sprayed over the top of the plant without injuring the plant. This system was called Roundup Ready and was to be the best weed control system because weeds could be controlled with one herbicide which simplified weed control practices. Roundup ready soybeans were released in 1996; cotton was released in 1997, and corn was released in 1998.

There were problems with crop injury in cotton by Roundup after it was introduced because of extreme environmental conditions. In the testing period, it was determined that Roundup could be sprayed on cotton up to a certain stage. Although farmers were spraying Roundup according to the label directions, there was considerable injury one year. It was determined that under the cool conditions, the plants could not metabolize the Roundup. Thus, the protocol was changed to avoid injury under the cool wet conditions. This problem was solved when the Roundup Ready Flex varieties were introduced.

This system was controversial because some people were afraid of genetic engineering. There were some potential problems that weed scientists were concerned about. The first and most important was the potential for weeds developing resistance. Weed resistance is developed when continuous spraying over a long period of time kills off plants that are susceptible to Roundup.

We now have a number of weeds that are resistant to Roundup in the United States and there are other species resistant to Roundup around the world. In the U.S., horseweed or marestail was the first major weed in cotton that developed resistance. Other weeds that are now resistant to Roundup include palmer Amaranth or pigweed, tall water hemp, common ragweed, giant ragweed, Italian Ryegrass, and Johnsongrass. Although marestail has been a problem in Southeast Missouri, the most serious problem that we face is Palmer Amaranth. This member of the pigweed family is very troublesome.

The reason that this weed will be our biggest problem is that it is resistant to Roundup and Staple, seed production of up to 450,000 seed per female, and the ability of the seed being spread by equipment and the pollen can be spread by the wind. Although a person?s field may not have any plants, it can be overtaken in a very short period of time.

However, this plant has three major weaknesses. These are shallow emergence depth, short seed life in the soil and a significant light requirement needed for germination. In conservation tillage, heavy residue crops can be used to essentially block sunlight required for Palmer germination and can greatly improve control. It is critical to note that in strip-tillage production, Palmer amaranth will emerge in the strip if herbicides are not activated in a timely manner by irrigation or rainfall. In conventional tillage, Palmer amaranth control can be improved by deep turning or using a yellow herbicide such as Treflan or Prowl preplant incorporated. The adoption of timely Ignite-based Liberty-Linked programs will likely improve control regardless of whether cotton is produced in conventional or conservation tillage systems.

After Roundup Ready crops became established, I began to see volunteer plants in the field the next season. After harvest, some seed were left in the field during the harvest operation. So the next year, I would notice cotton in soybean fields or corn or soybean fields. Spot spraying with Roundup would not control the ?weed? because it was resistant to Roundup. This has been a problem in many fields. There are chemicals available for spot spraying. Corn is killed by a grass killer which will not hurt the soybeans or cotton.

At the weed control field day, we learned that volunteer corn in corn can be a problem. It can cause problems with too many plants per acre and the potential for lodging. It can also be a problem in insect control since the field is not uniform. While control is complicated, there are solutions. If you planted Roundup Ready corn last year, you can use the Liberty link system and Ignite to control the volunteers. If Clearfield was used last year, you can use Roundup to control the volunteer corn. If the problem is really bad, you can go back to soybeans the following year and apply a grass herbicide.

There are weed control solutions for the Roundup Ready resistant weeds in conventional weed control programs and Roundup Ready systems. It should be noted that the cost of using other herbicides will be much more expensive.

One last bit of advice. If you have Roundup Resistant weeds, don?t let them go to seed. We have more fields that are being hoed this year due to the resistant weeds. It will take an effort but the reward will be worth it. It has been estimated that within 5-8 years, we will have cotton and soybeans with "stacked" or multiple resistance traits.

University of Missouri Extension programs are open to all.

Dr. Michael R. Milam is an agronomy specialist and county program director with University of Missouri Extension in Dunklin County.