On Wednesdays of this week, I had the opportunity to attend a Pigposium-Respect the Rotation Field Day at the Northeast Arkansas Research and Extension Center at Keiser. There were six stops on the field tour with excellent presentations. This event was also well attended.
The first Pigposium (Pigweed symposium) was held in Forrest City Arkansas .in 2010 to discuss the problems and solutions for controlling Roundup Ready Palmer Pigweed. While this is a major problem in our area, much of the State of Missouri and Illinois has problems with a closely related species, waterhemp.
One of the stops featured Dr. Aaron Hagar, Associate Professor of Weed Science at the University of Illinois who spoke about some very concerning changes in waterhemp herbicide resistance. Resistance in waterhemp is not new. Dr. Kevin Bradley at the University of Missouri has been researching this problem for years.
The United Soybean board has an on-line publication, "Waterhemp Management in Soybeans" which can be found at http://takeactiononweeds.com/wp-content/uploads/2014/01/waterhemp-management-in-soybeans.pdf. Dr. Bradley has edited this publication and part of this information is being used in this article.
Because waterhemp is dioecious, meaning there are male and female flowers on separate plants, there is potential for greater genetic diversity within a population than for most agronomic weeds. This genetic diversity increases the potential for evolving and spreading novel herbicide-resistance genes and other ecological traits that improve waterhemp survival in agronomic systems.
Waterhemp is a prolific seed producer and able to produce as many as 1.5 times more seeds than most other pigweed species. Waterhemp plants generally produce about 250,000 seeds per plant, although some plants can produce 1 million or more seeds under optimal conditions in noncompetitive environments.
Waterhemp has a remarkable ability to adapt to control tactics and has evolved resistance to many different classes of herbicides. To date, waterhemp has evolved resistance to six herbicide classes, including Group 5(e.g., triazines like atrazine and simazine), Group 2
(e.g., ALS-inhibiting herbicides like Pursuit® and Classic®), Group 14 (e.g., PPO-inhibiting herbicides like Ultra Blazer®, Cobra® and Flexstar®), Group 9 (e.g., glyphosate), Group 27 (e.g., HPPD-inhibiting herbicides like Callisto®, Laudis® and Impact®) and Group 4 (e.g., 2,4-D).
Many populations in the Midwest now exhibit multiple herbicide resistances that include several herbicide families. For example, Group 2 and 9 (e.g., ALS inhibitors and glyphosate, respectively) resistance in waterhemp is fairly common, and in many states resistance to as many as five herbicide groups now occurs in some waterhemp populations.
The greatest concern is how this species has developed resistance to these herbicides. While initially, there were mutations that helped this species overcome the herbicides, it has also been shown that some populations of waterhemp were able to increase their metabolism to overcome the herbicide.
One of the reasons that we have developed this resistance problem is because we are the victims of our success. Many producers started using no-till or reduced tillage to become more environmentally friendly. The use of Roundup Ready crops that allowed spraying over the top of the plants allowed this conservation tillage movement to expand. As more producers reduced tillage, more Roundup was used. There were many applications applied each year, some to weeds that were too large to kill. As a result, weeds developed resistance to Roundup or other herbicides.
Weeds are not resistant to steel or tillage. But reduced tillage not only helped the environment but it was very cost effective. Less equipment was needed, fewer trips across the field saved time and money. Some farmers have gone back to tillage to get some relief from the resistant weeds.
To control these resistant weeds, it is recommended that producers use pre-plant and residual herbicides. These herbicides control germination and help keep the population levels of resistant seed low.
They recommend that producers start with clean fields and keep them clean and don't let the escapes go to seed. In many instances, crews are hoeing out the remaining weeds. The seed do not survive in the soil for a long period of time.
So progress can be made against both waterhemp and Palmer pigweed by rotating the herbicide classes and following manufacturers and extension recommendations.
The University of Missouri Extension office is open Monday-Friday located in Kennett, Missouri at 233 North Main Street. Contact 573-888-4722 with questions or comments. MU is an equal opportunity/ADA institution.