Palmer Amaranth

Several other weeds have also developed glyphosate resistance in Kansas, including common waterhemp, marestail, kochia, common ragweed, and giant ragweed.

Glyphosate resistance can be confirmed with greenhouse and laboratory tests, but at this point that is probably no longer necessary. If a few Palmer amaranth plants or patches of plants survived where glyphosate was applied at the recommended rate with the appropriate adjuvants and spray coverage was good, there is a good chance those plants are resistant.

Figure 1. Surviving Palmer amaranth among dead plants treated with Roundup PowerMax at 32 fl oz/acre when plants were about 6 inches tall. Photo taken at Ashland Bottoms near Manhattan six days after treatment. Plants were confirmed to be resistant by an enzyme assay in the laboratory. Photo by Dallas Peterson, K-State Research and Extension.

Where glyphostate-resistant Palmer amaranth occurs, producers should plan on taking the necessary control measures. Another application of glyphosate alone probably will not help. It may be possible to use an alternative to glyphosate as a postemergence treatment in-season this year, although most postemergence alternatives are effective only on smaller plants.

In soybeans, the only alternatives would be PPO-inhibiting herbicides such as Cobra, FlexStar, Marvel, or Ultra Blazer. If there are just a few scattered plants, removing them by hand or other mechanical methods before they go to seed may help prevent them from spreading rapidly in the field. If those weeds remain growing in the field and set seed, the action of a combine will spread the seed throughout the field, as well as to other fields.

In fallow, it is best to use glyphosate as a tank-mix with 0.25 to 0.5 lb ae of 2, 4-D or dicamba. Treat fallow as soon as possible because the larger the glyphosate-resistant Palmer amaranth gets, the harder it is to control.

In the future, growers need to consider using a more integrated weed management approach that includes cultural practices and multiple herbicide modes of action, especially preplant and preemergence residual herbicides. Scout fields early after crop emergence and make timely postemergence applications with the appropriate herbicides for control of escaped weeds.

-Dallas Peterson, Weed Management Specialist

To attempt to very this one way or another, waterhemp and Palmer amaranth seed were collected in the fall of 2011 from various soybean and cotton fields in eastern and south central Kansas. These fields had abundant pigweed escapes after being treated with glyphosate.

Palmer amaranth escapes in soyeans treated with glyphosate. Photos by Josh Putman, Graduate Research Assistant, Agronomy, K-State Research and Extension.

Waterhemp and Palmer amaranth plants from these different populations were grown in the greenhouse and treated with glyphosate at 0.75 (1X), 1.5 (2X), and 3 (4X) lb/acre to evaluate for resistance. Response of the different populations was compared to susceptible populations of both species collected from the Ashland Bottoms experiment field south of Manhattan, KS. The susceptible check populations of both species was completely controlled by all rates of glyphosate.

As expected, a number of the waterhemp populations from across eastern Kansas survived glyphosate treatment up to a 4X rate, and appeared to be resistant. Two populations of Palmer amaranth from Cowley county in south central Kansas also had a high percentage of plants that survived the 1X and 2X rates of glyphosate and had some plants that survived the 4X rate. These Palmer amaranth populations did not appear to be as resistant to glyphosate as some of the Palmer amaranth from the Southeast U.S., but weren't being controlled by typical field rates in the greenhouse or with multiple applications of glyphosate in the field. Thus, it appears that glyphosate-resistant Palmer amaranth may now be present in Kansas.

Palmer amaranth seed was collected from additional fields this past fall and is now being evaluated in the greenhouse. Preliminary observations suggest that we are seeing similar survival after glyphosate treatments from Palmer amaranth collected from fields south of Great Bend, along with additional populations collected south of Wichita. Research on inheritance and the mechanism of resistance will need to be conducted to further characterize and confirm glyphosate resistance in these populations. Confirming herbicide resistance is a long and detailed process.

Regardless of whether glyphosate-resistant Palmer amaranth is present in your area now, there is a good chance it will develop at some point based upon what has happened in the Southeast U.S., especially if growers rely heavily on glyphosate for weed control.

Palmer amaranth is an extremely competitive weed and the development of glyphosate resistance means it will require an effective integrated weed management program to achieve acceptable control. Continuing to rely only on glyphosate for weed control, will only speed up the development of glyphosate-resistant weeds and diminish its effectiveness. Utilizing residual herbicides with different modes of action throughout the cropping system will help to manage existing glyphosate-resistant weeds and slow the development of new glyphosate-resistant weed populations.

-Dallas Peterson, Weed Management Specialist
dpeterso@ksu.edu

-Josh Putnam, Graduate Research Assistant, Agronomy
jputman@ksu.edu

-Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist
cthompso@ksu.edu