November 16, 2012

In early August, Panhandle producers observed a giant pigweed emerging above the corn canopy. With last summer's extremely high temperatures, Palmer amaranth had made the latest jump in its northward migration.

A troublesome weed in the southern U.S. for many years, Palmer amaranth (Amaranthus palmeri) had been found in Nebraska south of the Platte River and west of Hastings for several years. This year it was also found from the Panhandle to the Sandhills to Antelope County in northeast Nebraska.

It should be of particular concern to Nebraska growers as Palmer amaranth has developed resistance to several major herbicides. It will need to be strategically managed to prevent the further development of resistance.

The Newest Pigweed in Nebraska

Pigweeds, like Palmer amaranth, have been growing in western Nebraska for many years and most growers are familiar with redroot pigweed. In recent years, redroot pigweed has not been as prevalent as in the past due to an increase in common lambsquarters, which emerges earlier in the spring and has more tolerance to glyphosate.

Palmer amaranth is native to the lower Rio Grande Valley of Texas where cotton is an important crop. It was commonly found in the southern United States, and in recent years had expanded its range into Kansas.

Factors Affecting Its Spread

Temperature is the leading ecological factor determining which pigweed species will dominate. Palmer amaranth responds negatively to the low temperatures typical in the northern United States. Its ideal temperature range is between 85°F and 95°F and plant growth declines dramatically between 50°F and 60°F. Growth rate, biomass, and seed production of Palmer amaranth are greater than redroot pigweed at temperatures between 65°F to 95°F.

Palmer amaranth seed is moved via farm equipment, especially combines, or as a contaminant in crop seed or livestock feed. An interesting case in point occurred in 2011 when cottonseed cake used as a protein source for livestock was shipped north. Cotton fields infested with Palmer amaranth were harvested and as seed was removed from cotton fibers, Palmer amaranth seed was mixed in the seed cake. Cattle ate the protein and deposited the Palmer amaranth seed in an environment favorable for expansion.

Palmer amaranth has expanded its range into western and northern Nebraska for several reasons:

• higher temperatures,
• reduced use of herbicides at planting,
• its ability to develop resistance to herbicides, and
• a decrease in preplant tillage has provided an environment favorable to the weed.

Manage to Reduce Development of Herbicide Resistance

Palmer amaranth growing on your farm may already be resistant to some herbicide families. Triazine-resistant Palmer amaranth has already been identified in Nebraska and UNL researchers are testing for additional resistance development. As is always recommended, avoid relying on a single herbicide mode of action for weed control.

• Use production practices that do not spread the weed.
• Rotate herbicide modes of action to reduce the potential for resistance development.

In the south Palmer amaranth has developed resistance to the following herbicide families: dinitroanilines (Prowl, Sonalan), imidazolinones (Pursuit, Raptor), triazines (atrazine), PPO inhibitors (Reflex) and most recently EPSP synthetase inhibition (glyphosate).

Treatment

In the Panhandle, Palmer amaranth has been most noticeable in corn. Corn and weeds such as kochia, common lambsquarters, and hairy nightshade emerge earlier in the spring than Palmer amaranth. Using a herbicide with soil residual such as atrazine, Balance Flexx, Callisto, Dual Magnum, Outlook, Prowl, Permit, Verdict, or Warrant at corn planting will help control early season emergence. Following with a postemergence weed control program with herbicides such as dicamba, 2,4-D, glyphosate, Impact, or Laudis will help control later emerging plants.

In dry bean, herbicides applied at planting such as Dual Magnum, Outlook, Prowl, Sonalan, or Permit can provide early season control while Raptor and Reflex will provide postemergence control.

Robert Wilson
Extension Weed Specialist