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Palmer amaranth: Managing this new weed problem

Christy Sprague Published on 01 May 2014
Palmer amaranth seed

Herbicide-resistant Palmer amaranth continues to invade many areas of Michigan and the Upper Midwest. Our first encounter with Palmer amaranth in Michigan occurred in the fall of 2010.

A soybean producer in southwest Michigan was struggling with management of a weed he did not recognize. We were able to identify this weed as Palmer amaranth, and through greenhouse testing confirmed high levels of resistance to both glyphosate (Roundup) and ALS-inhibiting herbicides.

At this time, we also noticed several neighboring alfalfa fields where Palmer amaranth seed heads were poking just above the alfalfa canopy.

Knowing how glyphosate-resistant and multiple-resistant Palmer amaranth had been wreaking havoc in many cropping systems in the southern U.S., we were concerned about the impacts the introduction of this weed may have on Michigan cropping systems.

How was Palmer amaranth introduced into Michigan and how can it spread?
Since the original identification of Palmer amaranth in Michigan in 2010, we have now confirmed its presence in nine Michigan counties.

Palmer amaranth is not native to Michigan or to many areas of the Upper Midwest. So one of our first concerns was: How is this non-native pigweed species finding its way into Michigan?

We speculated that the herbicide-resistant Palmer amaranth populations found in Michigan were established by seed brought in from an outside source. What was this source?

While we may never know the direct source of this seed, when examining the field histories of many of our first reports of Palmer amaranth, we found that in many cases manure had been applied to these fields within a year or two of the growers noticing the plants.

This, along with some other observations, led me to speculate that the Palmer amaranth seed may have been brought in with cottonseed and byproducts fed to dairy cattle in many of these areas.

This may not be a surprise when you consider the millions of acres infested with glyphosate-resistant or multiple-resistant Palmer amaranth in the southern U.S., where a majority of the cotton is produced.

While this may help establish the origins of some of the initial reports of Palmer amaranth in the state, once Palmer amaranth establishes itself it is extremely difficult to control and seed can be moved from field to field with equipment and by other means.

Characteristics that make Palmer amaranth a management challenge
There are several characteristics that make Palmer amaranth a challenge to manage. An extended emergence pattern, rapid growth rate and resistance to several herbicide families make this weed extremely difficult.

In Michigan, we have tracked Palmer amaranth seedling emergence from mid-May through mid-August, with even a few seedlings emerging in early September.

The extended emergence pattern of this weed makes it difficult to pinpoint the most opportune time for management in several cropping systems.

Palmer amaranth’s rapid growth rate also makes timing of management strategies extremely difficult. From our research plots, we have observed Palmer amaranth growth from 3 to 7 inches in less than five days.

Herbicide resistance is the number one reason why Palmer amaranth has become such a challenge to control. Many of the Palmer amaranth populations we have tested in Michigan have multiple resistances to both glyphosate and the ALS-inhibiting herbicides.

This leaves a very few herbicide options available for control in several cropping systems.

Concerns with multiple-resistant Palmer amaranth in alfalfa
Annual weeds like Palmer amaranth are generally not a management concern for many forage producers. Some may argue that weeds often add to the overall nutritional value of the forage.

In many cases, annual weeds are controlled in forage production systems by mowing. Generally, these weeds do not recover after mowing, or if they are able to continue to grow, they do not have the opportunity to produce seed. However, this has not been the case for Palmer amaranth.

The rapid growth and recovery of this weed has led to thousands of Palmer amaranth seed heads above the alfalfa canopy at the end of the season.

The concern is that these seed heads may be able to produce viable seed that could contribute to the soil seed bank and perpetuate the spread of the devastating weed to other fields and crops.

In the fall of 2012, we randomly selected seed heads from three female Palmer amaranth plants from an alfalfa field prior to a fifth cutting. These plants were able to produce between 27 and 1,725 intact seeds per plant.

What was amazing, was that 98 percent of this seed was viable. While this is only a fraction of the possible seed that can be produced from a single female Palmer amaranth plant (average seed production 400,000 seeds), the fact that this plant can produce viable seed in less than 28 days gives Palmer amaranth an advantage to perpetuate its spread and survival in an alfalfa production system.

Palmer amaranth control

How do we manage multiple-resistant Palmer amaranth in alfalfa?
There are very few herbicide options available for control of pigweed species in alfalfa. The more commonly used herbicides, Raptor and Pursuit, are both ALS-inhibiting herbicides.

As mentioned previously, most of our Palmer amaranth populations are resistant to these herbicides.

Other options include possible dormant applications to established alfalfa, but with the later and extended emergence of Palmer amaranth, many of these applications may not last long enough to control Palmer amaranth and stop seed production.

From our experience, mature seed heads of Palmer amaranth generally appear after the fourth or fifth cutting of alfalfa.

This summer, we conducted research examining some possible solutions to manage Palmer amaranth and stop the seed production that occurs later in the growing season.

From our research, we were able to reduce the number of mature seed producing Palmer amaranth plants by a between-cutting application of Gramoxone 2.0 SL (paraquat) at 1 pt/A + surfactant at 0.25 percent v/v.

Our best results occurred when the herbicide application was made after the third or fourth alfalfa harvest. This application needs to take place within five days of cutting. There is also a pre-harvest interval for this type of application.

It is important to not cut or harvest alfalfa within 30 days of a Gramoxone application. While this may not be the best solution long term, it does provide us with an opportunity to help stop the possible spread of this weed through alfalfa production systems.

For more information on multiple-resistant Palmer amaranth or to help with its identification, we have developed several fact sheets that can be found on the Michigan State University Weed Science websiteFG

PHOTO 1: Palmer amaranth seed heads in a Michigan alfalfa field (photo taken October 8).

PHOTO 2: Palmer amaranth control after a between-cutting application of Gramoxone (after the 4th cutting) in the same field. Photos courtesy of Christy Sprague

Christy Spargue
Christy Sprague
Associate Professor
Extension Specialist Michigan State University