ESSEX, Ill. — Farmers going toe to toe against Palmer
amaranth will lose.
“This thing will outgrow us. We don’t have enough spray rigs
in this state. We don’t have enough folks that can operate them,” University of
Illinois weed specialist Aaron Hager said at a Respect the Rotation field tour.
“The way we win the battle against Palmer is not going
against the plant, but is by attacking this thing at the weakest point in its
life cycle — when it’s a seed.”
The Kankakee County research plots facilitated by Bayer
CropScience, tests various treatments in a field where Palmer amaranth was
established about three to four years ago.
One of the biggest advantages that can be used to combat
Palmer is that the seeds are not indefinitely viable in the soil
Unlike velvet weed or giant ragweed where seeds are viable
in the soil for 30 or 40 years, Palmer seed is viable for no more than 10 years.
“If you can prevent these seeds from being back into the
seed bank for three, four or five years, you can really begin to see these
population numbers go down. That’s why we focus so much of our time and effort
on talking about the seed introduction and seed prevention,” Hager said.
“The problem with preventing seed introduction is I don’t
think we can do it. There are so many ways that these small seeds can move
around quickly from area to area.”
Researchers are therefore focusing on what can be done to
prevent the plants from producing seeds.
“In some instances, a hoe is going to be the only thing that
we have to do that with,” Hager said. “Nobody likes to do that on a hot sunny
day, but if we can remove these plants before the seed population begins to
build, five years later we’re going to look back and say that’s the best thing
we ever did.”
Palmer amaranth has a higher growth rate and is more
competitive than other pigweed species.
Growth rates approaching three inches per day, and yield
losses of 78 percent in soybeans and 91 percent in corn attributed to Palmer
amaranth have been reported in the scientific literature.
“Once these populations do become established, your
herbicide cost will at least double. In many areas of Tennessee Palmer is now
about a $100-per-acre weed,” Hager said.
Palmer infestations vary in the research plots depending on
Hager noted one plot that featured various glyphosate rates
and application times.
The first application was when Palmer was about four inches
tall, and a second application was done seven days later.
“What we found this year is that we started with 22 ounces,
we then go to 44, 66, 88 ounces on up and found at this site that once we hit in
the range of 44 and 66 ounces our control ratings were about 95 percent,” Hager
“The challenge that we ran into is we were only able to
evaluate that experiment for about two weeks, because two weeks after either the
early timing or the last timing we couldn’t see the ground between the rows. We
had another flush of Palmer and very rapid growth rate.”
He said a minimum of three applications of herbicides would
be required before a combine could be used to harvest the field due to the heavy
“If you did one application, you’d never put a combine in
here. If you did two, you could put a combine in here, but you’d cuss yourself
for not making three. With three you have a fighting chance and four
applications would be even better,” he said.
“I don’t honestly remember how many germination events we’ve
had in this particular population. I think it’s somewhere over about 12.”
Hager recommends the following guidelines for Palmer
n If you discover a plant that you think may be Palmer amaranth, you can
verify its identity by sending a leaf tissue sample to the U of I for
identification using molecular biology techniques.
n Plants confirmed or suspected of being Palmer amaranth should be
physically removed from the field prior to flowering. Do not rely on herbicides
for control. Physical removal can include hoeing or hand-pulling plants from the
soil. If hoeing is used, be sure to sever the plant stem at or below the soil
surface to reduce the potential for re-growth, and remove plants from the field
as they will re-root from stem fragments.
n If Palmer amaranth plants are not identified until after brown-to-black
colored seeds are present on female plants, we suggest leaving the plants
undisturbed in order to avoid inadvertently spreading seed.
n Mark or flag areas where Palmer amaranth plants produced seed. These areas
should be intensively scouted the following season and an aggressive Palmer
amaranth management plan implemented to prevent future seed production.
* Do not mechanically harvest mature Palmer amaranth plants. Physically
remove the plants prior to harvest and either leave the plants in the field or
place in a sturdy garden bag and remove the plants from the field. Bury or burn
the bags in a burn barrel as soon as possible.
* Fields in which Palmer amaranth seeds were produced should not be tilled
during the fall or following spring. Leaving the seeds near the soil surface
increases the opportunities for seed predation by various granivores.
* Herbicides that control waterhemp also control Palmer amaranth. An
integrated herbicide program should include soil-residual herbicides applied at
full recommended use rates of within two weeks of planting and followed by
post-emergence herbicides applied before Palmer amaranth plants exceed 3 inches
“From a management standpoint, there is no one single tactic
that will effectively control this thing from reducing crop yield,” Hager
“This thing plays by its own rules. We have to adapt to its
rules, it will not adapt to what we want to do. If there’s ever been a good
example of a species that an integrated system is necessary for, this is it
without a doubt.”