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Got
Waterhemp? What now?
By Sam Custer
OSU Extension, Darke County
Mark Loux has spent a lot of time educating and warning about Palmer
amaranth, but while we were doing this, waterhemp has become a more
widespread problem.
AT the fair this past week I was asked what are we going to do about
the neighbor’s waterhemp. Thus I share with you this article
containing Mark Loux’s recommendations.
Waterhemp is only a little less fun to deal with than Palmer amaranth,
and has a tendency to fairly rapidly develop resistance to any new
sites of action used repeatedly against it in POST treatments.
Waterhemp infestations can be found around the state, with a
concentration in west central Ohio. We assume all waterhemp
populations are resistant to site 2 herbicides (ALS), but in our
screening so far, not all populations are resistant to glyphosate.
Populations in western Ohio tend to be glyphosate-resistant at this
point, but populations in other parts of Ohio not necessarily so.
Some populations that are glyphosate-resistant appear to have developed
at least a low level of resistance to site 14 (PPO) herbicides as well.
In areas of the corn belt further west where waterhemp has been the
major glyphosate-resistant weed problem for some time, the evolution of
multiple resistance is common.
Recent surveys by the University of Illinois determined that
approximately 60% of Illinois waterhemp populations were resistant to
glyphosate and also to site 2 and 14 herbicides. Due to the
tendency for waterhemp to evolve resistance to new sites of action used
against it, the U of I recommends a strategy of not using a new site of
action (usually PPO) until resistance to glyphosate has been confirmed.
So in a field situation where a grower is unsure about resistance, they
suggest applying glyphosate to small waterhemp plants, and then
monitoring to determine if the population is responding. If there
is no response, the next step is immediate application with a PPO
inhibitor.
The drawback here of course is that glyphosate-resistant waterhemp will
continue to grow during this brief period of evaluation following
glyphosate application, possibly reducing the effectiveness of the PPO
herbicide.
Waterhemp requires a combination of residual and POST herbicides, with
the goal of applying POST herbicides to plants not more than 6 inches
tall. In a number of Ohio fields with waterhemp that we know of,
growers were either unaware they had waterhemp and inadvertently let it
get far bigger than this.
Waterhemp produces a lot of seed and one or two years of this approach
will guarantee the presence of waterhemp for years to come.
It’s largely too late to provide much helpful guidance for this
summer’s waterhemp control programs. What’s essential to
accomplish still this year though, for future planning, is to figure
out whether a waterhemp population is resistant to glyphosate and/or
site 14 herbicides. Fortunately, there is a painless way to do
this, through a service offered by the University of Illinois Plant
Clinic. Link to the U of I newsletter article that provides the
needed info - http://bulletin.ipm.illinois.edu/?p=3619.
In a nutshell, you collect five new leaves that have developed
following an herbicide application, and send them to U of I via rapid
delivery along with one sample submission form per sample. Cost
is $50 per sample, which includes screening for both glyphosate and
site 14 herbicides.
We highly recommend making this investment where resistance
characteristics of a waterhemp population are unknown. This
information will inform future decisions on herbicides for control of
waterhemp, ensuring that appropriate herbicides are used and that the
population does not develop resistance to site 14 herbicides as
rapidly.
The LibertyLink soybean system is, of course, another option for
management of waterhemp, as well as Xtend and Enlist soybean systems
whenever they become available.
It should be noted that: 1) use of glufosinate, dicamba, or 2,4-D to
help manage this weed does not change the approach, and these
herbicides must be applied to small plants to control waterhemp; and 2)
the assumption is that inappropriate and continued use of these
herbicides would lead to the development of resistance to them.
And in fact there are already some populations of waterhemp farther
west with resistance to 2,4-D.
For more detailed information, visit the Darke County OSU Extension web
site at www.darke.osu.edu, the OSU Extension Darke County Facebook page
or contact Sam Custer, at 937.548.5215.
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