hello I'm Mike Owen I'm a university
Professor ameritus but happy to rejoin
you on crops TV and discuss herbicide
resistance past present and in the
future and as it turns out it's going to
be a lot of past and present but not so
much in the
future before I get too much into that I
want to remind every everybody that we
redid the herbicide guide for Iowa corn
and soybean production uh the URL is
available at the bottom of the screen uh
this is a good and I think very indepth
opportunity for you to read about the
herbicide recommendations that we have
made for porn and soybean and also there
are several articles that cover some of
the topics that uh we I won't be talking
about but I will talk a little bit more
about the information about herbicide
resistance you know I have to apologize
because a lot of these slides are you
know blasts from the past you know here
we have one that I have uh sort of
rhetorically entitled weed management
where we have Mar's tale or better said
horse weed field uh with some soybeans
planted in in reality it's more of a
weed field and this was attributable to
the farmer's inability or if you will
unwillingness to adapt to the changing
populations of the horse weed in that particular
particular
field let's start out with a little bit
of information about what we mean by
herbicide resistance uh the definition
that has been accepted by the weed
science Society of America although
challenged elsewhere is the fact that
this is an inherited
inherited
characteristic of
plants that
survive the dose as stated that normally
kills most of those weeds and this is a
selection process that was actually
discussed by Charles Darwin back in the
early or mid 1800s where he described
just how species
plants and animals respond to selective
forces that in involve them in their
environment and the cause of herbicide
resistance I guess this is like no great
surprise is simply because we use the
same control tactics um we have been
through a number of of waves if you will
of herbicide resistance starting out
with the ALS less resistance that was
brought forward primarily by American
cyanamid Products Pursuit on water hemp
and then became a very large and costly
feature in agriculture when everyone
switched to the genetically modified
glyphosate resistant soybeans and corn
and other crops and then all of a sudden
none of the plants water him Giant
Ragweed um and others would respond to
glyphosate this is not just a herbicide
feature however this evolutionary
process that weeds and as it turns out
pretty much every species adapts to here
we have a number of factors
grazing we see the response of certain
plants this types of growth that keep
the grazing process from a Hab in
tillage adaptation to tillage is the
development of of ryom and and root
stocks and I've also have said in the
past that water hemp has evolved
resistance to
cultivation and one might Ponder how can
that be well look at how water hemp has
the ability to germinate much later in
the grow growing season after the crops
have closed in and cultivation is no
longer an option this is a selection
pressure germination and dorcy is a
heritable trait and there is good
evidence in fact we published this many
many years ago about how different water
hemp populations germinate at different
times and probably have done so this
selectivity have done so because of the
strategies that were used to control it
mowing I think everybody is pretty well
aware of how weeds adapt uh the we we
see yellow foxtail for example that
grows in a prostate growth habit um dny
lions and plantain are the same thing
hand weeding there are many good
examples of how the
crop mimicry where the the weed has essential
essential
looked or evolved a a a phenotype to
look very much like the crop and
eliminate the effectiveness of hand
weeding then finally what we are now
dealing with Worldwide and in particular
as a problem in Iowa is the ability of
certain weeds to evolve resistance
whether it be metabolic resistance or
tget site resistance or even in some
degree translocation that
herbicides this is an oldy but a goodie
uh this is when there was a Dupont
selling agricultural products and they
recognized because of their uh ALS
inhibitor herbicide chemistries uh they
realized that there needed to be a
change in how we were managing weeds
with herbicides and they put forward
what was a very good uh brochure uh
managing herbicide or managing weed
populations to preserve herbicide value
this was in the late
1990s and as it was it was designed to
help farmers understand this
evolutionary process of herbicide
resistance and manage the evolving
resistance to preserve the herbicide
value the rhetorical question that I ask
and again this is from the 19 90s what
did we learn from this attempt by Dupont
to help preserve herbicide value and the
answer is not much and this is
unfortunate because we have now gone
through the third or the fourth wave of
resistance to different herbicides that
are being
used here is the glyphosate resistant
weeds that we have uh as major economic
problems we have common water pemp we
have common Ragweed further east and
south uh horse weed or May's tail in
Iowa Palmer pigweed has come into the
state through uh introduction using uh
contaminated weeds uh uh Prairie
plantings and also equipment giant
Ragweed and then further south we have
Johnson grass all of these are major
issues in their respective Crop
Production systems all have evolved
strong resistance to glyphosate and made
that herbicide essentially useless in
many instances and those of you who have
glyphosate resistant water hemp here in
Iowa fully appreciate what little
glyphosate now does in glyphosate resistant
resistant
crops before I retired in 2011 and
working in conjunction with the Iowa
soybean Association we started to do a
survey to determine how big of a problem
uh herbicide resistant weeds in Iowa
actually was and we focused on water
hemp and we focused on soybeans simply
because it was easier to find the
escapes later in the season uh prior to
harvest and we did the first sampling in
2011 we did a 2012 sampling and then I
recognized that while it was good
information it was not particularly
valuable from a predictive persp
perspective and so in
2013 we established a specific algorithm
that predicted certain fields in the
nine crop reporting districts across
Iowa and told us which fields we should
survey such that we would have a
predictive value at the
95% confidence interval what this means
is that these fields that we CH looked
at in in 2013
allowed us to predict at the
95% confidence interval meaning we were
right 95% of the time which Fields would
have resistance and we tested these
populations against five different
action and the reason we did this is try
to get a handle on how quickly the weeds
in those fields those escaped would
would expand and become major problems
across those fields and and here is
a essentially a diagram that shows after
so many years when you're going to have
herbicide resistance and we'll
say herbicide resistance in water hemp
how you know across the field and it
takes 5 to 7 years before a farmer
recognizes this is not just just an
escape it's not a sprayer skip or or
whatever and once this trait is
maintained in the field eight or nine
years a majority of the water hemp in
that field will have the herbicide resistance
characteristic now what does this mean
and here's a little slide and I think Dr
Hartzler actually put this together and
it talks about okay how many plants
escaping actually become a major problem
and I think you can look at the math I'm
not going to read this but essentially
even if you have 95% control with just a
few Escapes in a field 95% control
you'll end up with possibly one water
hemp plant per square foot across this
entire field and if those escapes had
have herbicide resistance it's going to
manage so right now I think this
represents the reality of herbicide
resistance in most Fields all right just
edges on the field maybe where you
introduced it when you brought a combine
in that had been in a field with
herbicide resistance uh maybe it's an
area where uh there was a a wet hole or
something like that but these plants did
experience whatever herbicide regime you
were using and it doesn't look to be
such a major problem that you can't just
go through and combine that area okay
but let's think about this if you go
through and the best machine I know to
spread weed seeds WI
and evenly across a field is an actual
flow combine and so if you go through
this and harvest this area those weed
seeds are going to get spread further
and further across the field and if you
do tillage that tillage will spread
those weed seeds further and further and
more evenly across the field until you
end up with a problem like that first
side that I showed you where it was not
weed this is perhaps a little bit
difficult but I'll go through it
hopefully very specific this is what
happened with the
2013 in the red bars and the
2019 repeat survey that was done at this
on the same field and again looking at
how resistance was changing over that
six-year production system we see in the
atrazine that uh 76 or so percent of the
fields that we picked at random which
again provides us with the statistical
ability to predict if resistance is
going to exist in any field and based on
that in
2013 the 1X rate of atrazine about 76%
of the fields had resistance when we
went back in 2019 and repeated the
survey and repeated the screening that
we did in the greenhouse we found that
resistance to atrazine at the 1X level
had increased somewhat but
importantly it had not declined
we go now to the right we look at
glyphosate we see glyphosate is
increased you know almost to 80 or
90% in
2019 IMAP piir representing the ALS
inhibitor herbicides was very high in
2013 and
2019 lactofen which represents the uh po
inhibitor herbicides we really didn't
see resistance in
2013 uh but in 2019 it is climbing into
the 20% range and then similarly at the
1X rate with we looked at a misot trione
uh as the representative of the hppd
inhibitor herbicides and the resistance
at the 1X has now increased up to about
15% of the fields are likely to have
water hemp with hppd resistant characteristics
characteristics
as we go to the right and we see under B
this is the same response but using four
times the herbicide rate and as you
might expect in many instances the
relative percentage of fields that had
resistance was a little less but if we look
look
at the pursuit or EMA here we see that
it's still very high and we are now seeing
seeing
a high level of resistance in about 5%
of the fields to the PO Inhibitors and
about 3% to the HPP inhibitor herbicides
this is important because these
indicate at the 95% confidence interval
being right 95 times out of a 100 what
percentage of fields are likely to have
herbicides this is the other issue that
using these herbicides and repeating
their use has caused and that is the
fact that the same herbicide resistant
population of water hemp probably has
resistance to other herbicides that were
used in the field again the same basic
layout on the left in the graph a red
2013 gold 2019 we look at resistance to
one of the five herbicides and in
2013 uh we had about
10% of the populations that we selected
had only one
resistance in 2019 at the 1X rate there were
were
no populations that we selected that had
one-way resistance that is because they
had evolved resistance to other herbicides
herbicides
two-way which wasn't which was a main
problem in
2013 two-way resistance
declined in 2019 down to about 20% of
the populations but three-way now is
greater in 2013 and even greater still
in 2019 forway resistance which wasn't a
major issue in
2013 the cardinal
red increased almost doubled in 2019 and
five-way resistance which we didn't
discover in 2013 is now showing up in
about 6 to 8% of the populations and in
graph B the the same thing but with the
Forex rat
of the herbicides and we can see that it
has become a a major feature in both
2019 so what is the prevalence of
multiple resistant water hemp comparing
2013 To the Left To
2019 on the right and and here in 2013
we see that essentially there's no five-way
five-way
resistance in
2019 one-way resistance went away
two-way resistance has increased a
little bit three-way resistance has
increased considerably and four-way and
five-way resistance are showing up
essentially in 2013
52% of the water hemp populations which
actually means
52% of the fields in Iowa have water
hemp with threeway or more herbicide
resistance and in
2019 this number has jumped up to
80% of the fields have water hemp with
threeway resistance traits or more the
four and the five-way resistance this
leaves PR ious little opportunity if
these water hemp populations have
increased to the point where they are a
field so we've looked at the
past we've looked at the present
situation and now let's look a little
bit at the future this is some work that
we published with Ryan Hamburg who is an
Iowa kid who is now at the Texas A&M
University working on his PhD and here
we looked at populations that were not
selected randomly but rather selected
arbitrarily and they
were challenged with
24d they were challenged with
damaa and they were challenged with
glufosinate and what we're looking at is
the survival frequency this is not to
suggest that this is actual resistance
but I think it gives a fairly strong
indication that there are populations
that are not going to respond very well
to most of these herbicides we look at
those that survived at less than
10% and the predominant number 75 of the
popul ations for
24d 108 of 133 total populations
survived dama and 145 populations out of
168 survived
glufosinate but as we go further and we
look at the frequency of of survival 20
to 30% and so forth we see that these
numbers increase up to almost
50% or more of the populations survived
24d none of the populations for damba
and none of the populations for
glufosinate we have to recognize however
that in other
areas not with water hemp but with
Palmer pigweed which again is becoming a
problem in Iowa we are finding high
levels of of diber resistance in
populations and we're finding high level
of populations or excuse me lower levels
glufosinate will we have options in the
30s for managing these herbicide
resistant weeds and let's recall that we
have not seen any new herbicide
mechanisms of action registered since
the late '90s and so we've gone almost
30 to 40 years without any new
herbicides Now new herbicides have been
registered but they are the same
mechanisms of action that we have had in
the past and so let's look at the traits
24d the enlist trait uh we look at
daa with the uh bear crop science daa
resistance and we look at the Liberty
resistance that exists in soybeans and
here we find that there was resistance
to 24d discovered in 2012 in
Nebraska we see with Liberty that Palmer
amaran resistant populations were
discovered in Arkansas in
2019 and daa resistance in water hemp in
Illinois and and in Tennessee is
2019 the
hg15 which is dual
metallic and others of the hg15
hg15
site resistance to that herbicide duel
was reported in Illinois in 2019 and we
have heard complaints about performance
for duel in Iowa and so these are the
herbicides that we have been using for
managing water hemp and Palmer amaran
and we can see that resistance already
exists in populations and so into the
30s we are going to have greater
challenges using those herbicides that
we have been using and the traits that
those herbicides are registered
for and with the recent this is
February 8th that we're recording this
on crops TV there there was an an
announcement that the daa registrations
have been
revoked uh by courts out of Arizona
because of failure for the EPA to follow
proper procedure and so whether or not
we're able to use daa this growing
season in corn or in soy for that matter
uh becomes a bit of a question and not a
good question as it
were so what can we do to manage
herbicide resistance and again we are
predicting that herbicide resistance in
water hemp those traits exist in a
majority of the fields that is not to
suggest that the majority of the weed
the water hemp plants in those fields
are resistant to herbicide
but it's a strong indication that the
likelihood is that they will if they
have not already evolved resistance to
those particular
herbicides well we have had if you will
a Fool's Paradise about the Simplicity
and the convenience of managing weeds
with single applications or multiple
applications of particular herbicides to
which the crops have
resistance and this is not
working Simplicity and convenience has a
price and the price is that herbicide
resistant water hemp is
evolving possible strategies include
multiple using multiple effective sites
but notice that I have highlighted
effective and given the multiple
resistances that exist in a high
percentage of the weed or in the weed
the water hemp populations in fields
whether or not they're effective that is
hard to suggest we could also rotate
effective sites of herbicide action but
again without knowing exactly which
weed or which herbicides are effective
becomes a bit of a question and so maybe
this is not such a good
strategy after all and again any
strategy that is effective regardless of
whether it is based on herbicides or
based on tillage such as
cultivation if it is effective it will
eventually fail because of the ability
of plants and in specifics water hemp to
evolve resistance to that effective
trait if that is the only thing that is
used to manage the water hemp which gets
back to the Simplicity and
convenience you need to use multiple
strategies herbicides will still be an
important consideration but without
including other techniques such
as we appropriate using cover crops and
we appropriate using
cultivation or increasing the diversity
of of crop rotation which is not
easy without using these other types of
strategies eventually the herbicide
fail in summary
summary
recognize that weeds and in specifics
water hemp will adapt to all management
practices that you employ in crop
production and what we have found
particularly with herbicide resistant
traits once a water hemp population
evolves resistance to a particular
herbicide characteristic it will more quickly
quickly
evolve resistance to other herbicide
traits and these traits
are additive they just Stack Up In The
Water hemp genome and they do not go
away whether or not you stop using that
particular herbicide they are still
there and that makes waiting a few years
and then coming back to the same
herbicide that was not
performing several years ago it still
will not
perform note that the diversity of the
systems is critically important the
simpler the system the less diverse the
crop production system is the more
rapidly the weeds will adapt and if this
happens to be based on herbicides the
more quickly resistance will
evolv we recognize that current Crop
Production systems are based on
herbicides this needs to change not to
eliminate the use of herbicides but to
add to the
herbicide other diverse management
practices and obviously changing tactics
using something like a cover crop or
rotating tillage or changing crop
rotation practices this is not simple
this requires time
but it is critical that this be done because
because
ultimately and
inevitably whatever herbicide program
fail
concluding resistance is a big problem
it's a global feature wherever
herbicides are used on whatever crop is
being produced resistance is evolving
and the re the evolution of herbicide
resistance is increasing at an
increasing rate and this is to
all herbicide mechanisms of action there
are none that do not have herbicide
resistant weed population
somewhere in
crops as a general statement Farmers
still feel that resistance is not their
issue they are
wrong industry is not really supporting
even though they give lip service to
weed management and product stewardship
in reality they at the grower level or
at the dealer level they are doing
little if
anything to promote product stewardship
to the extent that their customers are
changing their behavior this has to
change even if new herbicides or new
genetically engineered traits and crops
were developed and marketed this is not
going to fix the problem as I have
indicated once a wheat population
evolves resistance to a particular
herbicide they are now in a position
where they or it will more more quickly
evolve new resistances and as I've
indicated these resistances stack up
just like plant breeders Stack Up
genetically engineered traits in crops
and so new herbicides and new
genetically engineered traits are not
the cure to the problem regardless of
what the marketing people
say and it is
critical imperative that you focus and
develop action to manage the herbicide
resistant weeds that exist in fields in
Iowa and develop a more diverse weed Management
program these are the guys back in the
1800s that predicted water hemp
resistance in Iowa Chuck darw and Russ
Wallace they talked about the evolution
of characteristics attributable to selection
selection
pressure we are selecting herbicide
resistance in crops in Weeds using
herbicides this is what they predicted
and it is coming true this is Mike Owen
on crops TV and I thank you for the
opportunity to provide this information
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