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Herbicide resistance past, present and in the future

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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

information [Music]

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