Project Title: Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of
Further Herbicide Resistance Risk in Georgia
William Vencill, University of Georgia
Summary: The widespread use of herbicides such as glyphosate has resulted in the
development of herbicide-resistant weeds. GR Palmer amaranth is the most significant species
of concern in cotton production; more than 2 million ha of agricultural land in the Midsouth and
SE US are estimated to be infested. However, with the imminent introduction of other
herbicide-resistant crops such as auxinic-resistant cotton and soybean, weed scientists need to
better be able to predict herbicide-resistant weeds. To preserve other herbicide mechanisms of
action, we need to know more about the potential for other cases of glyphosate as well as
other herbicide resistance and multiple-resistant weed populations. The immediate goal of this
project is to 1) conduct a survey in 20 fields
geographically dispersed in Georgia and collect
mature weed seeds from as many species as
possible and determine sensitivity to seven
herbicide mechanisms of action (EPSP (WSSA
Group 9), GS (WSSA Group 10), PSII (WSSA Group
5), PPO (WSSA Group 14), ALS (WSSA Group 2),
HPPD (WSSA Group 27), and Auxinic (WSSA
Group 4)) and if any multiple herbicide resistance
is present and 2) determine a risk of major
agronomic weeds in Georgia to further herbicide
resistance development. In Fall 2013, 28 fields
were sampled in southwest, central, southeast,
and northeast Georgia (Figure 1).
Vencill, W.K.; T.L. Grey; Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of
Further Herbicide Resistance in Georgia; Project Type: Working Group; This project primarily
addresses the 2013 Special Priority
Problem and Justification: In 2009, growers in the United States harvested 3 million ha of
upland cotton with an estimated production value of $3.5 billion. Worldwide, the US ranks third
in cotton lint production (2.8 million MT) behind China (7.5 million MT) and India (3.8 million
MT). The profitability of US cotton has been greatly improved since the commercial
introduction of GM HT cultivars; between 1997 and 2008, the adoption of GM HT cotton
provided a cumulative net farm income benefit of $799 million. The majority of savings are
directly related to the replacement of expensive, soil-applied chemicals required for early
season weed control with the low cost, broad spectrum herbicide, glyphosate (Roundup). The
use of HT cotton cultivars has provided growers with supplementary returns beyond an
increase in farm profits (Norsworthy et al. 2012, Vencill et al. 2012). Studies evaluating the
implementation of GM crops state that the adoption of CT systems is one of the most
significant advantages of HT technology. Following the release of GM glyphosate-tolerant
cotton varieties (1997), the proportion of cotton acres managed to allow for >30% crop residue
remaining on the soil surface increased 109%. Proposed benefits of CT include: reduced soil
erosion, improved soil structure, increased carbon sequestration, enhanced moisture holding
capacity, and decreased surface water runoff. Studies also suggest that CT can reduce certain
grower input costs, such as those for fuel, labor, and machinery repair. The adoption of GM HT
cotton has also resulted in a decrease in herbicide use. Since 1997, the US, Australia, Argentina,
and South Africa have reduced the amounts of herbicide active ingredients applied in cotton by
3.4% (-6.3 million kg). Concurrently, the associated environmental impact quotient (EIQ) fell by
5.5%. The cumulative (1997 to 2008) value of non-pecuniary benefits derived from GM HT
cotton, such as the adoption of CT and reduced herbicide use, was an estimated $461 million
for US growers.
The rapid and widespread adoption of GM HT crop technology has not come without
problems; the unprecedented use of glyphosate over space and time has facilitated the
development of glyphosate resistance in 21 weed species worldwide, including Palmer
amaranth. In 2004, the existence of GR Palmer amaranth was confirmed at a 250 ha field site in
Macon County, GA. Production at this site had been a monoculture of glyphosate-tolerant
cotton where glyphosate, often applied at reduced rates, was employed as the sole means of
Vencill, W.K.; T.L. Grey; Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of
Further Herbicide Resistance in Georgia; Project Type: Working Group; This project primarily
addresses the 2013 Special Priority
weed control for at least seven years. Currently, GR Palmer amaranth infests more than 2
million ha in 10 states (Alabama, Arkansas, Georgia, Louisiana, Missouri, Mississippi, North
Carolina, New Mexico, South Carolina and Tennessee). Postemergence herbicide options for the
control of Palmer amaranth in cotton are extremely limited; Pyrithiobac has been, historically,
an effective means of controlling escaped Palmer amaranth however, there is now widespread
resistance to this herbicide throughout the SE US. Glufosinate is an additional tool for
controlling in-field Palmer amaranth populations that are resistant to glyphosate, pyrithiobac or
both, but only if it is applied to extremely small (<10 cm) weed seedlings.
GR Palmer amaranth has quickly become the single greatest threat to the economic
sustainability of cotton production. GA cotton growers spent in excess of $24/ha ($60/A or
$100 million statewide) for herbicides to control GR Palmer amaranth; this represents a cost
increase of at least 200%, relative to the expenditures for controlling the susceptible biotype of
the species. Concurrently, the amount of herbicide active ingredient applied increased by 150%
(4.25 lb ai to 10.625 lb ai). Even with this level of herbicide input, 92% of these growers hand
weeded 54% of the statewide crop at an average cost of $10/ha ($24/A). The number of
growers returning to tillage and in-field cultivation for the management of GR Palmer amaranth
is steadily increasing; in 2010 50% of the cotton acreage in GA received tillage prior to planting
(including deep tillage and/or herbicide incorporation). Widespread GR Palmer amaranth has
promoted the development of other herbicide-resistant crop cultivars such as glufosinateresistant and auxinic-resistant cotton and soybean. We need to know the risk of herbicide
resistance development to these herbicides.
Objectives:

Survey weed populations in 20 fields in five geographic regions of Georgia and
determine resistance to six herbicide mechanisms of action

Determine potential risk of most common weed species in Georgia agricultural
production to develop herbicide resistance
Procedures:
Vencill, W.K.; T.L. Grey; Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of
Further Herbicide Resistance in Georgia; Project Type: Working Group; This project primarily
addresses the 2013 Special Priority
1) A field survey was carried out in Fall 2013 (Table 1) once mature seed were ready to be
harvested. In each field, a transect was run and mature plants were collected, bulked in a
cotton bag, and air-dried. Plants from each site was hand-threshed to avoid sample crosscontamination. After a chilling period at 4 C, seeds were planted in 52 by 26 by 5 cm flats
containing a potting mixture. A minimum total of 100 F1 seedlings (3 to 5 cm tall) per
population (three flats or replicates per experiment run and repeated) were be sprayed with
glyphosate, imazapic , fomesafen, glufosinate, atrazine, 2,4-D and sethoxydim for grass weeds
according using moving-nozzle cabinet sprayer equipped with a flat-fan nozzle tip (TeeJet
8002VS, Spraying Systems Co., Wheaton, IL) calibrated to deliver 200 L ha-1143 of spray solution
at 200 kPa in a single pass over the foliage. A discriminatory dose (1x and 2x the labeled rate)
was applied when plants were at 2-4 leaf stage. Plants were visually rated for survival 2 WAT
using a scale of 0-100 with 0 as no injury and 100 complete kill. For those populations surviving
a discriminatory dose, a dose response study was carried out to further analyze the level of
herbicide resistance.
Vencill, W.K.; T.L. Grey; Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of
Further Herbicide Resistance in Georgia; Project Type: Working Group; This project primarily
addresses the 2013 Special Priority
Table 1. Location of sites, crop, tillage, and weeds collected.
Field
County
Latitude
SW001
Terrell
31.69333
SW002
Terrell
SW003
Longitude
Tillage
Weeds Collected
-84.4225
Crop
(2013)
peanut
convtl
31.655
-84.46583
cotton
convtl
Palmer amaranth,Texas millet,barnyardgrass, cutleaf
groundcherry, coffee senna
Palmer amaranth, crowfootgrass
Berrien
31.34917
-83.38806
cotton
convtll
Palmer amaranth, Florida beggarweed
SW004
Colquitt
31.16556
-83.63917
peanut
convtl
Florida beggarweed
SW005
Colquitt
31.07889
-83.81944
cotton
strip-till
Palmer amaranth
SW006
Berrien
31.14567
-83.17087
Corn
Convtl
Palmer amaranth
M001
Bleckley
32.4925
-83.34889
peanut
strip-till
M002
Bleckley
32.45972
-83.36722
cotton
strip-till
Palmer amaranth, Texas millet, crowfootgrass, sicklepod,
cutleaf groundcherry, coffee senna
Palmer amaranth
M003
Peach
32.56583
-83.91167
soybean
strip-till
Palmer amaranth, sicklepod
M004
Taylor
32.55583
-84.07139
cotton
strip-till
Palmer amaranth, sicklepod, coffee senna
M005
Macon
32.49028
-84.05472
cotton
strip-till
Palmer amaranth, sicklepod, coffee senna
M006
Dooly
32.21944
-83.93944
cotton
strip-till
Palmer amaranth, sicklepod, coffee senna
SE01
Burke
33.08727
-82.0483
peanut
convtl
Palmer amaranth, goosegrass, Texas millet
SE02
Burke
33.08657
-82.0461
peanut
convtl
Palmer amaranth, Texas millet., crowfootgrass
SE03
Burke
33.09179
-82.06795
cotton
convtl
Palmer amaranth
SE04
Burke
33.08806
-82.02694
peanut
convtl
large crabgrass, goosegrass
SE05
Burke
33.09884
-82.10721
peanut
convtl
hairy fleabane, yellow nutsedge, annual sedge
SE06
Burke
33.03575
-82.07956
noncrop
SE07
Burke
32.93642
-82.16745
cotton
convtl
Palmer amaranth
SE08
Burke
32.92808
-82.14848
cotton
convtl
Palmer amaranth
SE09
Burke
32.93078
-82.15701
cotton
convtl
Palmer amaranth
SE10
Jefferson
33.14567
-82.45687
soybean
convtl
Palmer amaranth
SE11
Glascock
33.30519
-82.52494
soybean
convtl
Palmer amaranth
NE01
Elbert
34.2198
-82.99521
soybean
convtl
Sicklepod, large crabgrass
NE02
Hart
34.26438
-82.89561
soybean
convtl
NE03
Madison
34.02975
-82.24683
soybean
convtl
Palmer amaranth, Cocklebur, sicklepod, large crabgrass,
crowfootgrass
Palmer amaranth
NE04
Madison
34.02975
-82.24683
soybean
convtl
Palmer amaranth
NE05
Elbert
34.20672
-82.91484
soybean
convtl
Palmer amaranth
Palmer amaranth
Vencill, W.K.; T.L. Grey; Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of
Further Herbicide Resistance in Georgia; Project Type: Working Group; This project primarily
addresses the 2013 Special Priority
Results
As expected, Palmer amaranth was the dominant weed (86% sampled fields had Palmer
amaranth present) found in fields during the sampling process (Table 2). This data agrees with
recent Southern Weed Science Society surveys that show Palmer amaranth to be the most
common as well as troublesome weed in Georgia annual crops (Webster 2009). In addition,
glyphosate-resistant Palmer amaranth was the most predominate herbicide resistance type
found in this survey with 69% of the Palmer amaranth populations sampled exhibiting
glyphosate resistance. No other weeds examined were found to glyphosate resistant.
Table 2. Results of screening of collected weed response to six herbicide MOA. Weeds were
considered resistant if <50% control at 4x labeled field use rate.
Weed
Resistance (% samples collected)
% Fields
Group 5
Group 9
Group 10
Group 14
collected (ACCase) (auxins)
(PS inhib)
(EPSP)
(GS)
(PPO)
86
NA
0
3
69
0
0
Sicklepod
24
NA
0
3
0
0
0
Coffee senna
21
NA
0
0
0
0
0
Cutleaf
7
NA
0
0
0
0
0
3
NA
0
0
0
0
0
3
NA
0
0
0
0
0
14
0
NA
0
0
0
0
Crowfootgrass 10
0
NA
0
0
0
0
Palmer
Group 2
Group 4
amaranth
groundcherry
Florida
beggarweed
Annual
fleabane
Texas millet
Vencill, W.K.; T.L. Grey; Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of
Further Herbicide Resistance in Georgia; Project Type: Working Group; This project primarily
addresses the 2013 Special Priority
Barnyardgrass 3
0
NA
0
0
0
0
Goosegrass
3
0
NA
0
0
0
0
Large
3
0
NA
0
0
0
0
3
0
NA
0
0
0
0
crabgrass
Annual sedge
Atrazine resistance was found in two weed species. A population of Palmer amaranth was
found to be atrazine resistant (ED50 = 3.57 lb ai/A atrazine). Laboratory tests indicated the
population has metabolic resistance (enhanced glutathione transferase) rather than target-site
resistance. This is the first case of atrazine-resistant Palmer amaranth outside a dairy operation
in Georgia. A sicklepod population from Taylor Co. was found to be atrazine-resistant (ED50 =
3.0 lb ai/A) and also appears to have metabolic atrazine resistance. This is the first case of
reported herbicide resistance in sicklepod (Heap 2014). As a result of these herbicide resistance
cases, we will conduct a survey of fields in cooperation with the extension specialist for corn
(Dr. Eric Prostko) and county extension agents in southwest Georgia this year to screen for
other cases of atrazine-resistant Palmer amaranth and sicklepod.
Fortunately, no resistance was found to Group 2, 4, 10, and 14 herbicides in any of the weeds
collected. This will be helpful information as Group 10 (glufosinate) herbicides are widely being
used in the state and the introduction of auxinic-resistant crops will increase the selection
pressure in Group 4 herbicides.
Potential Outcomes and Impacts
As a result of this work, the first case of herbicide resistance in sicklepod has been discovered
and atrazine resistance has been discovered in Palmer amaranth in a situation where crops are
rotated. These results have been shared with extension personnel in the state so that they and
growers can be on the lookout for other cases of atrazine resistance in these weeds and have
been shared through popular media (Prostko, E.P. 2014. Palmer amaranth resistant to atrazine
Vencill, W.K.; T.L. Grey; Extent of Multiple Herbicide-Resistant Weeds and Predicting Risk of
Further Herbicide Resistance in Georgia; Project Type: Working Group; This project primarily
addresses the 2013 Special Priority
confirmed in Georgia. Southeast Farm Press, Jan. 29, 2014). In addition, we will be working with
local extension personnel this year to screen for other cases of herbicide-resistance in these
weeds.
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