Growth and Seed Production of Horseweed (Conyza canadensis) Populations Resistant to Glyphosate,...

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Growth and Seed Production of Horseweed (Conyza canadensis) PopulationsResistant to Glyphosate, ALS-Inhibiting, and Multiple (Glyphosate + ALS-Inhibiting) HerbicidesAuthor(s): Vince M. Davis, Greg R. Kruger, Jeff M. Stachler, Mark M. Loux, and William G. JohnsonSource: Weed Science, 57(5):494-504. 2009.Published By: Weed Science Society of AmericaDOI: http://dx.doi.org/10.1614/WS-09-024.1URL: http://www.bioone.org/doi/full/10.1614/WS-09-024.1

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Growth and Seed Production of Horseweed (Conyza canadensis) PopulationsResistant to Glyphosate, ALS-Inhibiting, and Multiple

(Glyphosate + ALS-Inhibiting) HerbicidesVince M. Davis, Greg R. Kruger, Jeff M. Stachler, Mark M. Loux, and William G. Johnson*

Horseweed populations with mixtures of biotypes resistant to glyphosate and acetolactate synthase (ALS)–inhibitingherbicides as well as biotypes with multiple resistance to glyphosate + ALS-inhibiting herbicides have been documented inIndiana and Ohio. These biotypes are particularly problematic because ALS-inhibiting herbicides are commonly tankmixed with glyphosate to improve postemergence horseweed control in soybean. The objective of this research was tocharacterize the growth and seed production of horseweed populations with resistance to glyphosate or ALS-inhibitingherbicides, and multiple resistance to glyphosate + ALS-inhibiting herbicides. A four-herbicide by four-horseweedpopulation factorial field experiment was conducted in the southeastern region of Indiana in 2007 and repeated in 2008.Four horseweed populations were collected from Indiana or Ohio and confirmed resistant to glyphosate, ALS inhibitors,both, or neither in greenhouse experiments. The four herbicide treatments were untreated, 0.84 kg ae ha21 glyphosate,35 g ai ha21 cloransulam, and 0.84 kg ae ha21 glyphosate + 35 g ai ha21 cloransulam. Untreated plants from horseweedpopulations that were resistant to glyphosate, ALS-inhibiting, or multiple glyphosate + ALS-inhibiting herbicides producedsimilar amounts of biomass and seed compared to populations that were susceptible to those herbicides or combination ofherbicides. Furthermore, aboveground shoot mass and seed production did not differ between treated and untreated plants.Nomenclature: Cloransulam; glyphosate; horseweed, Conyza canadensis L. ERICA; soybean, Glycine max L. Merr.Key words: Herbicide-resistant, fitness, fecundity, growth habit, biotype.

Horseweed can be found throughout southern Canada, theUnited States, and tropical America (Gleason and Croquist1991). A single horseweed plant can produce nearly 200,000seeds (Bhowmik and Bekech 1993; Regehr and Bazzaz 1979),which are wind dispersed over very long distances (Dauer etal. 2006; Shields et al. 2006). Horseweed has been anincreasingly common weed in no-till agriculture systems(Weaver 2001) and is one of the most common andproblematic weeds in Indiana (Davis et al. 2008; Gibson etal. 2005; Johnson et al. 2004). It is difficult to control insoybean because of a lack of effective postemergenceherbicides prior to the introduction of glyphosate-resistant(GR) soybean (Bruce and Kells 1990; Moseley and Hagood1990; VanGessel et al. 2001). Horseweed is also among thetop 10 most important herbicide-resistant weed speciesworldwide (Heap 2008). Owen and Zelaya (2005) statedherbicide resistance in a weed like horseweed should beconsidered a worst-case scenario, given its incompletelyautogamous nature and its ability to disperse seeds over longdistances.

Horseweed biotypes resistant to glyphosate are nowdocumented in over one-third of the continental UnitedStates (Heap 2008) and have been found in up to 38% of no-till soybean production in some regions of Indiana (Davis etal. 2008). Biotypes resistant to acetolactate synthase (ALS)–inhibiting herbicides (ALSR) have also been documented tooccur frequently in the eastern corn belt (Kruger et al. 2008;Trainer et al. 2005). These different herbicide-resistantbiotypes sometimes have overlapping geographical confines(Loux et al. 2006). Furthermore, populations with multipleresistance to glyphosate + ALS-inhibiting herbicides have beendocumented from Ohio and Indiana (Heap 2008; Kruger etal. 2008). This development is problematic because glypho-

sate and ALS-inhibiting herbicides are the first and secondmost commonly used postemergence herbicides in soybeanproduction (U.S. Department of Agriculture–National Agri-cultural Statistics Service [USDA–NASS] 2008). Further-more, ALS-inhibiting herbicides are commonly tank mixedwith glyphosate to enhance postemergence horseweed controlin soybean.

Differences in growth and seed production betweenherbicide-resistant and -susceptible weed biotypes can affectthe persistence and frequency of herbicide-resistant biotypesin a population. If relative growth and reproductive fitness ofresistant biotypes are reduced, the frequency of resistantbiotypes will be reduced over time absent selection pressure(Green 2007; Maxwell et al. 1990). It has been shown inherbicide-resistant weeds with a negative fitness penalty likeatrazine-resistant jimsonweed (Datura stramonium L.) thatcrop management decisions can be used to exploit the fitnesspenalty to reduce the frequency of resistant biotypes (Williamset al. 1995; Zhang et al. 1999). No previous research hascompared the relative growth and seed production ofhorseweed populations with single resistance to glyphosate,ALS-inhibiting (ALSR) herbicides, or multiple resistance toglyphosate + ALS-inhibiting (G + AR) herbicides. Littlefitness penalty in horseweed might be expected, as Zelaya etal. (2004) reported that few differences were observed betweenGR and glyphosate-susceptible (GS) horseweed biotypes inthe greenhouse absent exposure to glyphosate. Furthermore,Davis and Johnson (2008) found GR horseweed biotypesproduced up to 72,000 seeds plant21 in competition withsoybean. In other weeds, Westhoven et al. (2008) found nofitness penalty in common lambsquarters (Chenopodiumalbum L.) biotypes with elevated glyphosate tolerance. Inaddition, Tranel and Wright (2002) stated biotypes rarelyhave a reduction in fitness to ALS-inhibiting herbicides. Alack of fitness penalty has been demonstrated in other weeds,such as common sunflower (Helianthus annuus L.) and prairiesunflower (Helianthus petiolaris Nutt.; Massinga et al. 2005)and downy brome (Bromus tectorum L. BROTE.; Park et al.

DOI: 10.1614/WS-09-024.1* First, second, and fifth authors, Department of Botany and Plant Pathology,

Purdue University, West Lafayette, IN 47907; third and fourth authors,Department of Horticulture and Crop Science, Ohio State University,Columbus, OH 43210. Corresponding author’s E-mail: [email protected]

Weed Science 2009 57:494–504

494 N Weed Science 57, September–October 2009

2004). The hypothesis of this experiment was growth and seedproduction of horseweed populations with single resistance toglyphosate and ALS-inhibiting herbicides, or multipleresistance to glyphosate + ALS-inhibiting herbicides, wouldbe similar without herbicide exposure. Therefore, resistantbiotypes would have a growth and seed production advantageover susceptible biotypes after exposure to labeled herbicideuse rates.

Materials and Methods

A field study was conducted in 2007 and repeated in anadjacent area of the same field in 2008 at the SoutheastPurdue Agriculture Center (SEPAC) located near Butlerville,IN (39u029N, 85u329W). Plots were established on a poorlydrained Avonburg silt loam soil (fine–silty, mixed, active,mesic Aeric Fragic Glossaqualfs) with 2.1% organic matter,cation exchange capacity (CEC) of 6.2, and pH of 7.Treatments were arranged as a four-by-four factorial in arandomized complete block design with four replications.Factors were four horseweed populations by four herbicidetreatments. One horseweed population was resistant toglyphosate (GR), one was resistant to ALS-inhibitingherbicides (ALSR), one was resistant to tank mixes ofglyphosate + ALS-inhibiting herbicides (G + AR), and onewas susceptible (SUS) to both herbicides. SUS, GR, andALSR field populations of horseweed were collected during asurvey of Indiana soybean fields during September andOctober of 2003, 2004, and 2005 (Davis et al. 2008). The G+ AR population was from Ohio and was provided by JeffStachler and Mark Loux with The Ohio State University.Populations were a composite collection of 40 flower heads asrecommended by Beckie et al. (2000). Populations werecharacterized for glyphosate resistance by Davis et al. (2008)and resistance to ALS-inhibiting herbicides by Kruger et al.(2008) prior to establishing this experiment. Furthermore, themechanism that confers resistance to ALS inhibitors wascharacterized as a substitution of Glu for Asp at position 376in the ALSR population (designated as population 116;Zheng et al. 2007). Mutations conferring resistance in the GRand G + AR populations are unknown. Herbicide treatmentswere untreated, 0.84 kg ae ha21 glyphosate,1 35 g ai ha21

cloransulam,2 and 0.84 kg ae ha21 glyphosate + 35 g ai ha21

cloransulam. Herbicides were diluted in tap water with theaddition of 2.8 kg ha21 of ammonium sulfate and 0.25% v/vnonionic surfactant.

Horseweed seeds were planted on the surface of acommercial potting soil3 in 25- by 50- by 5-cm variegatedplastic trays and placed in the greenhouse. Natural light wassupplemented with 400-watt clear high-pressure sodiumlamps that provided for a 12-h photoperiod. Daytime andnighttime greenhouse temperatures were 27 and 14 C,respectively. Plants were allowed to grow for 14 to 21 d toreach 0.5 to 1 cm wide and then were transplanted to 36-mm-diam peat pellets.4 Plants continued growth in the greenhousefor an additional 15 d in 2007, and 30 d in 2008. Thirtyhorseweed plants, each in peat pellets, were transplanted to thefield at 3- to 5-cm rosette width on May 23, 2007 and at 4- to7-cm rosette width on June 2, 2008. Germination timing andtransplant sizes closely simulated emergence and growth ofnatural populations at this field location, where populationsprimarily emerge in the spring (Davis and Johnson 2008).

Plot size was 1.5 by 1.8 m and replicated four times. Eachplot had 3 rows with 10 horseweed plants row21. Rows werespaced 0.6 m apart, and plants within a row were spaced15 cm apart.

The field study area was rotary tilled each year prior to thetransplant of horseweed seedlings. Plants were watered imme-diately after transplanting with sprinkler irrigation and as neededthroughout the season with drip irrigation.5 Additional weedsin the field area were controlled throughout the growingseason to limit interspecific competition. In 2007, broadleafweeds were removed by hand and grass weeds were controlledwith postemergence applications of 225 g ai ha21 fluazifop +fenoxaprop6 in mid-June, mid-July, and mid-August. Thestudy area for 2008 was sterilized with methyl bromide–chloropicrin7 mixture (33% chloropicrin) at 450 kg ai ha21 inmid-September 2007. Therefore, very few weeds were present in2008, so they were removed by hand. Herbicides were applied in140 L ha21 carrier volume at 138 kPa with a 2.3-m-wide CO2

pressurized backpack sprayer with XR11002 flat fan nozzles.8

Treatments were sprayed on July 16 and 24 in 2007 and 2008,respectively, when horseweed plants were between 5 and 10 cmtall. On the day of herbicide application (0 d after treatment[DAT]), plant rosette widths, heights, leaf surface areas, freshweights, and dry weights were measured for three plants inuntreated checks.

Visually determined control ratings, plant heights, leafsurface areas, fresh weights, and dry weights were measured forthree plants in each plot at 2, 6, and 12 wk after herbicidetreatments were applied (WAT). Whole plots and individualplants were visually rated by assessing general plant health andvigor incorporating visible biomass reduction and plantchlorosis and necrosis as compared to the untreated plants. Ascale of 0 to 100 was used, with 0 representing no apparentherbicidal effects and 100 representing plant death. Plantwidths were measured at the widest point across the center ofthe plant from leaf tip to leaf tip with green healthy leaves. Plantheights were measured from the base of the plant at the soilsurface to the top of the apical meristem. Plants in the rosettestage were recorded as 1-cm stem height. Leaves were removedfrom harvested plants and surface area plant21 was measuredwith the use of a LI-3100 model LI-COR leaf area meter.9

Plant leaves and stems were placed in paper bags and dried in aforced-air dryer for 7 to 9 d at 38 C. Seed production was alsoassessed at the 12 WAT timing. Flower heads (inflorescences)were severed from the primary and secondary apical meristemsand placed in separate paper bags to dry in the greenhousetemperatures for 10 to 14 d. Flower heads were dried, threshedby hand, and total seed mass per plant measured. Threeunopened capitula from each of the lower, middle, and upperportion of the horseweed inflorescences were harvestedseparately in the field and placed in plastic petri dishes.Capitula were allowed to dry in ambient room temperatures(23 C). Each capitulum was forced open with forceps, and theseeds were counted under a low-power dissecting microscope todetermine seeds capitulum21.

Statistical Analysis. All dependent response variables wereexamined, especially for the herbicide-resistant populations,which, when subjected to the herbicides to which they wereresistant, exhibited a high level of variability. Because thisvariability helps explain the difficult nature of biologicalresponses, especially during adaptation under selection

Davis et al.: Horseweed growth and fecundity N 495

pressure, variability was examined with the use of PROCUNIVARIATE in SAS version 9.1.10 Raw data are displayedin box-and-whisker plots. All data except visual ratings andseeds capitulum21 were log transformed prior to meanseparation as suggested by the Box-Cox procedure (Box et al.1978). Data were analyzed as a mixed model factorial with theuse of PROC MIXED in SAS (Littell et al. 2006). Horseweedpopulations and herbicide treatments were considered fixedeffects, whereas year and replications within year wereconsidered random effects. First-order interactions betweenhorseweed populations and herbicide treatments were alwayshighly significant (P , 0.001). The generated least-squaresmeans were sliced by main effects and separated with the useof the Tukey-Kramer multiple pairwise comparison test atP , 0.05 in the PDMIX800 SAS macro, which convertsleast-squares means output to letter groupings (Saxton 1998).Response variable least-squares means were backtransformedfor presentation.

Results and Discussion

Horseweed Growth and Seed Production Absent Exposureto Herbicides. Plant Width and Height. Horseweed rosettewidths ranged from 16 cm at the 25th percentile (Q1) to29 cm at the 75th percentile (Q3) for all populations at thetime herbicides were applied in mid-July each year (Figure 1).The average widths were 19, 23, 19, and 24 cm for the SUS,

ALSR, GR, and G + AR populations, respectively (data notshown). Horseweed plant heights ranged from 4 cm at thefirst quartile of the SUS and ALSR populations to 12 cm atthe third quartile for the G + AR population at the timeherbicides were applied (Figure 1). A few plants in the SUS,GR, and G + AR populations had not yet bolted by the sprayapplication timing indicated by the 1-cm height data(Figure 1). The average horseweed plant heights were 5, 5,5, and 8 cm for the SUS, ALSR, GR, and G + ARpopulations, respectively, and the G + AR population wastaller than the other three populations (Table 1).

Horseweed plant heights for untreated check plants at2 WAT averaged 7, 7, 7, and 11 cm for the SUS, ALSR,GR, and G + AR populations, respectively (Table 1). The G+ AR population was taller than the GR population, butnot taller than the SUS or ALSR populations, as was thecase at 0 DAT. There were no height differences amongthe four populations at 6 or 12 WAT, and they rangedfrom 18 to 43 and 50 to 98 cm tall at 6 and 12 WAT,respectively (Table 1). The (Q1 to Q3) height ranges at the12 WAT data-collection timing (mid-October) were 89 to110, 82 to 100, 80 to 95, and 91 to 118 cm for the SUS,ALSR, GR, and G + AR populations, respectively (Figure 2).For all untreated plants, plant heights were significantly(P , 0.05) taller by 138 to 140%, 260 to 390%, and 210 to340% between 0 to 2, 2 to 6, and 6 to 12 WAT, respectively(data not shown). Furthermore, horseweed plant heights were

Figure 1. Box-and-whisker plots for horseweed width (a), leaf area (b), height (c), and dry weight (d) accumulation offour horseweed populations grown in a field experiment near Butlerville, IN. Data were collected prior to herbicideapplication (0 DAT) in the middle of July 2007 and 2008. Horseweed populations were either susceptible (SUS),resistant to acetolactate synthase–inhibiting herbicides (ALSR), resistant to glyphosate (GR), or multiply resistant toglyphosate + acetolactate synthase–inhibiting herbicides (G + AR) and are labeled across the x-axes.


normally distributed and fit (P , 0.0001) a linear relation-ship with WAT in Equation 1:

horseweed height cmð Þ~{3:9z6:8x, r2~0:65, ½1�where x 5 weeks after treatment (WAT).

Leaf Area Accumulation. Horseweed leaf area plant21 rangedfrom 129 cm2 at the first quartile of the GR population to1,135 cm2 for the third quartile of the G + AR population atthe time herbicides were applied (0 DAT; Figure 1). Averageleaf area of the G + AR population was greater than the GRpopulation with 830 versus 280 cm2 plant21, respectively, at0 DAT (Table 2). The SUS and ALSR populations were notdifferent from the G + AR or GR populations. There were nodifferences in leaf area between populations at 2, 6, or 12WAT (Table 2). For all untreated plants from any popula-tion, leaf area was significantly (P , 0.05) higher at 2 WATcompared to the spray application timing (data not shown).Leaf area increased (P , 0.05) for all populations between 2and 6 WAT, and then decreased (P , 0.05) between 6 and12 WAT (data not shown). Therefore, highest average leafarea values were observed at 6 WAT and ranged from 1,110to 2,090 cm2 plant21 for all populations (Table 2).

Dry Weight Accumulation. Plant fresh weights and plant dryweights were highly correlated at 0 DAT (r2 5 0.9528), 2WAT (r2 5 0.9812), 6 WAT (r2 5 0.9746), and 12 WAT (r2

5 0.9890); therefore, only data for plant dry weights arepresented for brevity. Horseweed plant dry weights rangedfrom 3 g plant21 at the first quartile of the SUS population to16 g plant21 at the third quartile of the G + AR populationwhen herbicides were applied (0 DAT; Figure 1). There wereno differences in dry weight accumulation between allherbicide-resistant populations, and the G + AR populationhad more average (mean) dry weight plant21 at 0 DAT than

the SUS population (Table 3). However, there were nodifferences in g plant21 dry weight production between anypopulation at 2, 6, or 12 WAT (Table 3). Dry weightaccumulation for each of the populations was also analyzed bythe classical growth analysis (Radford 1967) as in Equation 2:

relative growth rate~ln W2ð Þ{ln W1ð Þ

t2{t1, ½2�

where W1 and W2 were the least-squares means of the plantdry weights in g plant21 at times t1 and t2. Equation 2 can berearranged to calculate biomass production between timeintervals with Equation 3:

W2~W1er t2{t1ð Þ, ½3�

where relative growth rate is represented by the parameter r.Relative growth rates (RGR) were 0.211, 0.126, 0.123, and0.168 for the SUS, ALSR, GR, and G + AR populations,respectively, for the 12-wk growth interval between applica-tion timing and harvest. These RGR values were not differentsignificantly different at P , 0.05; therefore, the grand meanRGR for untreated plants of all four populations was 0.157.

Seed Production. There were no differences in average (mean)horseweed seeds plant21 between populations for untreatedplants (Table 4). The G + AR population had the largestvariability in horseweed seeds plant21, with some untreatedplants producing a maximum of nearly 500,000 seeds plant21

(Figure 2). Horseweed log10 seeds plant21 for all untreatedplants were normally distributed and fit a linear relationship(P , 0.0001) with plant height in Equation 4:

log10 seeds plant{1~1:1129z0:0343x, r2~0:38, ½4�

where x 5 plant height (cm). Reproductive efficiency (seedsplant21 g21 aboveground plant dry weight) was also normally

Table 1. Plant height of horseweed populations that were either susceptible (SUS), resistant to acetolactate synthase–inhibiting herbicides (ALSR), resistant to glyphosate(GR), or multiply resistant to glyphosate + acetolactate synthase–inhibiting herbicides (G + AR) in a field experiment located near Butlerville, IN, in 2007 and 2008.Herbicides were applied when plants reached 5 to 10 cm in height.

Plant height of horseweed populationsa

Herbicide treatments Rate SUS ALSR GR G + AR

g ha21 ----------------------------------------------------------------- cm plant21 -----------------------------------------------------------------

0 DATb

Untreated — 5 B 5 B 5 B 8 A

2 WAT

Untreated — 7 aAB 7 aAB 7 aB 11 aAGlyphosate 840 ae 0.4 b 1 b 5 a 8 aCloransulam 35 ai 1 b 5 a 1 b 10 aGlyphosate + cloransulam 840 ae + 35 ai 0.2 b 1 b 1 b 8 a

6 WAT

Untreated — 27 aA 24 aA 18 aA 43 aAGlyphosate 840 ae 1 b 0.3 b 15 a 22 abCloransulam 35 ai 0.4 b 13 a 0.2 b 25 abGlyphosate + cloransulam 840 ae + 35 ai 0.3 b 1 b 1 b 15 b

12 WAT

Untreated — 78 aA 50 aA 62 aA 98 aAGlyphosate 840 ae 2 b 1 b 25 a 66 aCloransulam 35 ai 2 b 16 a 1 b 51 aGlyphosate + cloransulam 840 ae + 35 ai 1 b 1 b 2 b 55 a

a Least-squares means were separated with the use of the Tukey-Kramer multiple pairwise comparison test at P , 0.05 sliced by main effects. Therefore, means withina column followed by the same lower-case letter, and means within rows (presented for untreated checks only) followed by the same upper-case letter, are not significantlydifferent.

b Abbreviations: DAT, days after treatment; WAT, weeks after treatment.


distributed and fit a linear relationship (P 5 0.0006) withplant height in Equation 5:

seeds g{1 dry weight~{1,510z24:7x, r2~0:20, ½5�where x 5 plant height (cm).

Three capitula were harvested from each of the lower,middle, and upper portion of mature horseweed inflorescenc-es at the harvest collection timing to determine seedscapitulum21. There were no significant three- or two-wayinteractions between location and capitulum were harvestedfrom the inflorescences, herbicides, or populations. The maineffect of location from which the capitula were harvested wasalso not significant (P 5 0.54). Therefore, seeds capitulum21

data were combined for the three inflorescence locations fromwhich capitula were harvested. Herbicide and populationmain effects were significant at P 5 0.0282 and P , 0.0001,respectively. Average (mean) seeds capitulum21 was 32, 24,14, and 30 in untreated checks for the SUS, ALSR, GR, andG + AR population, respectively (Table 4). The mean seedscapitulum21 for the untreated check plants across allpopulations was 25 6 1.9 (data not shown).

Previous research documented that naturally occurringhorseweed plants at the location these experiments wereconducted primarily emerge in the spring and complete a lifecycle as summer annual weeds (Davis and Johnson 2008).

Germination timing in the greenhouse (mid- to late April)and transplant timing (mid-May to early June) was designedto simulate spring emergence and seedling sizes of naturalpopulations at this field location closely. The greenhouseemergence timing and early development of our greenhouse-propagated plants and natural, on-site plant emergence wassimilar during both years of the study (data not shown).However, after horseweed seedlings were transplanted to thefield site, the greenhouse-propagated plants did not bolt(advance vertical apical meristem growth) as rapidly as didnatural on-site spring-emerged horseweed plants. The firstquartile for plant heights ranged from 4 to 5 cm for allpopulations, and a few plants had not yet bolted in the middleof July, which was 6 to 8 wk after transplanting (Figure 1).Typically, spring-emerging horseweed plants in the field donot form a rosette, but they initiate vertical stem elongationquickly after emergence (Bhowmik and Bekech 1993; Buhlerand Owen 1997; Davis and Johnson 2008). Smisek (1995)examined fitness between paraquat-resistant and susceptiblehorseweed biotypes in Ontario, Canada, and also reportedthat 10% of horseweed plants that were transplanted from agreenhouse to a field study failed to bolt. Biotic or abioticfactors that affect the process of bolting are not wellunderstood. Nonetheless, overall growth was not impeded asplants were also wider than expected in relation to natural, on-

Figure 2. Box-and-whisker plots for horseweed height (a), leaf area (b), dry weight (c), and seed production (d) of fourhorseweed populations grown in a field experiment near Butlerville, IN. Data were collected 12 wk after application withfour herbicide treatments, in mid-October, 2007 and 2008. Horseweed populations were either susceptible (SUS),resistant to acetolactate synthase–inhibiting herbicides (ALSR), resistant to glyphosate (GR), or multiply resistant toglyphosate + acetolactate synthase-inhibiting herbicides (G + AR) and are labeled across the x-axes. These populationswere repeated on each x-axis for the four herbicide treatments (labeled across the top of the plots), which were untreated,0.84 kg ae ha21 glyphosate, 35 g ai ha21 cloransulam, or 0.84 kg ae ha21 glyphosate + 35 g ai ha21 cloransulam.


site plants for this time of year, with rosettes up to 41-cmwidth (Figure 1).

Untreated plants did bolt and significantly increased inheight by 138 to 140% between mid-July (0 DAT) and earlyAugust (2 WAT; Table 1). However, cm2 leaf area plant21

did not increase during the same interval, indicating theinitiation of bolting and early vertical stem elongation may bea major energy sink. There were no differences in plant height

at 6 or 12 WAT and no differences in leaf area plant21 at 2, 6,or 12 WAT between the populations (Tables 1 and 2). Atharvest timing in the middle of October, all untreated plantsaveraged nearly a meter tall, which is within the expectedrange of 0.1 to 2 m (Regehr and Bazzaz 1979). A difference inaboveground dry matter accumulation was also only observedat the spray application timing where the G + AR populationhad more dry weight plant21 than the SUS population with

Table 2. Leaf area production of horseweed populations that were either susceptible (SUS), resistant to acetolactate synthase–inhibiting herbicides (ALSR), resistant toglyphosate (GR), or multiply resistant to glyphosate + acetolactate synthase–inhibiting herbicides (G + AR) in a field experiment located near Butlerville, IN, in 2007 and2008. Herbicides were applied when plants reached 5 to 10 cm in height.

Leaf area production of horseweed populationsa


g ha21 ----------------------------------------------------------------------------- cm2 plant21 ------------------------------------------------------------------------------

0 DATb

Untreated — 400 AB 470 AB 280 B 830 A

2 WAT

Untreated — 560 aA 460 aA 290 aA 810 aAGlyphosate 840 ae , 10 b , 10 b 250 a 490 aCloransulam 35 ai , 10 b 140 a 10 b 890 aGlyphosate + cloransulam 840 ae + 35 ai , 10 b , 10 b 20 b 460 a

6 WAT

Untreated — 1,870 aA 1,120 aA 1,110 aA 2,090 aAGlyphosate 840 ae , 10 b , 10 b 310 a 240 aCloransulam 35 ai , 10 b 220 a , 10 b 870 aGlyphosate + cloransulam 840 ae + 35 ai , 10 b , 10 b , 10 b 340 a

12 WAT

Untreated — 720 aA 330 aA 680 aA 850 aAGlyphosate 840 ae , 10 b , 10 b 110 a 240 aCloransulam 35 ai , 10 b 50 a , 10 b 320 aGlyphosate + cloransulam 840 ae + 35 ai , 10 b , 10 b , 10 b 330 a



Table 3. Dry weight production of horseweed populations that were either susceptible (SUS), resistant to acetolactate synthase inhibiting herbicides (ALSR), resistant toglyphosate (GR), or multiple resistant to glyphosate + acetolactate synthase-inhibiting herbicides (G + AR) in a field experiment located near Butlerville, IN, in 2007 and2008. Herbicides were applied when plants reached 5 to 10 cm in height.

Dry weight production of horseweed populationsa


g ha21 ------------------------------------------------------------------------g plant21 ------------------------------------------------------------------------

0 DATb

Untreated — 5.4 B 9.8 AB 5.6 AB 10.4 A

2 WAT

Untreated — 13.7 aA 11.6 aA 7.3 abA 12.9 aAGlyphosate 840 ae 0.4 b 0.5 b 8.7 a 12.3 aCloransulam 35 ai 0.4 b 6.9 a 1.9 c 15.8 aGlyphosate + cloransulam 840 ae + 35 ai 0.03 b 0.4 b 2.5 bc 12.0 a

6 WAT

Untreated — 29.9 aA 24.6 aA 20.1 aA 32.4 aAGlyphosate 840 ae 0.1 b 0.1 b 7.8 a 12.1 aCloransulam 35 ai 0.02 b 11.0 a 0.1 b 25.4 aGlyphosate + cloransulam 840 ae + 35 ai 0.1 b 0.1 b 0.2 b 10.5 a

12 WAT

Untreated — 68.2 aA 39.1 aA 24.9 aA 81.7 aAGlyphosate 840 ae 1.0 b 0.3 b 9.7 a 36.0 aCloransulam 35 ai 0.9 b 9.4 a 0.6 b 35.0 aGlyphosate + cloransulam 840 ae + 35 ai 0.5 b 0.2 b 1.1 b 45.5 a




no difference between any herbicide-resistant population(Table 3). Furthermore, all populations had similar rates ofrelative growth (RGR). These data might suggest the mostimportant growth advantages for the herbicide-resistantbiotypes were prior to the first data collection timing (0DAT). Westhoven et al. (2008) also found accelerated earlyseason growth development was an advantage for biotypeswith increased tolerance to glyphosate in common lambs-quarters. Untreated plants from a glyphosate-resistant, ALSinhibitor-resistant, and multiple (glyphosate + ALS inhibitor)-resistant population grew and accumulated similar amounts ofbiomass to populations that were susceptible to thoseherbicides or combination of herbicides.

The average horseweed seeds plant21 of the G + ARpopulation was 2-fold, 6-fold, and 24-fold the amountproduced by the SUS, ALSR, and GR populations,respectively. However, the horseweed seeds per plant21 datawere variable (Figure 2) and there were no differences in theaverage seeds plant21 produced between populations (Ta-ble 4). Furthermore, the G + AR population produced arange maximum nearing 500,000 seeds plant21 (Figure 2),which is more seeds plant21 than has been documented fromprevious field research experiments. Bhowmik and Bekech(1993) conducted no-till field experiments with horseweed inDeerfield, MA and documented horseweed produced up to200,000 seed plant21. Regehr and Bazzaz (1979) documentedhorseweed plants grown in no-till field experiment in Urbana,IL produced an average between 1,300 and 50,700 seedsplant21 and a maximum of 230,000 seeds plant21. Further-more, in a previous no-till field experiment conducted at thesame site as this one, we documented glyphosate-resistanthorseweed produced less than 1,000 to 72,000 seeds plant21

when grown in competition with soybean (Davis and Johnson2008). Bhowmik and Bekech (1993) suggested seed productionwas density dependent and horseweed plants produced 200,000and 187,000 seeds plant21 at densities of 10 and 50 plantsm22, respectively. Plants in this experiment were establishedwithin that density range at 32 plants m22. In our previousresearch, we documented that glyphosate-resistant horseweed

produced less than 1,000 to 72,000 seeds plant21 when grownin competition with soybean (Davis and Johnson 2008). Wesuggested seeds produced plant21 was influenced by horseweedplant height in relationship to the surface of the soybean canopywhere plants above the soybean canopy ($ 50 cm) producedgreater than 88% of the total horseweed seed production insoybean (Davis and Johnson 2008).

Regehr and Bazzaz (1979) documented there was a linearrelationship between horseweed seeds produced plant21 tomature plant height for plants that ranged 40 to 200 cm tallin a no-till field experiment in Urbana, IL. Their Equation 6(which was interpreted based on the equation [log10 seedsplant21 5 2.705 + 0.001x] and the published figure, whichdid not match the figure in Regehr and Bazzaz 1979) was:

log10 seeds plant{1~2:705z0:01x ½6�where x 5 plant height in centimeters. Therefore, plants intheir study produced 300 seeds cm21 plant height for matureplants greater than 40 cm. In contrast, minimum (5%) andmaximum (95%) height levels across all untreated plants inthis experiment ranged from 61 to 127 cm. Therefore,Equation 4 would estimate horseweed seed production in thisexperiment was 4,400 seeds plant21 for mature plants greaterthan 61 cm. The research conducted by Regehr and Bazzaz(1979) was completed prior to the commercial sale ofglyphosate (Franz 1985) and over two decades prior to thefirst documentation of a GR horseweed population (Van-Gessel 2001). Therefore, it should be assumed that Equation6 modeled seed production for a horseweed populationsusceptible to glyphosate. The GR population in thisexperiment produced numerically the least average (mean)seeds at 3,900 seeds plant21 (Table 4) and had an average(mean) height of 62 cm (Table 1). Equation 6 documentedby Regehr and Bazzaz (1979) would estimate 2,100 seeds plant21

should be expected, which is approximately half the number ofseeds the GR population produced. Therefore, the GRpopulation in this experiment not only produced equivalentseeds plant21 to two populations susceptible to glyphosate, butproduced more than would be expected compared to a

Table 4. Seed production of horseweed populations that were either susceptible (SUS), resistant to acetolactate synthase–inhibiting herbicides (ALSR), resistant toglyphosate (GR), or multiply resistant to glyphosate + acetolactate synthase–inhibiting herbicides (G + AR) in a field experiment located near Butlerville, IN, in 2007 and2008. Herbicides were applied when plants reached 5 to 10 cm in height.

HerbicideSeed production of horseweed populationsa

Treatments Rate SUS ALSR GR G + AR

g ha21 ---------------------------------------------------------------------------- seeds plant21 -----------------------------------------------------------------------------

Untreated — 52,400 aA 16,300 aA 3,900 aA 94,100 aAGlyphosate 840 ae , 100 b 0 c 200 a 28,000 aCloransulam 35 ai , 100 b 200 b , 100 b 6,100 aGlyphosate + cloransulam 840 ae + 35 ai , 100 b 0 c , 100 b 27,000 a

----------------------------------------------------------------------- seeds capitulum21 ------------------------------------------------------------------------

Untreated — 32 24 14 30Glyphosate 840 ae 31 0 18 27Cloransulam 35 ai 29 19 15 25Glyphosate + cloransulam 840 ae + 35 ai 5 0 14 28Column meansb 24 AB non-est 15 B 27 A


b The herbicide-by-population interaction was not significant (P 5 0.12), and therefore column means followed by the same upper-case letter are not significantlydifferent under the Tukey-Kramer multiple pairwise comparison test at P , 0.05. No plants survived in the ALSR population when glyphosate was applied, so least-squares means for the ALSR population were not estimated (non-est).


glyphosate-susceptible horseweed population studied over threedecades prior.

Regehr and Bazzaz (1979) also documented that reproduc-tive efficiency had an inverse relationship with horseweedplant height. In contrast, this experiment found reproductiveeffort (seeds g21 aboveground dry weight) increased as plantheight increased (Equation 5). However, based on visualassessment of the figure supporting the negative relationshipreported by Regehr and Bazzaz (1979), as well as an r2 of 0.20(Equation 5) in this research, more dependent factors inaddition of g dry weight plant21 and plant height may beneeded to explain reproductive effort in horseweed. Reproduc-tive output per capitulum was also measured in this experiment.Untreated plants from all populations produced an average of25 seeds capitulum21 (Table 4) with no differences in seedscapitulum21 due to the location (lower, middle, or upper) onthe inflorescence from which capitula were harvested. Twenty-five seeds capitulum21 is similar to data collected during ourprevious experiment conducted at the same location with the useof a naturalized, on-site GR population. We previouslydocumented spring-emerging GR plants produced 32 seedscapitulum21 grown in competition with soybean (Davis andJohnson 2008). In contrast, Smisek (1995) reported 60 to 70horseweed seeds capitulum21 for plants grown in greenhouseconditions, and Thebaud and Abbott (1995) reported 68 seedscapitulum21 for plants grown in a common garden experiment.Nonetheless, differences in seeds produced capitulum21 wereonly significant when pooled across herbicide applications, andmay be an unfair comparison, because some populations wereaffected more by some of the herbicides than others. Inconclusion, horseweed populations with single resistance toglyphosate and ALS-inhibiting herbicides, or multiple resistanceto glyphosate + ALS-inhibiting herbicides produced similaramounts of seeds to populations that were susceptible to thoseherbicides or a combination of herbicides. Furthermore, allherbicide-resistant populations produced more seed than wouldbe expected based on plant height with a previous experimentconducted by Regehr and Bazzaz (1979) on a presumedsusceptible population.

Horseweed Growth and Seed Production FollowingExposure to Herbicides. Plant Height. The glyphosatetreatment reduced the average (mean) plant heights of theSUS and ALSR populations by greater than 85, 94, and 97% at2, 6, and 12 WAT, respectively (Table 1). However, theglyphosate treatment did not reduce average plant heights ofthe GR or the G + AR populations versus the untreated check atany of these timings. The cloransulam treatment reduced theaverage plant heights of the SUS and GR populations by greaterthan 85, 98, and 98% at 2, 6, and 12 WAT, respectively. Thecloransulam treatment did not reduce average plant heights ofthe ALSR or the G + AR populations versus the untreatedcheck at any of these timings. Likewise, the glyphosate +cloransulam treatment reduced plant heights of the SUS,ALSR, and GR populations by greater than 85, 94, and 97%observed at 2, 6, and 12 WAT, respectively. The glyphosate +cloransulam treatment also reduced plant heights of the G + ARpopulation at 6 WAT, but not at 2 or 12 WAT (Table 1).

Leaf Area Accumulation. Herbicide treatment effects observedon horseweed plant height were similar for leaf areaaccumulation. The glyphosate treatment reduced the average

(mean) leaf area plant21 of the SUS and ALSR populations bygreater than 98, 99, and 97% at 2, 6, and 12 WAT,respectively (Table 2). However, the glyphosate treatment didnot reduce average leaf area plant21 of the GR or the G + ARpopulations versus the untreated check at any of these timings.The cloransulam treatment reduced the average leaf areaplant21 of the SUS and GR populations by greater than 96,98, and 98% at 2, 6, and 12 WAT, respectively. Likewise, theglyphosate + cloransulam treatment reduced leaf area plant21

of the SUS, ALSR, and GR populations by greater than 93,99, and 97% observed at 2, 6, and 12 WAT, respectively.However, the glyphosate + cloransulam treatment did notreduce leaf area plant21 of the G + AR population at any datacollection timing (Table 2).

Dry Weight Accumulation. Herbicide treatment effects ob-served on plant height and leaf area accumulation were similarfor dry weight accumulation. The glyphosate treatmentreduced the average (mean) g dry weight plant21 of theSUS and ALSR populations by greater than 94, 99, and 98%at 2, 6, and 12 WAT, respectively (Table 3), whereas theglyphosate treatment did not reduce average g plant21 dryweight of the GR or the G + AR populations versus theuntreated check at any of these timings. The cloransulamtreatment reduced the average dry weight plant21 of the SUSand GR populations by greater than 97, 99, and 98% at 2, 6,and 12 WAT, respectively. Likewise, the glyphosate +cloransulam treatment reduced dry weight plant21 of theSUS, ALSR, and GR populations by greater than 65, 99, and95% observed at 2, 6, and 12 WAT, respectively. However,the glyphosate + cloransulam treatment did not reduce g dryweight plant21 of the G + AR population at any datacollection timing (Table 3).

Visual Plant Ratings. Visually assessed control ratings forwhole-plot evaluations and mean averages of ratings of theindividual plant evaluations were correlated at 2 WAT (r2 50.8765), 6 WAT (r2 5 0.8854), and 12 WAT (r2 5 0.8527).Therefore, mean control rating data collected from visualobservations of individual plants are reported for brevity. Forall horseweed populations that were susceptible to glyphosateor cloransulam, greater than 90% control was achieved at alldata collection timings (Table 5). Some control was alsonoted for herbicide-resistant populations as well, but the levelof control was always significantly less than susceptiblepopulations. Glyphosate provided 28, 64, and 68% controlon the GR population at 2, 6, and 12 WAT, respectively(Table 5). Glyphosate provided 27, 64, and 37% control onthe G + AR population 2, 6, and 12 WAT, respectively.Cloransulam provided 39, 66, and 62% control on the ALSRpopulation at 2, 6, and 12 WAT, respectively. Cloransulamprovided 29, 55, and 38% control on the G + AR populationat 2, 6, and 12 WAT, respectively. The glyphosate +cloransulam treatment provided $ 93% control on theSUS, ALSR, and GR populations and 36, 71, and 38%control on the G + AR population at 2, 6, and 12 WAT,respectively. The only time control was improved by tank-mixing glyphosate + cloransulam was noted for the G + ARpopulation at the 6 WAT data collection timing. Glyphosate+ cloransulam together provided 71% control versus 55%control from cloransulam alone (Table 5).


Seed Production. There were no seeds plant21 produced at thethird quartile for the SUS population for all three herbicidetreatments (Figure 2). However, there were a few plants in theSUS population that produced seeds following herbicidetreatments. Two plants survived the glyphosate treatment andproduced almost 7,000 seeds plant21, one plant survived thecloransulam treatment and produced 44,000 seeds, and oneplant survived the glyphosate + cloransulam treatment andproduced 143 seeds resulting in less than 100 mean seedsplant21 for all treatments (Table 4). Cloransulam reducedaverage (mean) seed production of the ALSR population to 200seeds plant21 versus 16,300 seeds plant21 for untreated plants(Table 4). The ALSR population produced 44,000 seedsplant21 following cloransulam treatment at the third quartile(Figure 2). The glyphosate treatment did not reduce seedproduction of the GR population versus untreated GR plantsand produced an average (mean) of 200 seeds plant21 (Table 4)and 26,000 seeds plant21 at the third quartile (Figure 2). Theglyphosate, cloransulam, or glyphosate + cloransulam treat-ments did not reduce the average (mean) seed production of theG + AR population (Table 4). The G + AR populationproduced 288,000, 190,000, and 197,000 seeds plant21 at thethird quartile following the glyphosate, cloransulam, andglyphosate + cloransulam treatments, respectively (Figure 2).

There was no interaction between herbicide treatments andhorseweed populations for seeds capitulum21. Average(mean) seeds capitulum21 across all herbicide treatmentswere 24 6 3.1, 15 6 3.6, and 27 6 1.8 for the SUS, GR, andG + AR populations, respectively (Table 4). The least-squaresmean for ALSR population was not estimated, because therewere no plants that produced capitula following herbicidetreatments with glyphosate, but the average seeds capitulum21

was 22 6 2.9 combined from the untreated and cloransulamtreatments. The G + AR population produced more seedscapitulum21 than the GR population (Table 4).

In summary, there were no differences between untreatedand treated average mean plant heights, leaf area accumula-tion, or biomass accumulation within any herbicide-resistantpopulation when treated with the normal use rate of theherbicide to which they were classified to be resistant.Furthermore, the only herbicide-resistant population withreduced seed production due to a herbicide with expectedresistance was the cloransulam treatment on the ALSRpopulation (Table 4). Therefore, the populations classifiedas resistant to glyphosate, cloransulam, or glyphosate +cloransulam responded as expected, resistant. However, in allherbicide-resistant populations low levels of herbicide injurywere visually observed following application of the herbicideof classified resistance (e.g., 28 to 68% visual control in theGR population was noted in the glyphosate treatment;Table 5). Moreover, there was always a slight suppression ofseeds plant21 for herbicide-resistant populations followingherbicide application.

The seeds plant21 averages for both the mean and thirdquartile were presented because it is likely these field-collectedpopulations were still segregating for the frequency andexpression of herbicide-resistant alleles. Therefore, the seedproduction of the 25% plants above the third quartile may bea better indicator of the seed production potential of the entireherbicide-resistant population following additional herbicideselection pressure events. Other research has demonstratedinherent variability in other weed species following herbicideexposure, particularly glyphosate. DeGennaro and Weller(1984) identified field bindweed (Convolvulus arvensis L.)biotypes that varied up to 70% in visual injury, root and shootdry weight, and shoot regrowth when exposed to a 2.24-kg ha21

application of glyphosate. Smith and Hallett (2006) showedcommon waterhemp (Amaranthus rudis Sauer) populationscollected from Illinois and Indiana had 10% plant survival to0.63 kg ae ha21 in populations suspected to be resistant. The

Table 5. Visual control of horseweed populations that were either susceptible (SUS), resistant to acetolactate synthase–inhibiting herbicides (ALSR), resistant toglyphosate (GR), or multiply resistant to glyphosate + acetolactate synthase–inhibiting herbicides (G + AR) in a field experiment located near Butlerville, IN, in 2007 and2008. Herbicides were applied when plants reached 5 to 10 cm in height.

Visual control of horseweed populationsa


g ha21 ----------------------------------------------------------------g plant21 ----------------------------------------------------------------

0 DATb

Untreated — 0 0 0 0

2 WAT

Untreated — 0 b 0 c 0 c 0 bGlyphosate 840 ae 96 a 97 a 28 b 27 aCloransulam 35 ai 91 a 39 b 93 a 29 aGlyphosate + cloransulam 840 ae + 35 ai 98 a 96 a 93 a 36 a

6 WAT

Untreated — 0 b 0 c 0 c 0 cGlyphosate 840 ae 98 a 99 a 64 b 64 abCloransulam 35 ai 99 a 66 b 99 a 55 bGlyphosate + cloransulam 840 ae + 35 ai 99 a 99 a 98 a 71 a

12 WAT

Untreated — 0 b 0 c 0 c 0 bGlyphosate 840 ae 95 a 100 a 68 b 37 aCloransulam 35 ai 96 a 62 b 99 a 38 aGlyphosate + cloransulam 840 ae + 35 ai 100 a 100 a 98 a 38 a

a Least-squares means were separated with the use of the Tukey-Kramer multiple pairwise comparison test at P , 0.05, sliced by main effects. Therefore, means withina column followed by the same lower-case letter, and means within rows (presented for untreated checks only) followed by the same upper-case letter, are not significantlydifferent.



10% survival rate was too low to consider these populations,as a whole, to be GR; however, surviving plants were clonedand found to be GR up to nine times the putative susceptibleclones and GR90 values were estimated to be 13 to 16kg ae ha21 (Smith and Hallett 2006). Moreover, Norsworthyet al. (2008) demonstrated survival data were variable in GRPalmer amaranth (Amaranthus palmeri S. Wats.) accessionsfrom Arkansas in field populations. Perhaps the variation inherbicide response (and not just the mean average response)should be of greater importance in future characterizations ofherbicide-resistant weed populations, as it demonstrates theprogression of selection pressure for particular traits. Further-more, the observable selection pressure responses for herbi-cide-resistant traits within these populations were mostdetectable as control based on visual observations rather thanany one quantitative plant measurement.

Implications of Growth and Seed Production of Herbi-cide-Resistant Horseweed Populations. This experimentcompared the relative growth and reproductive output ofhorseweed populations with single and multiple herbicideresistance to glyphosate and acetolactate synthase–inhibitingherbicides which are the two most widely used postemergenceherbicide mode-of-actions for the control of horseweed insoybean. Future herbicide-resistant crop technologies perhapsmay further encourage tank-mix combinations with these twoherbicides as a resistance management strategy (Green 2007).If relative growth and reproductive fitness of horseweed withsingle and multiple resistance to these herbicides werereduced, it might be expected that the frequency of resistancewould also decrease in the absence of selection pressure(Green 2007; Maxwell et al. 1990). One limitation of thisresearch was that only a limited number of compositehorseweed collections, designated as populations fromagriculture fields and classified as unique based on likeherbicide-resistant properties, were characterized. However,because of the similar variation in growth and seed productionpotentials of the populations absent exposure to herbicides, weconclude that horseweed populations composed of biotypeswith single resistance to glyphosate and ALS-inhibitingherbicides, or multiple resistance to glyphosate + ALS-inhibiting herbicides have similar growth and seed productionpotential. Furthermore, the variation within these herbicide-resistant populations following exposure to herbicides wouldsuggest that repeated applications will only increase the abilityof these populations to compete and reproduce followingrepeated applications of the same herbicide or combination ofherbicides.

To control these herbicide-resistant horseweed populations,and to offset the evolution of more herbicide-resistant weeds,multiple integrated weed management practices need to beimplemented with the idea that resistant biotypes will not justdisappear after growers stop the application of these herbicidemodes of action. Unfortunately, Boerboom (1999) suggestedthat tank-mixing multiple herbicide modes of action isperhaps the most common way producers alter theirmanagement practices to control herbicide-resistant weeds.This may be encouraged by future advancements in seedtechnologies to resist applications of multiple broad-spectrumherbicides (Green 2007). Green (2007) also suggested thesetank-mixing tactics usually only occur after the frequency ofresistance has reached high levels. As evidenced by the

competitive and prolific horseweed population with multipleresistance to glyphosate + ALS-inhibiting herbicides, post-emergence tank-mixing herbicide modes of action may not beenough to prevent the development of even more perplexingweed control challenges in horseweed. Glyphosate resistancein weed species evolved in crop production areas of the worldwith the most adoption of glyphosate-resistant crops (Powles2008). As we adopt future herbicide-resistant crop technol-ogies, producers and practitioners must remember thatrepeated use of the same herbicide tank-mix combination isstill repeated use of a single weed control strategy.Competitive and prolific multiple herbicide-resistant species,demonstrated by horseweed in this research, may evolve. Assuggested by other authors, strategies including herbicide andcrop rotations, tillage, preplant and preemergence applicationtimings, as well as using the appropriate rates for the mostdifficult-to-control weeds should be considered to control orreduce the occurrence of herbicide resistance evolution(Beckie 2006; Boerboom 1999; Green 2007; Knezevic andCassman 2003; Neve 2007; Roush et al. 1990; Sammons etal. 2007).

Sources of Materials1 Glyphosate, Roundup OriginalH, Monsanto Company, St.

Louis, MO 63167.2 Cloransulam, FirstRateH, Dow AgroSciences, LLC, Indianapo-

lis, IN 46268.3 Potting soil, Sun-Gro Redi-Earth Plug and Seedling Mix, Sun-

Gro Horticulture, Bellevue, WA 98008.4 Jiffy-7 peat pellets, Jiffy Products of America Inc., Norwalk,

OH 44857.5 T-Tape, T-Systems International, Inc., San Diego, CA 92121.6 Fluazifop + fenoxaprop FusionH, Syngenta Crop Protection,

Inc., Greensboro, NC 27409.7 Methyl bromide–chloropicrin MBC 67-33, Agriliance, LLC,

P.O. Box 64089, St. Paul, MN 55164.8 TeeJet flat fan XR11002 nozzles, Spraying Systems Company,

Wheaton, IL 60189.9 Model LI-3100 leaf area meter, LI-COR, Inc., Lincoln, NE

68504.10 SAS software, Version 9.1, 2002–2003, SAS Institute Inc.,

Cary, NC 27513.

Literature Cited

Beckie, H. J. 2006. Herbicide-resistant weeds: management tactics and practices.Weed Technol. 20:793–814.

Beckie, H. J., I. M. Heap, R. J. Smeda, and L. M. Hall. 2000. Screening forherbicide resistance in weeds. Weed Technol. 14:428–445.

Bhowmik, P. C. and M. M. Bekech. 1993. Horseweed (Conyza canadensis) seedproduction, emergence and distribution in no-till and conventional-tillagecorn (Zea mays). Agronomy (Trends Agric. Sci.) 1:67–71.

Boerboom, C. M. 1999. Nonchemical options for delaying weed resistance toherbicides in Midwest cropping systems. Weed Technol. 13:636–642.

Box, G.E.P., W. G. Hunter, and J. S. Hunter. 1978. Statistics for Experimenters:An Introduction to Design, Data Analysis, and Model Building. New York:Wiley.

Bruce, J. A. and J. J. Kells. 1990. Horseweed (Conyza canadensis) control in no-tillsoybean (Glycine max) with preplant and preemergence herbicides. WeedTechnol. 4:642–547.

Buhler, D. D. and M.D.K. Owen. 1997. Emergence and survival of horseweed(Conyza canadensis). Weed Sci. 45:98–101.

Dauer, J. T., D. A. Mortensen, and R. Humston. 2006. Controlled experimentsto predict horseweed (Conyza canadensis) dispersal distances. Weed Sci.54:484–489.


Davis, V. M., K. D. Gibson, and W. G. Johnson. 2008. A field survey todetermine distribution and frequency of glyphosate-resistant horseweed(Conyza canadensis) in Indiana. Weed Technol. 22:331–338.

Davis, V. M. and W. G. Johnson. 2008. Glyphosate-resistant horseweed (Conyzacanadensis) emergence, survival, and fecundity in no-till soybean. Weed Sci.56:231–236.

DeGennaro, F. P. and S. C. Weller. 1984. Differential susceptibility of fieldbindweed (Convolvulus arvensis) biotypes to glyphosate. Weed Sci. 32:472–476.

Franz, J. E. 1985. Discovery, development and chemistry of glyphosate. Pages3–7 in E. Grossbard and D. Atkinson, eds. The Herbicide Glyphosate.London: Butterwork and Company.

Gibson, K. D., W. G. Johnson, and D. E. Hillger. 2005. Farmer perceptions ofproblematic corn and soybean weeds in Indiana. Weed Technol.19:1065–1070.

Gleason, H. A. and A. Croquist. 1991. Manual of Vascular Plants ofNortheastern United States and Adjacent Canada. Bronx, NY: New YorkBotanical Garden. 592 p.

Green, J. M. 2007. Review of glyphosate and ALS-inhibiting herbicide cropresistance and resistant weed management. Weed Technol. 21:547–558.

Heap, I. M. 2008. International Survey of Herbicide Resistant Weeds. http://www.weedscience.com. Accessed: November 2008.

Johnson, B., J. Barnes, K. Gibson, and S. Weller. 2004. Late-season weed escapesin Indiana soybean fields. Crop Manag. DOI: 10.1094/CM-2004-0923-01-BR. http://www.plantmanagementnetwork.org. Accessed: November 2008.

Knezevic, S. Z. and K. G. Cassman. 2003. Use of herbicide-tolerant crops as acomponent of an integrated weed management program. Crop Manag. DOI:10.1094/CM-2003-0317-01-MG.

Kruger, G. R., V. M. Davis, W. G. Johnson, and S. C. Weller. 2008. Response ofselected Indiana horseweed (Conyza canadensis) populations to glyphosate andcloransulam. Proc. N. Cent. Weed Sci. Soc. 63:124.

Littell, R. C., G. A. Milliken, W. W. Stroup, R. D. Wolfinger, and O.Schabenberger. 2006. SAS for Mixed Models. 2nd ed. Cary, NC: SASInstitute.

Loux, M., J. Stachler, B. Johnson, G. Nice, V. Davis, and D. Nordby. 2006.Biology and Management of Horseweed. In The Glyphosate, Weeds, and CropSeries. GWC-9. http://www.ces.purdue.edu/extmedia/GWC/GWC-9-W.pdf.Accessed: March 2008.

Massinga, R. A., K. Al-Khatib, P. S. Amand, and J. F. Miller. 2005. Relativefitness of imazamox-resistant common sunflower and prairie sunflower. WeedSci. 53:166–174.

Maxwell, B. D., M. L. Roush, and S. R. Radosevich. 1990. Predicting theevolution and dynamics of herbicide resistance in weed populations. WeedTechnol. 4:2–13.

Moseley, C. M. and E. S. Hagood, Jr. 1990. Horseweed (Conyza canadensis)control in full-season no-till soybeans (Glycine max). Weed Technol.4:814–818.

Neve, P. 2007. Challenges for herbicide resistance evolution and management: 50years after Harper. Weed Res. 47:365–369.

Norsworthy, J. K., R. C. Scott, K. L. Smith, and L. R. Oliver. 2008. Response ofnortheastern Arkansas Palmer amaranth (Amaranthus palmeri) accessions toglyphosate. Weed Technol. 22:408–413.

Owen, M.D.K. and I. A. Zelaya. 2005. Herbicide-resistant crops and weedresistance to herbicides. Pest Manag. Sci. 61:301–311.

Park, K. W., C. A. Mallory-Smith, D. A. Ball, and G. W. Mueller-Warrant.2004. Ecological fitness of acetolactate synthase inhibitor-resistant and-susceptible downy brome (Bromus tectorum) biotypes. Weed Sci. 52:768–773.

Powles, S. B. 2008. Evolved glyphosate-resistant weeds around the world: lessonsto be learnt. Pest Manag. Sci. 64:360–365.

Radford, P. J. 1967. Growth analysis formulae—their use and abuse. Crop Sci.7:171–175.

Regehr, D. L. and F. A. Bazzaz. 1979. The population dynamics of Erigeroncanadensis, a successional winter annual. J. Ecol. 67:923–933.

Roush, M. L., S. R. Radosevich, and B. D. Maxwell. 1990. Future outlook forherbicide-resistance research. Weed Technol. 4:208–214.

Sammons, R. D., D. C. Heering, N. Dinicola, H. Glick, and G. A. Elmore.2007. Sustainability and stewardship of glyphosate and glyphosate-resistantcrops. Weed Technol 21:347–354.

Saxton, A. M. 1998. A macro for converting mean separation output to lettergroupings in Proc Mixed. In Proceedings of the 23rd SAS Users GroupInternational. Cary, NC: SAS Institute. Pp. 1243–1246.

Shields, E. J., J. T. Dauer, M. J. VanGessel, and G. Neumann. 2006. Horseweed(Conyza canadensis) seed collected in the planetary boundary layer. Weed Sci.54:1063–1067.

Smisek, A. 1995. Resistance to paraquat in Erigeron canadensis L. M.S. thesis.Ontario: University of Western Ontario.

Smith, D. A. and S. Hallett. 2006. Variable response of common waterhemp(Amaranthus rudis) populations and individuals to glyphosate. Weed Technol.20:466–471.

Thebaud, C. and R. J. Abbott. 1995. Characterization of invasive Conyza species(Asteraceae) in Europe: quantitative trait and isozyme analysis. Am. J. Bot.82:360–368.

Trainer, G. D., M. M. Loux, S. K. Harrison, and E. Regnier. 2005. Response ofhorseweed biotypes to foliar applications of cloransulam-methyl andglyphosate. Weed Technol. 19:231–236.

Tranel, P. J. and T. R. Wright. 2002. Resistance of weeds to ALS-inhibitingherbicides: what have we learned? Weed Sci. 50:700–712.

[USDA–NASS] U.S. Department of Agriculture–National Agricultural StatisticsService. 2008. Agricultural Chemical Use Database. www.pestmanagement.info/nass. Accessed: November 2008.

VanGessel, M. J. 2001. Glyphosate-resistant horseweed in Delaware. Weed Sci.49:703–705.

VanGessel, M. J., A. O. Ayeni, and B. A. Majek. 2001. Glyphosate in full seasonno-till glyphosate-resistant soybean: role of preplant applications and residualherbicides. Weed Technol. 15:714–724.

Weaver, S. E. 2001. The biology of Canadian weeds. 115. Conyza canadensis.Can. J. Plant Sci. 81:867–875.

Westhoven, A. M., G. R. Kruger, C. K. Gerber, J. M. Stachler, M. M. Loux, andW. G. Johnson. 2008. Characterization of selected common lambsquarters(Chenopodium album) biotypes with tolerance to glyphosate. Weed Sci.56:685–691.

Williams, M. M., N. Jordan, and C. Yerkes. 1995. The fitness cost of atrazineresistance in jimsonweed (Datura stramonium L.). Am. Midl. Nat.133:131–137.

Zelaya, I. A., M.D.K. Owen, and M. J. VanGessel. 2004. Inheritance of evolvedglyphosate resistance in Conyza canadensis (L.) Cronq. Theor. Appl. Genet.110:58–70.

Zhang, J., M. L. Salas, N. R. Jordan, and S. C. Weller. 1999. Biorationalapproaches to managing Datura stamonium. Weed Sci. 47:750–756.

Zheng, D., P. J. Tranel, V. M. Davis, G. R. Kruger, and W. G. Johnson. 2007.Target-site resistance to ALS inhibitors in horseweed. Proc. N. Cent. WeedSci. Soc. 62:34.

Received February 6, 2009, and approved April 26, 2009.


Growth and Seed Production of Horseweed (Conyza canadensis) Populations Resistant to Glyphosate,...

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