Biennial wormwood (Artemisia biennis) competition with soybean (Glycine max)

Transcription

1 Document generted on 01/21/2019 1:19.m. Phytoprotection Biennil wormwood (Artemisi biennis) competition with soyben (Glycine mx) Eric A. Nelson nd George O. Kegode Volume 86, Number 2, oût 2005 URI: id.erudit.org/iderudit/012512r See tble of contents Publisher(s) Société de protection des plntes du Québec (SPPQ) ISSN (print) (digitl) Explore this journl Article bstrct Biennil wormwood hs become serious weed of severl crops in the northern Gret Plins of the United Sttes nd Pririe Provinces of Cnd. Greenhouse replcement series experiments were conducted to investigte the effects of wtering regime (stressed nd nonstressed) nd nitrogen rte (50, 100, 150, nd 200 mg kg-1 of soil) on competition between soyben nd biennil wormwood. Soyben height ws reduced fter 9 weeks of competition with biennil wormwood compred with soyben grown in monoculture, wheres biennil wormwood plnts were tller when grown with soyben thn in monoculture. The chnge in plnt height indicted tht biennil wormwood height ws incresed due to interspecific competition, wheres soyben height ws reduced. When moisture ws limited, the reltive yield of biennil wormwood ws greter thn tht of soyben, indicting tht biennil wormwood ws more ggressive thn soyben. Soyben growth ws unffected by n increse in nitrogen rte, wheres biennil wormwood fresh weight ws 30% greter when the nitrogen rte ws incresed from 50 to 200 mg kg-1. Biennil wormwood ggressivity tended to increse s the nitrogen rte ws incresed from 50 to 200 mg kg-1. Overll results suggest tht the negtive impct of biennil wormwood competition with soybens under field conditions my increse when soil moisture is limited nd nitrogen fertility is incresed. Cite this rticle Nelson, E. & Kegode, G. (2005). Biennil wormwood (Artemisi biennis) competition with soyben (Glycine mx). Phytoprotection, 86 (2), Tous droits réservés L société de protection des plntes du Québec, 2005 This document is protected by copyright lw. Use of the services of Érudit (including reproduction) is subject to its terms nd conditions, which cn be viewed online. [ propos.erudit.org/en/users/policy-on-use/] This rticle is disseminted nd preserved by Érudit. Érudit is non-profit inter-university consortium of the Université de Montrél, Université Lvl, nd the Université du Québec à Montrél. Its mission is to promote nd disseminte reserch.

2 Biennil wormwood (Artemisi biennis) competition with soyben (Glycine mx) Eric A. Nelson 1 nd George O. Kegode 2 Received ; ccepted PHYTOPROTECTION 86 : Biennil wormwood hs become serious weed of severl crops in the northern Gret Plins of the United Sttes nd Pririe Provinces of Cnd. Greenhouse replcement series experiments were conducted to investigte the effects of wtering regime (stressed nd non-stressed) nd nitrogen rte (50, 100, 150, nd 200 mg kg -1 of soil) on competition between soyben nd biennil wormwood. Soyben height ws reduced fter 9 weeks of competition with biennil wormwood compred with soyben grown in monoculture, wheres biennil wormwood plnts were tller when grown with soyben thn in monoculture. The chnge in plnt height indicted tht biennil wormwood height ws incresed due to interspecific competition, wheres soyben height ws reduced. When moisture ws limited, the reltive yield of biennil wormwood ws greter thn tht of soyben, indicting tht biennil wormwood ws more ggressive thn soyben. Soyben growth ws unffected by n increse in nitrogen rte, wheres biennil wormwood fresh weight ws 30% greter when the nitrogen rte ws incresed from 50 to 200 mg kg -1. Biennil wormwood ggressivity tended to increse s the nitrogen rte ws incresed from 50 to 200 mg kg -1. Overll results suggest tht the negtive impct of biennil wormwood competition with soybens under field conditions my increse when soil moisture is limited nd nitrogen fertility is incresed. Keywords: Biennil wormwood, competition, moisture stress, nitrogen, replcement series, soyben. [Compétition entre l rmoise bisnnuelle (Artemisi biennis) et le soj (Glycine mx)] L rmoise bisnnuelle est devenue une importnte muvise herbe pour plusieurs cultures dns le nord des Grndes Plines des Étts-Unis et dns les provinces des Priries u Cnd. Des expériences vec des séries de remplcement ont été menées en serre fin d étudier les effets du régime d limenttion en eu (stressnt et non stressnt) et de l quntité d zote (50, 100, 150 et 200 mg kg -1 de sol) sur l compétition entre le soj et l rmoise bisnnuelle. Après 9 semines de compétition vec l rmoise bisnnuelle, l tille du soj étit inférieure à celle du soj en monoculture lors que l rmoise bisnnuelle étit plus grnde en présence de soj qu en monoculture. Les différences dns l tille des plntes montrent que l compétition interspécifique fit ugmenter celle de l rmoise bisnnuelle et diminuer celle du soj. Lorsque l eu étit un fcteur limittif, le rendement reltif de l rmoise bisnnuelle étit plus élevé que celui du soj, ce qui montre que l rmoise bisnnuelle étit plus gressive que le soj. L ugmenttion de l quntité d zote n ps ffecté l croissnce du soj. Cependnt, le poids de mtière frîche de l rmoise bisnnuelle étit 30 % plus élevé lorsque l quntité d zote est pssée de 50 à 200 mg kg -1. L gressivité de l rmoise bisnnuelle eu tendnce à ugmenter lorsque l quntité d zote est pssée de 50 à 200 mg kg -1. Globlement, les résultts montrent que les impcts négtifs de l compétition entre l rmoise bisnnuelle et le soj, dns des conditions nturelles, peuvent s ccroître lorsque l eu mnque et que l quntité d zote disponible ugmente. Mots clés : Armoise bisnnuelle, zote, compétition, séries de remplcement, soj, stress hydrique. 1. Deprtment of Plnt Sciences, North Dkot Stte University, Frgo, ND Current ddress: Hrvest Lnd Coopertive, 710 Front Street, Morgn, MN Deprtment of Plnt Sciences, North Dkot Stte University, 166 Loftsgrd Hll, Frgo, ND , USA; corresponding uthor e-mil: george.kegode@ndsu.edu 125

3 PHYTOPROTECTION 86 (2) 2005 INTRODUCTION Soyben (Glycine mx (L.) Merr.) hs become mjor oilseed crop of the northern Gret Plins of the United Sttes. Soyben production in North Dkot ws estimted t 1.5 x 10 6 h in 2004, suggesting 7.5 fold increse since 1990 (NDASS 2004). This increse in production re occurred prtly becuse of the decline in whet (Triticum estivum L.) nd brley (Hordeum vulgre L.) production, which ws prtly due to the incresed incidence of fusrium hed blight (Fusrium grminerum Schwbe) in the stte (US GAO 1999). In ddition, the Federl Agriculturl Improvement nd Reform Act brought severl chnges in the frm subsidy progrm in 1996 tht gve frmers more freedom in choosing crops (Bierlen et l. 2000). The expnded soyben production re hs provided new niches for invsive/problemtic weed species such s biennil wormwood. Biennil wormwood (Artemisi biennis Willd.) is ntive to the northwestern United Sttes nd western Cnd (Munz nd Keck 1968; Thoms 1961), but popultions hve been found everywhere within the United Sttes except in the southestern sttes (Hll nd Clements 1923). Biennil wormwood hs been problem in soyben nd dry edible ben (Phseolus vulgris L.) fields of North Dkot, South Dkot nd Minnesot for the pst severl yr (Kegode nd Christoffers 2003). Biennil wormwood hs been expnding its hbitt rnge nd hs recently been plced on Weeds to Wtch list by severl scientists of the North Centrl United Sttes (Johnson et l. 2004). Biennil wormwood exhibits both n nnul nd biennil life cycle (Fernld 1950). Mture biennil wormwood is tll, woody plnt nd prolific seed producer (Mhoney nd Kegode 2004; Stevens 1932). Biennil wormwood cn grow over 3 m tll nd produces thick stems, up to 5 cm in dim, tht my impede crop hrvest. Seson-long competition from 10 biennil wormwood plnts m -2 decresed the yield of soyben seeded in 76-cm rows by 44% (Nelson nd Kegode 2000). This reduction in soyben yield ws presumed to be both ttributble to direct competition with biennil wormwood nd impeded hrvest. Biennil wormwood infesttions hve incresed due to combintion of fctors. Mny herbicides used for brodlef weed control in soyben nd dry ben do not provide cceptble biennil wormwood control. Biennil wormwood is frequently misidentified s common rgweed (Ambrosi rtemisiifoli L.), nd herbicides used to control common rgweed do not control it (Kegode nd Christoffers 2003). Time of weed germintion nd emergence reltive to crop emergence sets in plce the dynmics of competition. Germintion of biennil wormwood hs not been well-chrcterized. Freshly hrvested biennil wormwood seeds pper to germinte best when seeds re exposed to mximum temperture of 37 C with diurnl temperture fluctution of 24 C or greter (Kegode et l. 2004). Biennil wormwood field emergence usully occurs in mid- to lte June in estern North Dkot (Kegode nd Cierni 2003) nd seedlings tht emerge lte re cpble of producing numerous seeds (Mhoney nd Kegode 2004). Fronning nd Kegode (2004) reported, however, tht biennil wormwood emergence cn vry depending on soil type. In tht study, emergence of biennil wormwood occurred in lte April in sndy lom soil, lte My to erly June in lom soil, nd lte June in silty cly soil. The hzrds rising from weeds in ny plnt production system my be viewed t two levels: reduced bsolute yield nd reduced vlue of tht product (Cousens nd Mortimer 1995). Typiclly, crop yield loss will ber proportionl reltionship to the bundnce of weeds nd the durtion nd severity of interference with the crop (Cousens 1985). The outcome of interference is determined by the time of emergence of weeds nd crops in reltion to diminishing resources such s soil moisture nd nitrogen. Soil moisture conditions cn gretly ffect seedling growth nd the competitive bility of weeds (Wiese nd Vndiver 1970). Ogg et l. (1994) reported tht pe (Pisum stivum L.) ws sensitive to soil wter content, nd its bility to compete with myweed chmomile (Anthemis cotul L.) ws impired by drought stress. Soil nitrogen level is n importnt fctor in weed crop interference (Okfor nd De Dtt 1976). Chnges in dry mtter production of perennil ryegrss (Lolium perenne L.) nd white clover (Trifolium repens L.) following nitrogen ppliction were ssocited with n increse in ggressivity of perennil ryegrss rther thn nitrogen-relted reduction in the reltive growth of white clover (Mrtin nd Field 1984). Weed competition studies re bsed on binry species mixtures, nd different indices re used to evlute competition. For instnce, the reltive yield totl (RYT) index consists of the ddition of crop nd weed reltive yields nd indictes resource complementrity (de Witt 1960; Rdosevich et l. 1997), wheres the ggressivity index includes the effect of competition on both the crop nd weed nd indictes competitive bility (Rdosevich et l. 1997). In the following experiments, the spect of reduction of yield ws evluted in the greenhouse under vrious conditions. The objectives of this reserch were to determine the effect of wtering regime nd nitrogen level on competition between biennil wormwood nd soyben. MATERIALS AND METHODS Generl greenhouse methodology Two greenhouse experiments were conducted to investigte the effects of wtering regime nd soil nitrogen concentrtion on competition between biennil wormwood nd soyben. Both experiments used replcement series (De Wit 1960) with five soyben:biennil wormwood rtios of 4:0, 3:1, 2:2, 1:3 nd 0:4 in rndomized complete block design. Biennil wormwood seeds hrvested from plnts tht hd grown in soyben field ner Fergus Flls, MN, in 1999 nd Trill soyben were used for both experiments. Soyben nd biennil wormwood were pregerminted to ensure uniformity of plnts t the commencement of competition. Biennil wormwood seeds 126

4 NELSON, KEGODE: BIENNIAL WORMWOOD COMPETITION IN SOYBEAN were plced on the soil surfce in 5-cm-dim by 8-cm-deep pet pots filled with 75:25 mixture of Foldhl lomy fine snd (sndy over lomy, mixed quic hploboroll):potting mixture (70-80% Cndin sphgnum pet moss, perlite, dolomitic limestone (for ph djustment), gypsum, nd wetting gent) nd wtered to field cpcity. Cler plstic ws plced over the pots for 3 d to increse the humidity nd temperture for uniform biennil wormwood germintion nd emergence (Mhoney nd Kegode 2004). This method of incubtion resulted in reltively fst nd uniform biennil wormwood emergence. Soyben ws seeded directly into the 28-cm-dim by 30-cm-deep pots filled with 10 kg of soil-potting mixture s outlined bove, t twice the number needed to conduct the experiment, nd thinned to the desired density 4 d fter emergence. Upon seedling emergence, the cler plstic ws removed nd biennil wormwood seedlings were wtered s needed. Biennil wormwood seedlings were thinned to three similr-sized plnts per pot 1 wk fter emergence nd trnsplnted into the pots contining soyben when both species were 5 cm tll. Becuse emergence time for biennil wormwood ws longer thn for soyben, biennil wormwood ws seeded 2 wk before soyben to chieve uniform height of both species t time of trnsplnting. To minimize the trnsplnt shock, the soil surrounding the roots of the biennil wormwood seedlings ws mintined intct during trnsplnting. Upon trnsplnting of biennil wormwood, the pots were wtered nd fertilized s dictted by the protocol for ech study nd plced in greenhouse set t temperture of 25 ± 2 C with 16-h photoperiod under nturl sunlight supplemented with high-pressure mercury hlide lmps (230 µmol m -2 s -1 ). The pots were rndomly moved round periodiclly to void position effects within the greenhouse. Effect of wtering regime on competition A two-fctor replcement series ws conducted in the greenhouse to evlute the effect of wtering regime nd plnt rtio on competition between soyben nd biennil wormwood. The five soyben:biennil wormwood plnt rtios described previously were used nd the wtering regimes were non-stressed, in which pots were wtered to field cpcity dily, nd wter-stressed, in which wter ws withheld until 75% of ll plnts showed signs of wilting. In this experiment, wtering ws ccomplished by dding wter until pots leked from below. The experiment ws configured s completely rndomized design with four replictes nd ws repeted three times. Soyben nd biennil wormwood were llowed to compete for 9 wk before plnt mesurements were mde. Field studies indicted tht significnt reduction in soyben yield occurred following 6 wk of competition with biennil wormwood (Nelson nd Kegode 2000). Consequently, the 9 wk period of competition in this study ws rbitrrily chosen to encompss the time frme in which soyben could compete with biennil wormwood nd show loss in productivity. After 9 wk, plnts were removed from pots, their roots wshed using wter from low pressure hose spry, nd whole plnt fresh weight, shoot nd root dry weights nd shoot length were mesured. Soyben nd biennil wormwood plnts were dried t 55 C for 7 d prior to obtining dry weights. Effect of nitrogen rte on competition A two-fctor replcement series experiment ws conducted to evlute the effect of nitrogen concentrtion nd plnt rtio on competition between soyben nd biennil wormwood. There were five soyben: biennil wormwood rtios nd four nitrogen levels of 50, 100, 150 nd 200 mg kg -1 soil. Nitrogen rtes were chosen bsed on the mount of nitrogen lredy in the soil nd djusting upwrds in even increments. The desired nitrogen rtes were obtined by dding 0, 500, 1000 nd 1500 mg of nitrogen s mmonium nitrte immeditely following trnsplnting of biennil wormwood seedlings. Ech pot contined 10 kg of Sioux sndy lom (sndy-skeletl mixed udorthentic hploboroll) which contined 50 mg kg -1 nitrogen s indicted by soil tests. The soil tests indicted tht soil phosphorus nd potssium levels were dequte for soyben growth: soil ph ws 6.5 nd soil orgnic mtter ws 3.5%. Pots were seled so s to prevent nutrient loss nd wter ws dded cutiously ech time so s to void flooding the pots. The experiment ws configured s completely rndomized design with three replictions nd ws repeted. All pots, fter plnting, were wtered to field cpcity dily. The two species were llowed to compete for 9 wk before plnts were removed from the pots. Their roots were wshed using wter from low pressure hose spry nd mesurements were tken, including soyben nd biennil wormwood whole plnt fresh nd dry weight, shoot length from the soil surfce, nd shoot, root, nd lef dry weight. Soyben nd biennil wormwood plnts were dried s previously described. Sttisticl nlysis Prior to dt nlysis, reltive yield, reltive yield totl nd ggressivity indices were determined from some of the growth prmeters mesured. Reltive yield (RY) of ech species ws clculted ccording to Rdosevich et l. (1997) s follows: RY = Y mix / Y mono where RY = reltive yield of species (), Y mix = yield for species () in mixture, nd Ymono = yield of species () in monoculture. Reltive yield totl (RYT) describes how species use resources in reltion to ech other (Rdosevich et l. 1997) nd ws determined by dding the reltive yields of biennil wormwood nd soyben for ech plnt rtio. Vlues of RYT ner 1 indicte tht the sme resources re being used, while RYT < 1 indictes mutul ntgonism nd RYT > 1 suggests voidnce or symbiosis. When RYT = 1, differences between the two species re still possible. For exmple, one species my be more ggressive thn the other nd contribute more thn expected to the totl yield, while the other contributes less thn expected. This unequl contribution indictes n interction between species for common resource nd tht one species gins more thn the other. Aggressivity defines the reltive success of two species in using resources nd provides mens to 127

5 evlute interference mong n rry of species (Rdosevich et l. 1997). Biennil wormwood ggressivity indices were clculted by subtrcting the soyben reltive yield from the biennil wormwood reltive yield, nd the indices cn rnge from 1 to -1. When two species re grown in 50:50 mixture, the ggressivity vlue will equl zero if the two species re competing eqully. The ggressivity vlue for biennil wormwood will be 0.5 when it outnumbers soyben by 3:1 rtio when both species re competing proportiontely. Likewise, the vlue for biennil wormwood will be -0.5 when the two species re competing proportiontely in 3:1 rtio of soyben: biennil wormwood. Anlysis of vrince procedures were conducted on dt from these two experiments nd mens were seprted using Fisher s protected LSD test t the 0.05 significnce level. The min effects for both greenhouse experiments were fixed effects. No tretments by experiment interctions were present; therefore, dt were combined over repetitions in time for ech experiment. Student s t-test t P 0.05 ws used to determine whether RYTs differed significntly from unity (1.0) nd whether ggressivity indices differed significntly from zero (equl competitors). Dry weight dt were used in our discussion primrily, though where there were no differences, fresh weight dt tht showed significnt differences were used ppropritely. RESULTS AND DISCUSSION Effect of wtering regime on competition Soyben plnts were 14% shorter when only one biennil wormwood plnt ws growing in competition with three soyben plnts compred with soyben grown lone nd were 25% shorter when three biennil wormwood plnts were growing with one soyben plnt (Tble 1). Inversely, biennil wormwood plnts were tllest when grown with two (2:2 rtio) or three (3:1 rtio) soyben plnts. Biennil wormwood plnts reched mximum height of 62 cm when one plnt grew with three soyben plnts. Biennil wormwood height in monoculture ws 11% shorter compred with biennil wormwood growing in mixture t the 3:1 soyben:biennil wormwood rtio. This indicted tht biennil wormwood height ws decresed more by intrspecific competition thn by interspecific competition with soyben. Bsed on dry plnt weights, ggressivity of biennil wormwood t 2:2 plnt rtio ws 0.27 nd not significntly different from 0, which indicted tht biennil wormwood ws eqully s ggressive s soyben t plnt rtios contining t lest 50% biennil wormwood (Tble 1). As expected, biennil wormwood ws more ggressive thn soyben when there were three biennil wormwood plnts in the mixture. Wtering regime, however, did not ffect the ggressivity of biennil wormwood reltive to soyben. In contrst, Ogg et l. (1994) reported tht decreses in vilble moisture incresed the ggressivity of myweed chmomile (Anthemis cotul L.) reltive to pe (Pisum stivum L.). Biennil wormwood under non-stressed conditions contributed 12% more thn expected to the reltive fresh weight yield t the 3:1 soyben:biennil wormwood rtio (Tble 2). All other plnt rtios in the nonstressed wtering regime provided the expected reltive yields of biennil wormwood. When moisture ws limited, biennil wormwood contributed 16, 25 nd 13% more thn expected to the reltive yield t the 3:1, 2:2 nd 1:3 soyben:biennil wormwood rtios, respectively. The increse in biennil wormwood reltive yield indictes tht biennil wormwood gined more of the common resources thn soyben, prticulrly when the two species were moisturestressed. The RYT of soyben plus biennil wormwood, competing t the five plnt rtios, ws not sttisticlly different from 1.0 when bsed on totl plnt dry weight (Fig. 1A), which indictes tht the sme resources were being used eqully by the two species. Similrly, myweed chmomile nd pe competed like t both low nd high wter levels (Ogg et l. 1994). Even though under moisture stress conditions myweed chmomile ws more ggressive Tble 1. Soyben nd biennil wormwood plnt height nd biennil wormwood ggressivity indices s influenced by five soyben:biennil wormwood rtios fter 9 wk of competition in replcement series experiment, verged over stressed nd non-stressed wtering regimes Plnt height (cm) Biennil Soyben:biennil Biennil wormwood wormwood rtio Soyben wormwood ggressivity,b PHYTOPROTECTION 86 (2) 2005 b 4:0 44 3: : : * 0: * LSD (0.05) Aggressivity bsed on plnt dry weights. Aggressivity defines the reltive success of the two species in using resources, where zero signifies the two species re competing eqully, -1 signifies complete dominnce of soyben, nd +1 signifies complete dominnce of biennil wormwood. Vlues followed by n sterisk re significntly greter thn zero ccording to one-tiled t-test. 128

6 NELSON, KEGODE: BIENNIAL WORMWOOD COMPETITION IN SOYBEAN Tble 2. Influence of soyben:biennil wormwood rtio nd stressed nd non-stressed wtering regime on the reltive yield of biennil wormwood, bsed on fresh weight fter 9 wk of competition with soyben in replcement series experiment Reltive yield Soyben:biennil Predicted Wtering regime b wormwood rtio reltive yield Non-stressed Stressed 4:0 3: : : : LSD (0.05) c Reltive yield clculted s the individul plnt fresh weight in mixture s proportion of the corresponding individul plnt fresh weight in monoculture. b Non-stressed tretments were wtered to field cpcity dily. Stressed tretments were wtered to field cpcity ech time 75% of plnts showed signs of wilting. c Lest significnt difference between wtering regimes nd mong plnt rtios t 5% probbility. thn pe, the RYTs of pe nd myweed chmomile were similr in vrying moisture levels regrdless of plnting rrngement. Biennil wormwood reltive yield t the 2:2 rtio ws greter thn expected bsed on shoot dry weight (dt not shown), nd t the 3:1 nd 2:2 soyben: biennil wormwood rtios when bsed on totl dry weight (Fig. 1A). Conversely, soyben t the 2:2 plnt rtio contributed less totl dry weight thn would be expected. Uneven contribution to the RYT indictes tht biennil wormwood is gining more of the common resources thn soyben. At the 2:2 plnt rtio, biennil wormwood contributed 12% more to the RYT thn soyben; this is the plnt rtio where both species would be contributing eqully if not for biennil wormwood being more ggressive thn soyben when moisture ws limited. The differentil response of the two species to soil moisture stress suggests tht under field conditions biennil wormwood would hve the competitive edge during periods when moisture ws most limited. These dt lso imply tht biennil wormwood my thrive best when growing with crops, such s soyben, tht do not compete ggressively when soil moisture is limited, nd for which there re few options for biennil wormwood control. Effect of nitrogen rte on competition Soyben fresh weight ws reduced by 16% t the 1:3 soyben:biennil wormwood rtio compred with soyben in monoculture (Tble 3). Similrly, soyben root dry weight ws reduced by 17 nd 21% in the 1:3 nd 2:2 soyben:biennil wormwood rtios, respectively, compred with soyben in monoculture. The low soyben fresh nd dry root weights t high biennil wormwood rtios re indictive of interspecific competition between soyben nd biennil wormwood. A LSD (0.05): Soyben 0.10 Biennl wormwood 0.11 Totl NS B LSD (0.05): Soyben 0.13 Biennl wormwood 0.06 Totl NS 1 1 Reltive yield Reltive yield :0 3:1 2:2 1:3 0:4 0 4:0 3:1 2:2 1:3 0:4 Soyben:biennil wormwood rtio Soyben:biennil wormwood rtio Figure 1. Influence of soyben:biennil wormwood rtio on soyben, biennil wormwood nd totl yield expressed s reltive yield bsed on dry weight fter 9 wk of competition in replcement series experiment verged cross A) non-stressed nd stressed wtering regimes, nd B) four nitrogen rtes. LSD (0.05) vlues compre reltive yields mong plnt rtios within species. 129

7 Tble 3. Influence of soyben:biennil wormwood rtio on soyben totl fresh nd root dry weights verged cross four nitrogen rtes, nd influence of plnt rtio nd nitrogen rtes on biennil wormwood lef dry weight fter 9 wk of competition in replcement series experiment Biennil wormwood lef dry weight (g per plnt) Soyben Soil nitrogen (mg kg -1 soil) Soyben:biennil wormwood rtio Totl fresh weight Root dry weight (g per plnt) (g per plnt) 4: : : : : LSD (0.05) Lest significnt difference mong soil nitrogen rtes nd plnt rtios t 5% probbility. Biennil wormwood lef dry weight incresed s the nitrogen rte incresed for ll soyben:biennil wormwood rtios (Tble 3). Biennil wormwood lef dry weight in monoculture ws gretest t the 150 nd 200 mg kg -1 nitrogen rtes, nd lowest with 50 mg kg -1 nitrogen. Biennil wormwood lef dry weight for both the 50 nd 150 mg kg -1 nitrogen rtes decresed s the plnt rtio chnged from 3:1 to 2:2 soyben:biennil wormwood rtio, nd incresed t the 150 mg kg -1 nitrogen rte s the plnt rtio chnged from 2:2 to 1:3. Biennil wormwood plnt height nd totl dry weight incresed s nitrogen incresed from 50 to 150 mg kg -1 but did not increse further when nitrogen incresed to 200 mg kg -1 (Tble 4). Biennil wormwood plnt height with 150 mg kg -1 of nitrogen incresed by 7% nd totl dry mtter incresed by 28% when compred with plnts grown in soil with 50 mg kg -1 nitrogen. Similrly, myweed chmomile, in competition with pe, grew much lrger when nitrogen ws dded compred with no dded nitrogen (Ogg et l. 1994). The ctul consequence of biennil wormwood s response to dded nitrogen my be more evident in the field thn in the greenhouse becuse soyben growth ceses before tht of biennil wormwood. Biennil wormwood in soyben fields grows substntilly while soyben senesces nd dries down, nd is therefore ble to interfere with soyben hrvest. The ggressivity index of biennil wormwood incresed both s the nitrogen rte incresed nd s the plnt rtio chnged from soyben monoculture to biennil wormwood monoculture (Tble 4). The ggressivity index of biennil wormwood for the 2:2 plnt rtio t 50 mg kg -1 nitrogen ws zero nd incresed to 0.12 t the 100 mg kg -1 nitrogen rte. Biennil wormwood ggressivity did not increse ny further s the nitrogen rte ws incresed to 150 or Tble 4. Influence of nitrogen rte on biennil wormwood plnt height nd totl plnt dry weight verged over plnt rtios, nd nitrogen rte nd soyben:biennil wormwood rtio on biennil wormwood ggressivity fter 9 wk of competition with soyben in replcement series experiment Aggressivity indices b PHYTOPROTECTION 86 (2) 2005 b c Soyben:biennil wormwood rtio Soil nitrogen rte Plnt height Totl dry weight 4:0 3:1 2:2 1:3 0:4 (mg kg -1 soil) (cm) (g per plnt) * 1.00* * 0.64* 1.00* * 1.00* * 0.53* 1.00* LSD (0.05) c Biennil wormwood plnt heights nd totl dry weights were verged over the five plnt rtios becuse there ws no significnt interction between plnt rtio with either plnt height or totl dry weight. Aggressivity indices define the reltive success of the two species in using resources, where zero signifies the two species re competing eqully, -1 signifies complete dominnce of soyben, nd +1 signifies complete dominnce of biennil wormwood. Vlues followed by n sterisk re significntly greter thn zero ccording to one-tiled t-test. Lest significnt difference mong soil nitrogen rtes nd plnt rtios t 5% probbility. 130

8 NELSON, KEGODE: BIENNIAL WORMWOOD COMPETITION IN SOYBEAN 200 mg kg -1. For the 1:3 soyben:biennil wormwood rtio, biennil wormwood ggressivity index incresed from 0.47 t 50 mg kg -1 nitrogen to 0.64 t 100 mg kg -1, but decresed therefter to 0.53 t 200 mg kg -1. Aggressivity indices for biennil wormwood were sttisticlly greter thn zero for the 2:2 plnt rtio t 100 nd 200 mg kg -1, nd for the 1:3 plnt rtio t ll nitrogen rtes, which, s expected, indictes tht biennil wormwood ws competing more ggressively thn soyben. Soyben reltive yield t the 2:2 plnt rtio decresed from 0.5 to 0.4 s the nitrogen rte incresed from 50 to 200 mg kg -1 (Tble 5). Theoreticlly, the reduced ggressivity of soyben could hve been due to hindering of soyben by n incresing nitrogen rte. However, n dverse response to nitrogen seems unlikely becuse the contribution of soyben t the 3:1 soyben:biennil wormwood rtio to the RYT incresed from 0.73 to 0.85 s the nitrogen rte ws incresed from 50 to 200 mg kg -1. Most likely, this response ws due to incresed biennil wormwood growth t the higher nitrogen rtes since biennil wormwood ggressivity t the 2:2 plnt rtio incresed s the nitrogen rte incresed (Tble 4). The RYTs were nerly equl when bsed on dry shoot nd totl dry weights, which indictes tht soyben nd biennil wormwood competed eqully for wter nd nitrogen (Fig. 1B). When soyben plnts comprise 75% of the plnt rtio, soyben would be expected to contribute 0.75 to the RYT. In this cse, soyben contributed 0.8 when bsed on both dry shoot nd totl dry weights, which ws not sttisticlly greter thn 0.75 (Fig. 1B). For the reltive yields of biennil wormwood bsed on dry shoot nd totl dry weights, the vlues were not sttisticlly different from wht would be expected for even competition. The difference from expected contribution to ctul contribution ws similr t ll rtios nd not sttisticlly significnt. When the RYT ws clculted for dry shoot nd totl plnt dry weight, ll vlues were sttisticlly equl to 1.0 (Fig. 1B), which indictes tht nitrogen nd other common resources were being used similrly by both species. These findings re similr to those of Ogg et l. (1994) who reported tht pe nd myweed chmomile did not compete for nitrogen. Biennil wormwood infesttions under field conditions re usully greter thn those used in this study nd therefore competitive effects re likely more intense thn wht ws observed. For exmple, Fronning nd Kegode (2004) reported biennil wormwood densities between 84 nd 194 plnts m -2 in soyben weed mngement studies t three loctions. Nelson nd Kegode (2000) reported tht seson-long competition from 10 biennil wormwood plnts m -2 reduced hrvestble soyben yields by 44%. Prt of the yield loss ws probbly due to hrvesting inefficiency since t hrvest, biennil wormwood ws 137 cm tll compred to 66-cm-tll soyben. Furthermore, biennil wormwood plnts hd stems tht verged 3 cm in dim, which impeded hrvest. Soyben nd biennil wormwood compete similrly for wter nd nitrogen nd therefore high densities of biennil wormwood will likely led to lrge reduction in soyben yield. In our study, biennil wormwood nd soyben were seeded so they emerged within the sme time period. However, in North Dkot fields, soyben is seeded in mid- to lte My nd emerges in lte My to erly June wheres biennil wormwood typiclly emerges in lte June to erly July (Kegode nd Cierni 2003). Aprt from cusing hrvesting inefficiency due to lte mturtion of biennil wormwood (Kegode nd Christoffers 2003), it is uncler how competitive lte-emerging biennil wormwood plnts cn be in the presence of soyben. Allelopthic interference by biennil wormwood on soyben is possibility since biennil wormwood lef, stem nd root biomss dded to soil cused 14, 10 nd 32% reduction in fresh weight of soyben, respectively, fter 14 d (Cierni nd Kegode 2003; Kegode nd Cierni 2005). Biennil wormwood llelopthy ppered to increse s the plnt incresed in size, which might explin why this species emerges lte nd yet competes with crops to cuse yield loss. Future reserch needs to ddress the ggressivity of lte-emerging biennil wormwood plnts with respect to llelopthic interference. Tble 5. Influence of soyben:biennil wormwood rtio nd nitrogen rte on soyben reltive yield bsed on fresh weight fter 9 wk of competition with biennil wormwood in replcement series experiment Reltive yield Soyben:biennil Soil nitrogen rte (mg kg -1 soil) wormwood rtio : : : : :4 LSD (0.05) b b Reltive yield describes how species use resources in reltion to ech other. Lest significnt difference mong soil nitrogen rtes nd plnt rtios t 5% probbility. 131

9 ACKNOWLEDGEMENTS The uthors thnk M. Cierni, K. Mhoney, B. Fronning nd S. Mrk. This mteril is bsed upon work supported by the Coopertive Stte Reserch, Eduction, nd Extension Service, U.S. Deprtment of Agriculture, under greement No REFERENCES Bierlen, R., L.D. Prsch, B.L. Dixon, nd B.L. Ahrendsen The 1996 FAIR Act: mesuring the impcts on lnd lesing. Rev. Agric. Econ. 22 : Cierni, M.G., nd G.O. Kegode Allelopthic potentil of biennil wormwood. North Centrl Weed Sci. Soc. 58 : 69 (bstr.). Cousens, R A simple model relting yield loss to weed density. Ann. Appl. Biol. 107 : Cousens, R., nd M. Mortimer Dynmics of weed popultions. Cmbridge University Press, Cmbridge, Gret Britin. 332 pp. De Wit, C.T On competition. Versl. Lndbouwkd. Onderz. 66 : Fernld, M.E Gry s mnul of botny. Volume 2: Biosystemtics, floristic nd phylogeny series. 8th ed. Dioscorides Press, Portlnd, OR. p Fronning, B.E., nd G.O. Kegode Biennil wormwood (Artemisi biennis) postemergence control in soyben. Weed Technol. 18 : Hll, H.M., nd F.E. Clements The phylogenetic method in txonomy. The North Americn species of Artemisi, Chrysothnmus, nd Atriplex. Crnegie Inst. Wsh., Wshington, DC. 355 pp. Johnson, W.G., R.G. Hrtzler, nd D.E. Nordby Weeds to wtch: weeds tht seem to be expnding their hbitt rnge. North Centrl Weed Sci. Soc. 59 : 133 (bstr.). Kegode, G.O., nd M.J. Christoffers Intriguing world of weeds: Biennil wormwood (Artemisi biennis Willd.). Weed Technol. 17 : Kegode, G.O., nd M.G. Cierni Weed seedling emergence ptterns in North Dkot row crops. North Centrl Weed Sci. Soc. 58 : 70 (bstr.). Kegode, G.O., nd M.G. Cierni Biennil wormwood llelopthic potentil. Weed Sci. Soc. Am. 45 : 187 (bstr.). Kegode, G.O., M.J. Christoffers, L.W. Mengistu, nd A.G. Thoms Genetic diversity nd lte germintion my contribute to biennil wormwood weediness. Proc. 4 th Int. Weed Sci. Congress, Durbn, South Afric, June p. 57 Mhoney, K.J., nd G.O. Kegode Biennil wormwood (Artemisi biennis) biomss lloction nd seed production. Weed Sci. 53 : Mrtin, M.P.L.D., nd R.J. Field The nture of competition between perennil ryegrss nd white clover. Grss Forge Sci. 39 : Munz, P.A., nd D.D. Keck A Cliforni flor. University of Cliforni Press, Berkeley, Cliforni pp. Nelson, E.A., nd G.O. Kegode Interference of biennil wormwood in soyben. Proc. West. Soc. Weed Sci. 53 : 74 (bstr.). NDASS (North Dkot Agriculturl Sttistics Service, USDA) Soyben cres. Avilble from [Accessed Mrch 29, 2005]. Ogg, A.G., R.H. Stephens, nd D.R. Gely Interference between myweed chmomile (Anthemis cotul) nd pe (Pisum stivum) is ffected by form of interference nd soil wter regime. Weed Sci. 42 : Okfor, L.I., nd S.K. De Dtt Competition between uplnd rice nd purple nutsedge for nitrogen, moisture, nd light. Weed Sci. 24 : Rdosevich, S., J. Holt, nd C. Ghers Weed ecology: Implictions for mngement. John Wiley nd Sons, New York. 589 pp. Stevens, O.A The number nd weight of seeds produced by weeds. Am. J. Bot. 19 : Thoms, J.H Flor of the Snt Cruz Mountins of Cliforni. Stnford University Press, Stnford, Cliforni. 434 pp. US GAO (U.S. Generl Accounting Office) Grin fungus cretes finncil distress for North Dkot brley producers. GAO. Rep. GAO/RCDE Grin Fungus (B ). U.S. Gov. Print. Office, Wshington, DC. Wiese, A.F., nd C.W. Vndiver Soil moisture effects on competitive bility of weeds. Weed Sci. 18 : PHYTOPROTECTION 86 (2)