Manipulating Crop Row Orientation to Suppress Weeds and Increase Crop Yield

Transcription

1 Weed Science : Mnipulting Crop Row Orienttion to Suppress Weeds nd Increse Crop Yield Ctherine P. D. Borger, Abul Hshem, nd Shhb Pthn* Crop rows oriented t right ngle to sunlight direction (i.e., est west within the winter cropping system in Western Austrli) my suppress weed growth through greter shding of weeds in the interrow spces. This ws investigted in the districts of Merredin nd Beverley, Western Austrlin (ltitudes of 31u nd 32uS) from 2002 to 2005 (four trils). Winter grin crops (whet, brley, cnol, lupines, nd field pes) were sown in n est west or north south orienttion. Within whet nd brley crops oriented est west, weed biomss (verged throughout ll trils) ws reduced by 51 nd 37%, nd grin yield incresed by 24 nd 26% (compred with crops oriented north south). This reduction in weed biomss nd increse in crop yield likely resulted from the incresed light (photosyntheticlly ctive rdition) interception by crops oriented est west (i.e., light interception by the crop cnopy s opposed to the weed cnopy ws 28 nd 18% greter in whet nd brley crops oriented est west, compred with north south crops). There ws no consistent effect of crop row orienttion in the cnol, field pe, nd lupine crops. It ppers tht mnipultion of crop row orienttion in whet nd brley is useful weed-control technique tht hs few negtive effects on the frming system (i.e., does not cost nything to implement nd is more environmentlly friendly thn chemicl weed control). Nomenclture: Brley, Hordeum vulgre L.; cnol, Brssic npus L.; field pe, Pisum stivum L.; lupine, Lupinus ngustifolius L.; whet, Triticum estivum L. Key words: Light interception, row orienttion, row spcing, weed biomss, grin yield, nnul ryegrss, wild rdish. Light vilbility is n importnt fctor in regulting the competitive reltionship between crops nd weeds becuse light influences the growth nd development of neighboring plnts (Bllre nd Csl 2000; Bllre et l. 1990; Ghers et l. 1994; Holt 1995; Rousseux et l. 1996). During erly growth stges, there is interference between crop nd weed plnts becuse of reflected light. The reflection of fr-red photons by the stem of one plnt lowers the red to fr red photon rtio of light experienced by the stems of neighboring plnts. This modifies the light environment in the plnt stem tissue, which results in n incresed stem elongtion rte. As plnts ge, the crop cnopy closes, nd mutul shding further increses the competition for photosynthetic light. Shded leves lower in the cnopy hve ccess to low levels of photosyntheticlly ctive rdition nd low-red to fr-red photon rtio. Light lso influences flowering nd fruit set. Therefore, light is significnt determinnt of crop productivity. Crops cn be mnipulted to increse shding of weeds by the crop cnopy, to suppress weed growth, nd to mximize crop yield. One possible wy to reduce light interception by weeds nd to increse light interception by the crop cnopy is to mnipulte the crop row spcing nd orienttion (Holt 1995). Reducing the spce between crop rows or orientting crop rows t ner right ngle to the sunlight direction increses the shding of weeds between the rows. The growth of poison ryegrss (Lolium temulentum L.), littleseed cnrygrss (Phlris minor Retz.), wild ot (Aven ftu L.), nd common vetch (Vici stiv L.) in whet ( 308 ) crops nd blck nightshde (Solnum nigrum L.) in vineyrds (Vitis vinifer L.) were influenced by crop row spcing nd orienttion (Angirs nd Shrm 1996; Shrm nd Angirs 1996,b; Shresth nd Fidelibus 2005). Furthermore, in the bsence of weeds, orienttion ffected crop yield or soil DOI: /WS * First uthor: Deprtment of Agriculture nd Food Western Austrli, Drylnd Reserch Institute, P.O. Box 432, Merredin, WA, Austrli 6415; second uthor: Deprtment of Agriculture nd Food Western Austrli, Centre for Cropping Systems, P.O. Box 483, Northm, WA, Austrli 6401; third uthor: Deprtment of Agriculture nd Food Western Austrli, 10 Doney Street, Nrrogin, WA, Austrli Corresponding uthor s E-mil: cborger@gric.w.gov.u moisture reltions in olive (Ole europe L.) nd pple (Mlus domestic Borkh.) orchrds nd ot (Aven stiv L.) crops (Connor et l. 2009; Mohler 2001; Plmer 1977, 1989; Pendleton nd Dungn 1958). The effect of row orienttion vries with ltitude nd with the sesonl tilt of the erth in reltion to the sun. Ner the equtor, north south (s opposed to est west) orienttion gives crops higher levels of light bsorption for most of the yer. At higher ltitudes (up to 55u), bsorption is highest in north south crops in summer nd est west crops for the rest of the yer. From 65u upwrds, est west orienttion gives gretest light bsorption ll yer (lthough the difference between orienttions is minor) (Mutsers 1980). The ltitude of the Western Austrlin Whet Belt (brodscle grin cropping region) rnges from 28u to 33uS. The cropping seson occurs during winter nd spring, indicting tht est west crops should receive gretest light bsorption (Mutsers 1980). The ngle of the sun (in reltion to the horizon) cn be s low s 35u during the winter cropping seson, lthough it rnges from 39 to 61u in spring, when crops rech mturity (Geoscience Austrli 2009). Therefore, lthough solr energy is bundnt in Austrli, within winter cerel grin crops, there is still intense competition for light between crops nd weed species (Lemerle et l. 1995; Vndeleur nd Gill 2004). The mnipultion of crop row orienttion to reduce the competitive bility of weeds hs not been investigted in Western Austrli. It is likely tht crops oriented in the est west direction could shde weeds in the interrow spces to greter extent thn crops oriented north to south. The objective of this study ws to exmine whether crop row orienttion nd row spcing could chnge the light vilbility to crops nd weeds nd, consequently, ffect weed growth nd crop yield. Mterils nd Methods Beverley Trils. Two trils were estblished t Beverley, Austrli (Tble 1), on June 4, 2002 (herefter referred to s Beverley 2002) nd June 11, 2004 (herefter referred to s Beverley 2004). Crops, including whet ( Westoni ) t 75 kg h 21, brley ( Stirling ) t 75 kg h 21, lupines ( Kly ) 174 N Weed Science 58, Mrch April 2010

2 Tble 1. Detils of tril sites t the Deprtment of Agriculture nd Food Western Austrli (DAFWA) t Beverley nd Merredin, Austrli, including site loction, soil type, nd verge climte. Soil type is ccording to the Austrlin soil clssifiction system from Isbell (2002), nd climte dt is from the Bureu of Meteorology (2009). District Beverley Merredin Loction DAFWA Avondle Reserch Sttion DAFWA Merredin Reserch Sttion Ltitude 32u S 31u S Longitude 116u E 118u E Distnce from Perth, WA, Austrli 92 km ESE 232 km ENE Soil type Red chromosol (deep-red, lomy snd) Yellow kndosol (sndy lom) Averge rinfll mm mm Averge cropping seson (My to November) rinfll mm mm Averge mximum cropping seson dily tempertures 16.8 to 28.4 C 16.3 to 28.4 C t 100 kg h 21, field pe ( Helen ) t 80 kg h 21, nd cnol ( Kroo ) t 8 kg h 21 were sown t 18- or 36-cm row spcing, in n est west or north south orienttion. Crops were estblished using minimum-tillge cultivtion (knife points nd press wheels), on unit plot size of 2 m by 10 m. Stndrd fertilizers were pplied t sowing nd fter emergence (N P S Zn, kg h 21 for whet, brley, nd cnol; P S C Cu Zn, kg h 21 for lupines nd field pe). Rigid ryegrss (Lolium rigidum Gudin) nd wild rdish (Rphnus rphnistrum L.) were the predominnt weeds t the site, emerging from nturl weed seed bnk. Weeds tht emerged before sowing were controlled with mixture of prqut t 270 g i h 21 nd diqut t 230 g i h 21, but no other PRE or POST herbicides were pplied. Merredin Trils. Two trils were estblished t Merredin, Austrli (Tble 1), on June 2, 2004 (herefter referred to s Merredin 2004), nd My 11, 2005 (herefter referred to s Merredin 2005). Crops were sown nd fertilized s for the Beverley, Austrli, site. For Merredin 2004, crop cultivrs remined the sme s used t the Beverley, Austrli, site, but sowing rte ws ltered to 70 kg h 21 for whet nd brley, 100 kg h 21 for lupines nd field pe, nd 7 kg h 21 for cnol. Becuse the weed seed bnk ws low t this site, 200 seeds m 22 of nnul ryegrss ( Sfegurd ) nd 300 podsegments m 22 of wild rdish were spred before the sowing opertion nd incorported by shllow cultivtion. For Merredin 2005, crops were sown t the sme rtes s Merredin 2004, but cultivrs were ltered to Wylktchem whet, Hmelin brley, Mndelup lupines, Helen field pes, nd Kroo cnol. Row spcing ws chnged to 23 nd 60 cm. Annul ryegrss seeds were introduced nd incorported s for Merredin For both trils, weeds tht emerged before sowing were controlled with mixture of prqut t 405 g i h 21 nd diqut t 345 g i h 21. Mesurements Tken during the Experimentl Period. Density of crop nd weed plnts ws recorded 3 to 4 wk fter emergence from two, 50-cm by 100-cm, fixed qudrts per plot. Density nd boveground dry biomss of crops nd weeds were recorded from the sme qudrts when crops were t the lte-flowering stge. Photosyntheticlly ctive rdition (PAR) ws lso mesured t the lte-flowering stge of crops (i.e., just before biomss ssessment) t middy on sunny dy t the center of the crop row spce with liner ceptometer 1 (Percy 1991). PAR mesurements were tken from bove the crop cnopy nd bove the weed cnopy t two loctions within ech plot. Light vilbility is expressed s percentge of light interception by the crop cnopy (rther thn by the weed cnopy or bre ground in the interrow spce). PAR mesurements were tken when the crop ws mture to ssess mximum light interception by the crop. At tht ge, crops hd reched mximum height but hd not yet entered senescence or shed leves. Visul ssessment indicted tht the crops were helthy (no signs of lef curling or shedding from stress). At hrvest, crop yield, grin size, nd grin protein were recorded. Design nd Anlysis. Trils were rrnged in rndomized complete-block design with three replictions t ech site. Dt from the four trils were combined nd nlyzed s split-plot design using ANOVA. 2 Tril (yer/loction) ws the min plot fctor, nd crop type, orienttion, nd row spcing were the subplot fctors. As row spcing vried between trils (i.e., 18 to 36 cm nd 23 to 60 cm), row spcing ws recorded s wide or nrrow when the entire dt set ws considered. The difference between mens within ech fctor nd mens within ll possible interctions mong fctors were determined using Fisher s Protected LSD test. Differences for ny mesured prmeter were considered significnt t P, Results nd Discussion Weed nd Crop Emergence. In ll trils, the dominnt weed species were nnul ryegrss nd wild rdish, lthough cpeweed [Arctothec clendul (L.) Levyns.] nd threecornered jck (Emex ustrlis Steinheil) were lso evident. Annul ryegrss nd wild rdish re mong the predominnt weeds in cerel nd brodlef grin cropping systems throughout Austrli (Wlsh nd Powles 2007). Initil weed density ws significntly different between trils, rnging from 280 plnts m 22 t Beverley 2004 to 19 plnts m 22 t Merredin 2005 (LSD ; P, 0.001), but there ws no consistent difference between weed density becuse of tril loction. Weed density ws unffected by crop species, crop row orienttion, or row spcing. Likewise, initil crop density ws not ffected by orienttion (dt not presented). Row spcing hd significnt effect on initil crop density (s seeding rte ws not ltered between row spcings), but the effect ws not consistent mong crop type, yer, or loction (dt not presented). Weed nd Crop Biomss. Dry weed biomss ws significntly different mong trils nd ws relted to initil weed density (rnging from 248 g m 22 t Beverley 2004 to 28 g m 22 t Merredin 2005; LSD ; P, 0.001). Averged throughout ll trils, weed biomss ws significntly lower in crops grown in n est west, rther thn north south, orienttion (91 g m 22 for est west crops nd 109 g m 22 Borger et l.: Crop row orienttion N 175

3 Tble 2. Men weed dry biomss, mesured t the flowering stge of brley, whet, cnol, field pe, nd lupine crops sown in n est west or north south orienttion in four trils: Beverley 2002, Beverley 2004, Merredin 2004, nd Merredin 2005 (P, 0.001; LSD ). Men weed biomss is verged throughout the row spcing (23 to 60 cm nd 18 to 36 cm). Weed biomss Crop Loction Yer Est west North south g m Brley Beverley Merredin Whet Beverley Merredin Cnol Beverley Merredin Field pe Beverley Merredin Lupine Beverley Merredin Not vilble. for north south crops; P ; LSD ). Whet crops oriented est west hd significntly lower weed biomss thn north south crops, except for the Merredin 2004 tril, where the difference ws not significnt (Tble 2). Similrly, est west brley crops hd lower weed biomss thn north south crops, except for the Merredin 2005 tril, where the difference ws not significnt. For both trils t Merredin, cnol crops oriented est west hd lower weed biomss thn north south crops. However, t Beverley 2002, there ws no significnt effect of orienttion, nd t Beverley 2004, the est west cnol hd greter weed biomss thn the north south crop. Weed biomss in field pe oriented est west ws only significntly lower thn tht of north south field pe t Beverley Likewise, weed biomss of lupines oriented est west ws only significntly lower t Merredin Averged throughout ll trils, weed biomss ws lower in crops with nrrow row spcing (nrrow row spcing, 93 g m 22 ; wide row spcing, 107 g m 22 ;P50.016; LSD ). However, ll possible interctions mong row spcing nd the other fctors were not significnt. Weed biomss ws significntly ffected by crop type nd by the interction between crop type nd tril, but the reltionship ws not consistent mong trils (dt not presented). As expected, dry crop biomss ws significntly different mong crops, with brley hving the gretest biomss (490 g m 22 ), cnol hving the lowest biomss (223 g m 22 ), nd field pe, lupine, nd whet hving similr biomsses (378, 373, nd 382 g m 22 ;P, 0.001; LSD ). Crop biomss ws lso significntly different mong trils, with the lowest verge biomss t Merredin 2004 (153 g m 22 ) nd highest verge biomss t Beverley 2002 (526 g m 22 ; P, 0.001; LSD ). The biomss of ll crop species, except cnol, were consistently lower t Merredin 2004 (P, 0.001, dt not presented), probbly becuse Merredin rinfll in 2004 ws below verge (i.e., 279 mm totl rinfll Tble 3. Men grin yield from brley, whet, cnol, field pe, nd lupine crops sown in n est west or north south orienttion (P ; LSD ). Crop yield Crop Est west North south kg h Brley 1, Whet 1, Cnol Field pe Lupine Men grin yield is verged throughout tril (Beverley 2002, Beverley 2004, Merredin 2004, nd Merredin 2005) nd row spcing (23 to 60 cm nd 18 to 36 cm). t Merredin in 2004, mm growing seson rinfll, 12.5% below growing seson verge)(bureu of Meteorology 2009). Dry crop biomss ws not consistently ffected by orienttion or row spcing nd ws not relted to weed biomss. Grin Yield nd Qulity. Clen grin yields, verged throughout ll trils, were significntly greter for crops sown in n est west, rther thn north south orienttion (793 nd 656 kg h 21 ;P, 0.001; LSD ). However, the interction between crop type nd orienttion indicted tht this difference ws predomintely due to the difference in whet nd brley yield. Whet yield (verged throughout ll trils) ws 24% greter in est west, rther thn north south, crop orienttion, nd brley yield ws 26% greter in est west, rther thn north south, crops (Tble 3). Yield of cnol, field pe, nd lupine crops in n est west orienttion were not significntly different thn those in north south orienttion. Within individul trils, brley yield (verged throughout row spcings) from est west crops were greter thn those of north south crops t Beverley 2002 (2,180 nd 1,720 kg h 21 ), Beverley 2004 (2,070 nd 1,720 kg h 21 ), nd Merredin 2005 (1,150 nd 910 kg h 21 ). Whet yield from est west crops ws greter thn those of north south crops t Beverley 2002 (2,850 nd 2,020 kg h 21 ) nd Merredin 2005 (960 nd 590 kg h 21 )(P ; LSD ). Differences mong yields of other crops were not significnt. The higher grin yields observed in whet nd brley crops growing in n est west orienttion were probbly relted to the reduced weed biomss in the est west cerel crops (Tble 2). As expected, the tril hd significnt effect on crop yield (verged throughout ll crops), with the gretest yield t Beverley 2002 (verge yield of 1,034 kg h 21, compred with 626 kg h 21 t Merredin 2004, 549 kg h 21 t Beverley 2004, nd 689 kg h 21 t Merredin 2005; P ; LSD ). Row spcing nd the interction mong row spcing nd the other fctors hd no effect on grin yield (dt not presented). Grin protein nd size remined unffected by tril, orienttion, or row spcing (dt not presented). Light Interception. Within ll trils, the crop ws tller thn the weeds throughout the growing seson. Averge percentge of light interception by the crop cnopy (rther thn by the weed cnopy) ws significntly greter in crops oriented est west, compred with north south crops (72 nd 61% light interception; P, 0.001; LSD ). Averged through- 176 N Weed Science 58, Mrch April 2010

4 out individul crops, the percentge of light interception by the cnopy of lupines (58%) ws significntly lower thn tht of brley, cnol, field pe, nd whet, which intercepted 70, 69, 71, nd 66% of light (P ; LSD 5 6.6). The interction between crop type nd orienttion indicted tht whet nd brley crops in n est west orienttion t the lteflowering stge intercepted 28 nd 18% more light thn the crops in the north south orienttion. The difference between light interception by other crops in n est west or north south orienttion ws not significnt (Tble 4). The interction mong crop type, orienttion, nd tril indicted tht there ws significntly greter light interception by est west whet nd brley crops, compred with north south crops, t ll trils except Merredin Of the other crops, only cnol t Merredin 2005 hd greter light interception in n est west, rther thn north south orienttion (dt not presented). Therefore, reduced light vilbility to weeds in crops oriented est west occurred consistently in the cerel crops, where suppression in weed growth nd increses in grin yield were lso observed. Orienttion did not hve consistent effect on light interception, weed biomss, or yield of brodlef crops. The cnopy rchitecture of brodlef crops is generlly wider thn the cnopy of cerel crops, which my negte the effect of crop row orienttion. Further reserch is required to determine the effect of crop orienttion on the competitive bility of weeds in brodlef crops. Averged throughout ll trils, light interception by crops with nrrow rows ws greter thn tht intercepted by crops t wide row spcings (70 nd 63%; P, 0.001, LSD ). However, the interction mong tril, crop type, nd row spcing indicted tht there ws only significnt difference between light interception under nrrow or wide row spcing in cnol nd field pe t Avondle 2002 nd whet t Merredin 2005 (dt not presented). This my indicte why row spcing did not ffect weed biomss or crop biomss nd yield. Shrm nd Angirs (1996,b) nd Angirs nd Shrm (1996) found tht reduced row spcing incresed light interception by crops nd reduced weed biomss, incresing crop yield. Likewise, previous work t Merredin (in the bsence of weeds) indicted tht reduced row spcing my increse light interception by the crop cnopy (lthough this did not result in improved grin yield nd reduced soil evportion from shding of the ground) (Yunus et l. 1993). Alterntively, Roberts et l. (2001) found tht whet row spcing did not influence growth nd seed production of rye (Secle cerele L.) in Oklhom. In generl, reduced row spcing ppers to improve crop light interception nd competitive bility ginst weeds, but the effect is not consistent. The results of these trils (regrding orienttion) confirm those of Shrm nd Angirs (1996,b) nd Angirs nd Shrm (1996), who found tht the effect of mnipulting the row orienttion of whet reduced biomss of poison ryegrss by 22.5% nd littleseed cnrygrss by 28% nd incresed whet yield 7.8 to 8.7%. However, s the tril site ws t 31u519N, 77u099E (Indi), nd the crops were grown from winter to summer (October to June, rbi seson), north south crops received gretest light bsorption towrd the end of the growing seson (t mximum tillering), nd so, experienced gretest yield (Mutsers 1980; Shrm nd Angirs 1996b). Western Austrlin winter grin crops rech mximum height (reproductive stge) during winter nd spring, where est west oriented crops receive the gretest light bsorption (Mutsers 1980). In the current study, weed biomss Tble 4. Men percentge of light (photosyntheticlly ctive rdition) interception by cnopies of brley, whet, cnol, field pe, nd lupine crops sown in n est west or north south orienttion (P ; LSD ). Light interception Crop Est west North south % Brley Whet Cnol Field pe Lupine Percentge of light interception ws mesured t noon on cler dy in the center of the interrow spce t the lte-flowering stge of the crops. Percentge of light interception ws verged throughout tril (Beverley 2002, Beverley 2004, Merredin 2004, nd Merredin 2005) nd row spcing (23 cm to 60 cm nd 18 cm to 36 cm). production ppered to be influenced by cerel crop light interception, with the subsequent reduction in weed biomss influencing crop yield but not crop biomss. This suggests tht weed growth ws not delyed until lte in the development of the crop, when the lrger crop plnts could most effectively shde weeds in the interrow spce. Angirs nd Shrm (1996) nd Shrm nd Angirs (1996b) noted reduction in weed growth rte (in response to crop orienttion) between 120 nd 150 d fter crop sowing, i.e., t the mximum tillering stge of whet. Light interception in the current study ws only mesured t one time of yer, when crop plnts were mture. Future studies mesuring light interception during severl stges of crop development would more comprehensively indicte the reltionship between crop nd weed competition under vrying crop orienttions. Mnipultion of row orienttion hd consistent effect only on whet nd brley. However, in Austrli, whet nd brley re the most commonly grown brodscle grin crops. For exmple, the 5-yr verge (2003/2004 to 2007/2008) for grin production in Western Austrli ws 7.7 million metric tonnes of whet nd 2.4 million tonnes of brley, s compred with 1.3 million of cnol, field pe, nd lupine combined (Deprtment of Agriculture nd Food Western Austrli 2009). Mnipultion of row orienttion is n idel method to incorporte into n integrted weed-mngement progrm becuse it does not cost growers nything to implement, nd it is environmentlly friendly compred with chemicl weed control tctics (Mohler 2001). Further reserch is required to determine whether these results will be consistent cross similr brodscle grin cropping systems (t ltitudes where orienttion cn ffect light vilbility), both ntionlly nd interntionlly. The effect of crop row orienttion is likely to vry, depending on the griculturl system, crop vriety, nd the mjor weed species present. However, given tht sunlight is bundnt in Western Austrli (with 171 sunny d yr 21 in Merredin, Austrli, nd 146 sunny d yr 21 in Beverley, Austrli; Bureu of Meteorology 2009), the effect of mnipulting crop row orienttion my be greter in other systems where competition for light between crop nd weeds is greter limittion to crop growth. Sources of Mterils 1 Sunfleck Ceptometer Delt-T Devices LTD, 128 Low Rod, Burwell, Cmbridge CB5 OEJ, Englnd. Borger et l.: Crop row orienttion N 177

5 2 GenStt softwre, Version 11.1, VSN Interntionl, Ltd., 5 Wterhouse Street, Hemel Hempsted HP1 1ES, Englnd. Acknowledgments The uthors thnk the Grins Reserch nd Development Corportion for funding this reserch (projects DAW713 nd DAW00114). Thnks re lso due to stff t the Deprtment of Agriculture nd Food Western Austrli for their ssistnce, including Nerys Wilkins nd the Reserch Support Units t Merredin nd Beverley, Austrli. Literture Cited Angirs, N. nd V. Shrm Influence of row orienttion, row spcing nd weed-control methods on physiologicl performnce of irrigted whet (Triticum estivum). Indin J. Agron. 41: Bllre, C. L. nd J. J. Csl Light signls perceived by crop nd weed plnts. Field Crop Res. 67: Bllre, C. L., A. L. Scopel, nd R. A. Snchez Fr-red rdition reflected from djcent leves: n erly signl of competition in plnt cnopies. Science 247: Bureu of Meteorology Western Austrlin Climte Dt. bom.gov.u/climte/verges/tbles/c_w_nmes.shtml. Accessed: June 8, Connor, D. J., A. Centeno, nd M. Gomez-del-Cmpo Yield determintion in olive hedgerow orchrds. II. Anlysis of rdition nd fruiting profiles. Crop Psture Sci. 60: Deprtment of Agriculture nd Food Western Austrli Western Austrli s Agri-Food, Fibre nd Fisheries Industries w.gov.u/servlet/pge?_pgeid5639&_dd5portl30&_schem5portal30. Accessed: September 14, Geoscience Austrli Compute Sun nd Moon Azimuth nd Elevtion. Accessed: September 17, Ghers, C. M., M. A. Mrtinez-Ghers, J. J. Csl, M. Kufmn, M. L. Roush, nd V. A. Deregibus Effect of light on winter whet (Triticum estivum) nd Itlin ryegrss (Lolium multiflorum) competition. Weed Technol. 8: Holt, J. S Plnt responses to light: potentil tool for weed mngement. Weed Sci. 43: Isbell, R. F The Austrlin Soil Clssifiction. 2nd ed. Cnberr, Austrli: CSIRO Publishing. 152 p. Lemerle, D., B. Verbeek, nd N. Coombes Losses in grin yield of winter crops from Lolium rigidum competition depend on crop species, cultivr nd seson. Weed Res. 35: Mohler, C. L Enhncing the competitive bility of crops. Pges in M. Liebmn, C. L. Mohler, nd C. P. Stver, eds. Ecologicl Mngement of Agriculturl Weeds. Cmbridge, UK: Cmbridge University Press. Mutsers, H.J.W The effect of row orienttion, dte nd ltitude on light bsorption by row crops. J. Agric. Sci. 95: Plmer, J. W Diurnl light interception nd computer model of light interception by hedgerow pple orchrds. J. Appl. Ecol. 14: Plmer, J. W The effects of row orienttion, tree height, time of yer nd ltitude on light interception nd distribution in model pple hedgerow cnopies. J. Hortic. Sci. 64: Percy, R. W Rdition nd light mesurements. Pges in R. W. Percy, L. Ehleringer, H. A. Mooney, nd P. W. Rundel, eds. Field Methods nd Instrumenttion. New York, NY: Chpmn nd Hll. Pendleton, J. W. nd G. H. Dungn Effect of row direction on spring ot yields. Agron. J. 50: Roberts, J. R., T. F. Peeper, nd J. B. Solie Whet (Triticum estivum) row spcing, seeding rte, nd cultivr ffect interference from rye (Secle cerele). Weed Technol. 15: Rousseux, M. C., A. J. Hll, nd R. A. Snchez Fr-red enrichment nd photosyntheticlly ctive rdition influence lef senescence in field-grown sunflower. Physiol. Plnt. 96: Shrm, V. nd N. N. Angirs Effect of row orienttions, row spcings nd weed-control methods on light interception, cnopy temperture nd productivity of whet (Triticum estivum). Indin J. Agron. 41: Shrm, V. nd N. N. Angirs. 1996b. Light interception, weed growth nd productivity of irrigted whet s influenced by crop geometry nd weed control methods. Indin J. Plnt Physiol. 1: Shresth, A. nd M. Fidelibus Grpevine row orienttion ffects light environment, growth, nd development of blck nightshde (Solnum nigrum). Weed Sci. 53: Vndeleur, R. K. nd G. S. Gill The impct of plnt breeding on the grin yield nd competitive bility of whet in Austrli. Aust. J. Ag. Res. 55: Wlsh, M. J. nd S. B. Powles Mngement strtegies for herbicideresistnt weed popultions in Austrlin drylnd crop production systems. Weed Technol. 21: Yunus, I.A.M., R. K. Belford, D. Tennnt, nd R. H. Sedgley Row spcing fils to modify soil evportion nd grin yield in spring whet in dry Mediterrnen environment. Aust. J. Agric. Res. 44: Received June 8, 2009, nd pproved October 19, N Weed Science 58, Mrch April 2010