Wevd RiVi-ari/i. I'Q'Wi. Volume .^6. I7.'.1.H7 Competftion between winter wheat and Veronica hederifolia: influence of weed density and the amount and timing of nitrogeii application C. .ANGONIN, J.-P. CAUSSANEL AND J.-M.MEYKARD" Imtrtii! National de la Hecherrhe Agftmntmifue, Lahoratoire de Malherhologw, BVI5'4V. 2!(B4 France and *lnstimt Natifimil de fa grofiomiqia; Lahoniloirc d'Agrimomic, 78850 Tbiverval-Chignon. France Received 30 January vereioin accepted 18 October 1995 Sununan The effccisofVtronr™/i'MT(/rj//ML. densities lOn the yield of winterwhcal {Triticumut-siivum L.) were studied in field trials Ciimducted at tht sjnue location im 1992 and 1993, In 1991, nitrogen at (i) kg ha"' was applied ,al tillering followed by n further SO kg ha" ' at the first node of stem elon- gatiorj. In 1993. four rtgiimes of nitrogen i^pplica- tioois were compared: a totai ot 140 kg ha" ' supplied al three dales,, 6(1 kg ha ^ supplied at tillering, 60 kg ha " sopplied at the first node of stem eloTigiiliion and no nitrogen as the control. Competittve effects from V. hederifoiia (ranging fromnto ]92plantsimi" "),weregreateslin 1W3, she year-widi, ihe lt>es( tarty gr<.>wth development of thts weed., In 1993, yiieid losses, as .described with a:ncn-linearmt>d,ell.cha:ngedfor each regitne of fcrtilijation,. In lx>ith years. V. hederifolia de- creased wheat earnumbcrper unit area for each nitrogen treatment. This is explained by an in- crease in tiller moriailiiy and a nilrogen defi- ciency in wheat at the stem elongation and flowering stages, Nevertheless, wilh late applica- •lon of nitrogen,, indmdual grain weight in- creased and the effect of V. hecierifohM on wheat yield Jos^was lowest The results are discussed in ^ 1W6 Europeao Weed Researdi Society relation to the effects on all yield components, and show the importaince of choosing the right nitrogen ferlilizatiio-n for specific yicidi iargeis of whcaf prnductior ivhen infesled bv weeds. lntirudiiction Regardiess olcDvironmental fact'tits. the biology lof weeds deSersrtines their a^gressivity agaiosi itbe crop i(NorrLs, \9*^I). I', hederifoiia. a 'wide- spread weed in winter wheat., is characterkedl by a shod life cycle and a iow canopy in comparison with wiinler wheat. The ear number of whtjai can be reduced bv this weed, but grain yield is noi always cnrrcspondmgly affecied (Caussanet et al. 1 ?73). in intensive wheat culture, ,around 6% of yield lo.ss dueiO' I-'- liederifolm has been found (Geniwiit. 1993: Wilson & Wiiphl. 1990),, Never- theless, with ihe extensification of wlie,at culturc in western Europe and associated low prices planned by the Common Agricultural Policy (CAP), if is fikcly thai: Iht- amounts of nitrogen and herbicides fojr wheat growing will be rc- do,ced. Thie influeiiicc of nitrogen lenilizaiiion o{ Ihc crop on the aggressiviity of the weed* needs !o be investigated,, in order lo define suiiiibie Wfcd manageniE'iil. M echia n isms of yi el d el,a bora tioiii in wheat iia ve been studied in diffiereni situalioris of intraspeci- fic c*>mpciirion and nitrogen nutnlion (Ellen & Spiertz. 1980: Mask-, li)ffi;Me™ard& Sebilkrtte, 1994). Tht" yield components at harvest reflect the construction of the populalioia by estab- lishtnt'itt of siuccessive oreans (Masle. 1981),. Wheat yield (V} is ihe product of grain niimber (GN) and weig^ht per grairsi (WIG): F=GNXW1G: grain riunibcr ean be broJicii down again as ihc product of car number (EN)