The influence of winter oilseed rape (Brassica napusssp.oleiferavar.biennis) canopy size on grass weed growth and grass weed seed return

Abstract

SUMMARYFour experiments conducted over three seasons (2002–05) at the Crops Research Unit, University of Reading, investigated effects of canopy management of autumn sown oilseed rape (Brassica napusL. ssp.oleiferavar.biennis(DC.) Metzg.) on competition with grass weeds. Emphasis was placed on the effect of the crop on the weeds.Rape canopy size was manipulated using sowing date, seed rate and the application of autumn fertilizer.Lolium multiflorumLam.,L.×boucheanumKunth andAlopecurus myosuroidesHuds. were sown as indicative grass weeds.The effects of sowing date, seed rate and autumn nitrogen on crop competitive ability were correlated with rape biomass and fractional interception of photosynthetically active radiation (PAR) by the rape floral layer, to the extent that by spring there was good evidence of crop: weed replacement.An increase in seed rate up to the highest plant densities tested increased both rape biomass and competitiveness, e.g. in 2002/3,L. multiflorumhead density was reduced from 539 to 245 heads/m2and spikelet density from 13 170 to 5960 spikelets/m2when rape plant density was increased from 16 to 81 plants/m2. Spikelets/head ofLoliumspp. was little affected by rape seed rate, but the length of heads ofA. myosuroideswas reduced by 9% when plant density was increased from 29–51 plants/m2.Autumn nitrogen increased rape biomass and reducedL. multiflorumhead density (415 and 336 heads/m2without and with autumn nitrogen, respectively) and spikelet density (9990 and 8220 spikelets/m2without and with autumn nitrogen, respectively). The number of spikelets/head was not significantly affected by autumn nitrogen.Early sowing could increase biomass and competitiveness, but poor crop establishment sometimes overrode the effect. Where crop and weed establishment was similar for both sowing dates, a 2-week delay (i.e. early September to mid-September) increasedL. multiflorumhead density from 226 to 633 heads/m2and spikelet density from 5780 to 15 060 spikelets/m2.