Competition between maize and Datura stramonium in an irrigated ®eld under semi-arid conditions J CAVERO, C ZARAGOZA*, M L SUSO  & A PARDO   Dept Genetica y Produccion Vegetal, Estacio Ân Experimental de Aula Dei (CSIC), Apdo. 202, 50080 Zaragoza, Spain, *Unidad de Sanidad Vegetal, Servicio de Investigacion Agroalimentaria (DGA), Apdo. 727, 50080 Zaragoza, Spain, and  Centro de Investigacion y Desarrollo Agrario, Gobierno de La Rioja, Apdo. 1056, 26080, Logron Äo, Spain Received 12 October 1998 Revised version accepted 9 March 1999 Summary Crop growth of maize (Zea mays L.) and Datura stramonium L. in monoculture and competition was studied over 4 years in a ¯ood irrigated ®eld in Zaragoza (Spain). Plant density was 8.33 m ±2 for maize and 16.66 m ±2 (1994 and 1995) and 8.33 m ±2 (1996 and 1997) for D. stramonium. Maize yield was decreased by 14±63% when competing with the weed. Yield reduction increased as the time between crop and weed emergence decreased. The development of leaf area per plant during the exponential growth phase was faster in maize primarily because the leaf area of maize seedlings at emergence time was greater than that of the weed. The faster growth of maize in leaf area and height reduced the photosynthetically active radiation received by the weed. Datura stramonium had a lower radiation use eciency (RUE) than maize. Competition from the weed slightly decreased the maximum leaf area index (LAI) of the crop, and leaf senescence of maize was accelerated. The weed competed with the crop late in the season reducing crop growth rate, grain number per ear and grain weight. Competitive ability of D. stramonium for light was mainly due to its growth habit, with the leaves concentrated in the upper part of the canopy (more than 75% of LAI in the upper 25% of its height), its higher light extinction coecient (0.89) and its indeterminate growth habit. The N plant content of maize was not in¯uenced by the presence of the weed. The weed had a higher N plant content than the crop throughout the season and took up more N in monoculture. Keywords: leaf area growth, height, radiation use ef®ciency, canopy architecture, N plant content. Introduction Datura stramonium L. is a weed that can be found in maize (Zea mays L.) ®elds of the Iberian peninsula (Saavedra et al., 1989; Conesa, 1992; Sa et al., 1992) and many other parts of the world (Malan et al., 1982; Weaver & Warwick, 1984; Covarelli, 1994). Its importance as a weed in this crop is increasing in the Mediterranean area (Covarelli, 1994) and is probably related to the Correspondence: Dr Jose Cavero, Dept Genetica y Produccion Vegetal, Estacion Experimental de Aula Dei (CSIC), Apdo. 202, Zaragoza 50080, Spain. Tel: (+34) 76 576511; Fax: (+34) 76 575620; E-mail: jcavero@eead.csic.es Ó Blackwell Science Ltd Weed Research 1999 39, 225±240 225