Spatial distribution of Lolium rigidum seedlings following seed dispersal by combine harvesters J M BLANCO-MORENO*, L CHAMORRO*, R M MASALLES*, J RECASENS   & F X SANS* *Departament de Biologia Vegetal, Universitat de Barcelona, Barcelona, Spain, and  Departament d’Hortofructicultura, Bota `nica i Jardineria, Escola Te`cnica Superior d’Enginyeria Agra `ria, Universitat de Lleida, Lleida, Spain Received 13 October 2003 Revised version accepted 27 May 2004 Summary This paper considers the relationships between the dispersal of seeds and the distribution pattern of an annual weed. A comparative study of seed dispersal by combine harvesters, with and without a straw chopper attached, was established using Lolium rigidum, a common weed in Mediterranean cereal crops. Seed dispersal distance was quantified and the relationships between dispersal and fine-scale seedling distribution evaluated. Primary dispersal of L. rigidum seeds occurs in a very limited space around the parent plants, but the density of seed is low because most seeds do not fall from spikes spontaneously. In contrast, many seeds are spread by combine harvesters. In this study the maxi- mum dispersal exceeded 18 m from established stands in cereal fields, although the modal distance was close to the origin. In addition, the action of the combine harvesters tended to accumulate L. rigidum seeds predominantly under the straw swath, with some lateral movement. This action could explain the fine-scale banded pattern of L. rigidum in cereal fields. Although the treatment of straw by the standard and straw chopper combines differed, the resultant seed distribu- tion showed few differences. Keywords: seed dispersal, combine harvester, straw chopper, spatial distribution, Lolium rigidum. Introduction Many weeds exhibit an uneven distribution in fields, and sometimes patchiness may differ depending on the sampling scale (Rew et al., 1997; Cousens & Croft, 2000; Rew & Cousens, 2001). The spatial distribution may be related to the interaction of numerous factors, such as soil type (Ha¨ usler & Nordmeyer, 1995; Dieleman et al., 2000), cultivation (Marshall & Brain, 1999; Colbach et al., 2000a), harvesting (Ballare´ et al., 1987; McCanny & Cavers, 1988), herbicide efficacy (Dieleman et al., 2000) and crop interference (Weiner et al., 2001), all of which can affect seed distribution, germination and survival. In recent years, the uneven distribution of weeds has lead to the development of site-specific weed control (Gerhards et al., 1997; Paice et al., 1998) within precision agriculture (Heisel et al., 1997; Nordmeyer et al., 1997), which optimizes agricultural inputs (e.g. herbicides) by varying application rates to match within- field requirements. However, the effect of agricultural factors on seed distribution must be quantified to improve our predictions of weed populations. Many studies have addressed the spread of weeds (McCanny & Cavers, 1988; Howard et al., 1991; Rew & Cussans, 1997; Thill & Mallory-Smith, 1997; Woolcock & Cousens, 2000; Colbach & Sache, 2001; Wallinga et al., 2002), as there is great interest in evaluating the dynamics of weed populations. Moreover, an improved knowledge and understanding of seed dispersal could be useful in terms of theoretical ecology, in order to explain the uneven distribution of weed populations within fields (Colbach et al., 2000b). Mathematical modelling of seed dispersal and of movement by farm equipment has been used to explain the rates of spread of target weed species, and to suggest implications for weed management. For example, combine harvesting promotes the rapid spread of weeds (McCanny & Cavers, 1988), which causes the patches to extend over time. Combine harvesting has been analysed as a cause of dispersal within (Ballare´ et al., 1987; McCanny & Cavers, 1988) and among fields Correspondence: JM Blanco-Moreno, Departament de Biologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Av. Diagonal 645, 08028, Barcelona, Spain. Tel: (+34) 934 021 471; Fax: (+34) 934 112 842; E-mail: jmblanco@ub.edu Ó European Weed Research Society Weed Research 2004 44, 375–387