Relay cropping for sustainable intensification of agriculture across temperate regions: Crop management challenges and future research priorities

Relay cropping (RC) is a multiple cropping system that consists in planting a second crop into a standing first crop prior to its harvest. RC has potential to address food security and environmental sustainability via spatio-temporal diversification of cropping systems. Key potential benefits of RC include increased crop productivity, net economic returns, land use efficiency, soil fertility, efficient nutrient cycling, and pest control (pests sensu lato that include animal pests, pathogens and weeds). Despite these potential benefits, RC is poorly adopted worldwide, especially in Europe, compared to other forms of innovative cropping systems including cover-and inter-cropping. Here we review the literature to assemble information on key factors affecting the performance of RC by focusing on field crops grown for grain harvest purpose across temperate regions. We found severe knowledge gaps on crop management issues affecting the RC performance including the crop species and genotypic traits to be efficiently combined; the RC configuration including row number and width; optimal sowing dates and densities; irrigation, fertilization, and pest management; and technical problems in the har-vesting phase. We identified that while RC may represent "a very versatile system" for farmers in terms of crop management -via an efficient utilization of time, labor, and equipment -this cropping system also requires a lot of anticipation and organization even before the primary crop sowing. This increases crop management complexity for the farmers including the difficulty in managing " the competition phase " and the need for specific farming equipments. Finally, we propose three research priorities to fill the current knowledge gaps in RC research and implementation, worldwide, viz. varietal evaluation and selection, RC modelling and development of decision support systems, and technological innovation for an improved RC management.