Crop yields in intercropping: meta-analysis and virtual plant modelling Yang Yu Thesis committee Promotor Prof. Dr N.P.R. Anten Professor of Crop and Weed Ecology Wageningen University Co-promotors Dr W. van der Werf Associate professor, Centre for Crop Systems Analysis Wageningen University Dr T.J. Stomph Assistant professor, Centre for Crop Systems Analysis Wageningen University Other members Prof. Dr E. Hoffland, Wageningen University Prof. Dr F.S. Zhang, China Agricultural University, Beijing, China Prof. Dr E.S. Jensen, Swedish University of Agricultural Science, Sweden Dr L. Hemerik, Wageningen University This research was conducted under the auspices of the C. T. de Wit Graduate School for Production Ecology and Resource Conservation Crop yields in intercropping: meta-analysis and virtual plant modelling Yang Yu Thesis submitted in fulfillment of the requirements for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. Dr A.P.J. Mol, in the presence of the Thesis Committee appointed by the Academic Board to be defended in public on Wednesday 20 April 2016 at 1.30 p.m. in the Aula. Yu, Y. (2016) Crop yields in intercropping: meta-analysis and virtual plant modelling 172 pages. PhD thesis, Wageningen University, Wageningen, NL (2016) With references, with summary in English ISBN: 978-94-6257-676-6 Abstract Intercropping, the cultivation of two or more crop species simultaneously in the same field, has been widely practiced by smallholder farmers in developing countries and is gaining increasing interest in developed countries. Intercropping can increase the yield per unit land compared to sole cropping. The yield advantage of intercropping is often assessed using the land equivalent ratio (LER). LER may be interpreted as the relative area required by sole crops to produce the same yields as achieved in a unit area of intercrop. LER>1 means intercropping is more efficient in land use than sole cropping. A large variation of LER has been found in the literature. However, few studies attempted to investigate reasons for this variation in LER. This thesis aims to reveal how temporal niche difference, crop type combination, and agronomic practices affect LER, productivity and interspecific interactions in annual intercrops. LER increased with temporal niche differentiation according to our meta-analysis of literature data. This positive relationship was valid in mixtures of C3 and C4 species but not in C3/C3 mixtures. Application of N fertilizer in intercropping decreased LER when the intercropped species were sown and harvested simultaneously. However, reducing overlap in growing periods of the intercropped species mitigated the negative effect of N fertilizer on LER. A functional-structural plant (FSP) model was developed to investigate the interplay between temporal and spatial complementarity and plant traits in mixed plant systems. The results showed that complementarity of light use in time and space likely determine productivity of species mixtures. The early-sown plants benefited from later sowing of the late-sown plants due to the relaxed competition for light from the late-sown plants until a plateau when the growth durations of the intercropped species overlapped less than 50% of the total growth period of the intercrop. By contrast, the late-sown plants suffered a great reduction in biomass due to the competition for light from the early-sown plants especially at moderately delayed sowing time and when spatial arrangement of the intercrop allowed strong interspecific competition. The shading effect from the early-sown plants on the growth and productivity of the late-sown plants was smaller if the late-sown plants had the potential to grow tall and if it had a high maximum CO2 assimilation rate. A meta-analysis of relative yields in cereal/legume intercrops was conducted to investigate the relationship between performance of intercropped species and management. Earlier sowing of one species increased its competitiveness towards the other species while later sowing decreased it. Application of N fertilizer enhanced the competitiveness of a cereal towards a legume, resulting in overall low productivity of legumes in intercrops. However, sowing legumes earlier than cereals mitigated the negative effect of N on productivity of legumes. Overall, this thesis shows that the complementary resource use resulting from plant traits diversity and temporal and spatial arrangements of plant mixtures is one of the key factors for high productivity of intercropping. This finding strengthens the basis for further research on the possible contribution of species diversity in agricultural systems to meeting the demand for food and other agricultural products while mitigating the environmental impacts of modern agriculture. Contents Chapter 1 General introduction 1 Chapter 2 Temporal niche differentiation increases the land equivalent ratio 11 of annual intercrops: A meta-analysis Chapter 3 Robust increases of land equivalent ratio with temporal niche 37 differentiation: A meta-analysis with quantile regression Chapter 4 Plant traits drive spatially and temporally complementary 53 light capture, photosynthesis, and productivity in plant mixtures Chapter 5 A meta-analysis of relative crop yields in cereal/legume 77 mixtures suggests options for management Chapter 6 General discussion 99 References 115 Appendix A 129 Appendix B 149 Appendix C 153 Appendix D 155 Summary 159 Acknowledgements 163 Publication list 167 PE & RC Training and Educational Statement 169 Curriculum vitae 171 Chapter 1 General introduction Chapter 1 General introduction What, where and why is intercropping? Intercropping is defined as the cultivation of two or more crop species in the same field for the whole or part of their growing period (Willey, 1990; Hauggaard-Nielsen et al., 2008). Intercropping is an ancient agronomic practice and was applied worldwide. Before the 1940s, intercropping was commonly practiced in the United States and Europe (Andersen, 2005; Machado, 2009). It however has gradually disappeared in developed countries due to mechanization and the availability of cheap synthetic fertilizers and pesticides which make sole cropping an efficient way to go (Horwith, 1985; Machado, 2009). In developing countries in Asia, Africa and Latin America, where farmers have limited access to mechanization and agricultural chemicals, intercropping is still widely used (Machado, 2009; Lithourgidis et al., 2011). In Latin America, 70-90% of beans are intercropped with maize, potatoes and other crops (Lithourgidis et al., 2011). Vandermeer (1992) reported that almost all of the cowpeas grown in Africa were grown in intercrops. It is clear from the ecological literature that ecological functioning and ecological services tend to increase with species richness (Tilman, 1999; Loreau et al., 2001). In this sense, intercropping is no exception (Cardinale et al., 2007) and intercropping has been shown to have clear advantages over sole crops in many aspects. For instance, intercropping may utilize land and other resources more efficiently than sole cropping (Reddy and Willey, 1981; Zhang and Li, 2003); intercropping may suppress pests and diseases (Andow, 1991; Trenbath, 1993), and weeds (Ayeni et al., 1984; Banik et al., 2006); and intercropping can increase stocks of organic soil carbon and nitrogen (Cong et al., 2015). One of the challenges facing the world is to match the rapidly changing demand for food from an increasing population with limited land, using environmentally friendly agricultural methods (Godfray et al., 2010). Sustainable intensification of agriculture is one way to tackle the challenge (Tilman et al., 2011). Sustainable intensification is defined as a set of agricultural practices and technnologies that increase crop production and resource use efficiency on croplands, while reducing the environmental impact of agriculture. Given the advantages of intercrops, intercropping has the potential to contribute to sustainable intensification of modern agriculture (Bedoussac et al., 2015; Jensen et al., 2015). Therefore, intercropping is currently receiving renewed interest as an environmentally friendly agronomic practice in developed countries. 2 General introduction Land-use efficiency of intercrops Land-use efficiency is one of the most widely studied aspects in intercropping research. Land- use efficiency of an intercrop may be compared to that of sole crops using the so-called land equivalent ratio (LER) (Mead and Willey, 1980). The LER is calculated as the sum of relative yields of component crops in an intercrop versus sole crops (Eq. 1). Y1 Y2 LER PLER1 +=+= PLER 2 (1) M1 M 2 where Y1 and Y2 are the yields (per unit of total area of the intercrop) of species 1 and 2 in the intercrop, and M1 and M2 are the yields of the species in sole crops (per unit area of the sole crops), and PLER1 and PLER2 are the partial land equivalent ratios of the intercropped species, i.e. the relative yields of the species. If the relative yield of one species in a two species intercrop is 70% and the relative yield of the other species in the intercrop is 50%, the LER is 120%, or 1.2. LER indicates the relative land area required by sole crops to produce the same yield/biomass of component species as achieved on one unit area of intercrops (Mead and Willey, 1980; Reddy and Willey, 1981). LER > 1 means that, in order to produce the same component crop yield as in a unit area of intercrop, a greater land area of sole crops