Investigating resource competition in cereal-legume intercropping systems
von William Nelson
Datum der mündl. Prüfung:2019-02-08
Betreuer:Prof. Dr. Anthony Whitbread
Gutachter:Prof. Dr. Reimund P. Rötter
Gutachter:Prof. Dr. Johannes Isselstein
EnglischIntercropping has long been a key crop production strategy for low input farming systems, such as those of smallholders, and organic farmers, where two or more crops are used to allow for the complementary use of resources, such as light, water, and nutrients. Although some intercropping systems have shown success, our understanding of them is not fully complete. The increasing need to utilise ecosystem resources as efficiently as possible demands a more comprehensive understanding, in particular as large-scale, high production farming systems begin to show interest in mixed specie crop production. A more site-specific approach is required, which looks at management aspects, such as planting practices, fertiliser and irrigation use, soil type, weather conditions, and cultivar choice. This thesis looks at cereal-legume row intercropping with very different management and environmental conditions in India and Germany. A major challenge with intercropping research is to efficiently investigate all potential scenarios using traditional experimentation. Process-based agro-ecosystem modelling is explored and used to conduct virtual experiments using the crop simulation model (CSM) APSIM. The general discussion develops overall conclusions for intercropping strategies and the scope of their future use. Chapters two and three are based on the use of intercropping as part of a climate resilient, risk reducing cropping strategy in semi-arid regions. As there is little knowledge of how and to what extent intercropping can be a viable option under future conditions, a field experiment in the dry season offered an opportunity to test this system under extreme but real-world conditions. Consequently, a field trial was run in semi-arid India over a two-year period (2015 and 2016) in the dry and hot (summer) season. These trials were set up as a split-split-plot experiment with four replicates to assess the performance of simultaneously sown sole versus intercropped stands of pearl millet and cowpea, with two densities (30 cm and 60 cm spacing between rows - both with 10 cm spacing within rows), and three drip irrigation treatments (severe stress, partial stress, and well-watered). Results showed that intercropping pearl millet led to a significantly lower total grain yield in comparison to the sole equivalent. Pearl millet’s highest yields were 1,350 kg/ha when intercropped and 2,970 kg/ha when grown as a sole crop; for cowpea, 990 kg/ha when intercropped, and 1,150 kg/ha as a sole crop. Interestingly, even when maximum daily temperatures reached up to 42.2 °C (julian day 112 in 2016), well-watered, pearl millet produced reasonable yields. We conclude that sole as opposed to intercropping systems could be a more efficient and therefore suitable practice under similar temperature regimes. However, more research would be needed to identify a suitable cowpea genotype and planting density that could allow for higher intercropped pearl millet yields. Clearly, despite the bulk of the literature promoting the benefits of intercropping, the system per se is not a ‘silver bullet’ solution to agricultural production and sustainability needs. We believe it is important to link such detailed experimental data with models to help reveal the mechanistic processes beneath system performance. This third chapter illustrates this approach using the data presented in chapter two, which was tested against APSIM simulations. After rigorous model calibration and validation, simulation experiments evaluated the manipulation of genetic traits, such as maximum plant height, i.e. ‘ideotyping’. The model showed distinct interactions and our approach the ability to highlight insights into intercropping systems. For the pearl millet-cowpea intercrop simulations, the cereal was the driver of total intercrop yield. To achieve this, the cereal intercrop component had to be taller than the cowpea. Chapter four of this thesis looked at a mechanised system using intercrop combinations of eight winter faba bean genotypes and three winter wheat varieties on two sites in temperate central Germany. Intercropping was higher yielding than sole crop equivalents, especially on the more marginal site and soil. Limited leaf area index and canopy height of faba bean are key traits for high yielding winter bean-wheat intercrops. Based on the genetic material used in this experiment, marginal sites are better cultivated with vigorous genotypes, while fertile sites require less vigorous ones. Mirrored in the simluation experiment of chapter three, this strategy works to ensure good cereal yields as excessive legume biomass growth must not out compete the cereal for light. Winter faba beans in particular should be bred to restrain from excess vegetative biomass development under more fertile conditions. While intercropping can play a role in robust food production systems, further experimentation is needed to help design specific GxExM combinations of intercrop systems. This thesis finishes with a detailed discussion that looks not only into findings and discussion points of the core chapters, but also some of the challenges and ways forward for intercropping research and CSM.
Keywords: Intercropping; Resource competition; Crop simulations modelling; Systems agronomy