Analysis and Design of a Leek-Celery Intercropping System using Mechanistic and Descriptive Models

Intercropping leek (Allium porrum L.) and celery (Apium graveolens L.) was recognized as an option to reduce growth and reproductive potential of weeds while maintaining yield and product quality of both crops on a high level. To optimise the intercropping system for yield, quality and weed suppression a combined use of mechanistic and descriptive models, together with experimental work, was applied. An eco-physiological model was used to improve understanding of interplant competition based on physiological, morphological and phenological processes. The model was parameterised based on characteristics of the plants in monocultures and its performance was evaluated for the crop mixtures using experimental data from different growing seasons. After validation the model was used to simulate biomass production and quality of leek, celery and seed production of Common Groundsel (Senecio vulgaris L.) for a wide range of crop densities and times of weed emergence. In a second step, the results of the simulations where summarized using a descriptive hyperbolic yield-density model, which then allowed evaluation of the intercropping system in terms of productivity, product quality, and the ability to suppress weeds. The paper will explain this combined modelling approach and how it was used to design and optimise the leek-celery intercropping system. Moreover, this study shows that functional biodiversity, as represented by the intercropping system, can contribute to the improvement of the economical potential while increasing the sustainability of highly developed agricultural production systems. INTRODUCTION Many field vegetables such as leek (Allium porrum L.) are weak competitors against weeds, causing high costs for labour intensive weed management practices. Recently a number of studies have addressed intercropping as an option for an integrated weed management strategy, particularly in low-external input farming systems (Caporali et al., 1998; Itulya and Aguyoh, 1998; Liebman and Davis, 2000; Rana and Pal, 1999; Schoofs and Entz, 2000). An intercropping system using celery (Apium graveolens L.) as a companion cash crop was developed to improve the weed suppression of leek (Baumann et al., 2000). In glasshouse and field experiments it was shown that the increased competition of light by the intercrop canopy compared to a leek monoculture significantly reduced the biomass and the seed production of late-emerging Senecio vulgaris L., an important annual weed in vegetable production (Baumann et al., 2001b). However, the strong relative competitive ability of celery in the intercropping system resulted in a loss of leek quality because stem diameter was reduced to < 20 mm (market criterion) (Baumann et al., 2001a). The authors, therefore, concluded that optimization of the intercropping system with respect to crop quality and weed suppression was needed for successful implementation of the intercropping system and suggested the application of eco-physiological simulation models to optimize the system. Earlier, (Kropff and Van Laar, 1993) advocated the use of modeling to develop and optimize weed management systems with respect to cost effectiveness and minimization of environmental effects. Eco-physiological crop growth models can be very effective to evaluate and Proc. XXVI IHC – Sustainability of Horticultural Systems Eds. L. Bertschinger and J.D. Anderson Acta Hort. 638, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 60 develop complex systems, such as multi-species plant communities (Kropff and Van Laar, 1993). Based on physiological, morphological, and phenological processes, such models provide insight into the competitive relationships of the system. These models facilitate the exploration of complex systems without extensive field experimentation to investigate all options in a wide range of conditions. Empirical models and regression techniques can then help to analyze the final outcome of modeling studies and to describe plant interference in cropping systems. The current study attempts to combine a mechanistic and descriptive modeling approach to optimize the system. A well evaluated eco-physiological model, such as INTERCOM (Kropff and Van Laar, 1993), provides the necessary insight into the processes and plant characteristics determining mutual competitive effects and allows generating a large number of data sets for a wide range of densities and environments. Subsequent application of a statistical descriptive model to the generated data sets can help to summarize the results, to calculate the relative competitive ability of the system components, and to describe yield and product quality of the component crops in relation to plant density and mixing ratios. The objective of this study was to evaluate the use of combined modeling approaches for analysis and design of a leek and celery intercropping system with the main aim of optimizing this system in respect to yield and quality, while improving weed suppression. MATERIALS AND METHODS An adapted version of the eco-physiological competition model INTERCOM (Kropff and Van Laar, 1993) was used to simulate interplant competition between leek, celery and S. vulgaris in pure and mixed stands for various conditions and a wide range of crop densities and different relative times of weed emergence. The model was simplified with respect to physiological processes but included the original detailed simulation of competition for light (Baumann et al., 2002). Because water and nutrients were available in ample supply in the experimental system, competition for these resources was not simulated in this version of the model. The competition model was parameterized using experimental data from pure stands of the crops. Validation with independent data showed that the model simulated growth in both monocultures and mixtures accurately. For a detailed description of the model, the eco-physiological characteristics of the crops and the underlying experiments the authors refer to (Baumann et al., 2002). To study the growth of S. vulgaris and its effect on intercrop performance, the model was extended to include this weed species. Parameter values were derived from field experiments (Baumann et al., 2002) and additionally from earlier studies carried out by (Schnieders, 1999). The model was validated with independent data from monocultures and mixtures of the three species collected in two field experiments, carried out in 1997 and 1998 (Baumann et al., 2001b). Simulation Studies After validation of the model, the performance of pure and mixed crop stands with and without S. vulgaris was simulated for local environmental conditions. Plant density for leek was varied between 0 to 25 plants m, and plant density of celery was varied between 0 and 20 plants m. Plant density of S. vulgaris remained constant at 50 plants m at a relative emergence time of 0, 10, 20, 30 and 40 d after crop establishment. Simulation runs were conducted with weather data of 1997 and 1998 from Wädenswil, Switzerland for all combinations of crop densities with and without S. vulgaris. Biomass production and per-plant mass of the species after a growing period of 88 for 1997 and 92 d for 1998, were output of the model. For leek, the diameter of the pseudostem, which is used as a quality parameter, was calculated based on the per-plant mass, because a high correlation (r=0.92) between the dry mass of above-ground organs and pseudostem diameter had been found in earlier experiments (Baumann et al., 2002). For celery, the per-plant fresh mass was calculated based on an average dry matter content of 7.3%,