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Intercropping, defined as growing two or more crops simultaneously on the same field (Federer, 1993), is widespread all over the world. Especially in smallholder farming like in Africa (e.g., Malawi: 80 – 90% of soybean cultivation), India (17% of arable land) or China (25% of arable land), intercropping is a common cropping system. In times of climate change, rising food prices, shortage of arable land and food in third world countries and countries with a rapidly increasing population, adjusted traditional cropping systems become more and more important. Farmers tend to utilize every square centimetre of available arable land for production and for diversification of their families diet. Besides, there is a so-called unconscious intercropping: Because fields and farm size are very small (0.1 – 2 ha), the sum of field borders can be considered as intercropping in a larger scale (Figure 1).
Figure 1. In China, the average field size is very small and fields alternate as stripes with different crops grown on it, turning field boundaries into a kind of unconscious intercropping at a larger scale (A, B). For illustration, field boundaries are marked with white lines (A) and field length and width are between 5 to 20 m.
Competition results not only in a survival of the fittest, but also in an optimal use of ecological niches. Agriculture can utilize interspecific competition in order to adjust cropping systems. Some attempts have been made to investigate and improve the various forms of intercropping. An increasing number of these research efforts, especially during the 1990’s, were done by modeling studies in order to simulate interspecific competition. Most models dealing with interspecific competition are common crop grow or crop/weed models extended with a submodel or additional algorithms. In most cases, modeling a cereal-cereal interaction, the crops of choice are a cereal-legume mixture as on one hand, this crop combination is a common and widespread intercropping system due to the advantages of nitrogen supply by the legume and on the other hand, these species are already included in most crop growth models.
Nevertheless, intercropping has always been considered as a secluded cropping system within one field so far. But in African and Asian countries, where intercropping is widespread, the system can be extended to a much larger scale: common on-field intercropping goes along with small field size on average, low mechanization level and hence, small field boundary distances. For example in China, where the average farm size is around 0.1 ha, small fields alternate as stripes with different crops grown on it and turning field boundaries into a kind of unconscious intercropping at a larger scale. To simulate not the secluded intercropping system explicitly, but the field boundaries could turn modeling of a single cropping system into modeling of more regional considered cropping patterns.
Competition for light seems to be the most palpable, both, for measurements in the field and for submodeling. However, intercropping cannot be considered solely as a change in available solar radiation within a dominant and understorey canopy, as it influences also soil properties like temperature and moisture, root distribution, microclimate conditions like wind speed and humidity, pests and diseases and nutrient availability for the plants standing next to each other. The possibilities to model intercropping are various: modified weather and climate-, soil-, growth factors and plant health-indices are imaginable. But as data collection in the field is difficult in intercropping systems, modelers often restrict modeling of intercropping to competition for solar radiation (Ball & Shaffer, 1993; Baumann et al., 2002; Lowenberg-DeBoer et al., 1991; Wiles & Wilkerson, 1991). Even there, weekly plant samples and data collection or samples in more frequent intervals are necessary. However, most models simulate the effect of intercropping using a similar model approach for simulating e.g., competition for solar radiation (see subsection ‘background’).