Multiple cropping is a form of Ecological Intensification that is potentially highly sustainable when two or more crops are grown at the same time or in a sequence. It does this by balancing three key ecological processes: competition, on the one hand, and commensalism (one plant gaining benefits from the other) or mutualism (both plants benefitting each other) on the other. Typically, farmers will plant crops as close together as possible to utilise all the available land. When different crop species or varieties are grown together, the competition may be fierce; trees grown in a maize field, for example, may shade out the crop. But this can be compensated for by determining the optimal spacing and by exploiting various forms of commensalism or mutualism, for example where the tree may be a legume, providing nitrogen for the crop plant beneath.
There are numerous examples of multiple cropping:
Intercropping: interspersion of different crops on the same piece of land, such as a home garden, either at random or more commonly in alternate rows usually designed to minimise competition but maximise the potential for both crops to make use of the available nutrients, such as nitrogen supplied by a legume.
Rotations: the growing of two or more crops in sequence on the same piece of land.
Agroforestry: annual herbaceous crops are grown interspersed with perennial trees or shrubs. The deeper-rooted trees can often exploit water and nutrients otherwise unavailable to the crops. The trees may also provide shade and mulch, creating a microenvironment, whilst the ground cover of crops reduces weeds and prevents erosion.
Sylvo-pasture: similar to agroforestry, but combines trees with grassland and other fodder species for livestock grazing. The mixture of shrubs, grass and crops often supports mixed livestock populations.
Green manuring: the growing of legumes and other plants to fix nitrogen and then incorporate the nutrients into the soil for the following crop. Commonly used green manures are Sesbania and the fern Azolla, which contains nitrogen-fixing, blue-green algae in ricefields.Contribution to Sustainable Intensification
Multiple cropping is highly sustainable as it relies on reducing competition and increasing mutual benefits between crops. It can provide a more efficient use of resources, such as soil nutrients, that would not otherwise be available to a single crop; support or shade a companion crop; or host a great diversity of insects, bacteria and other organisms that contribute to pest and disease control. Nitrogen is essential for plant growth but it is often lost from the soil through unsustainable agricultural and soil management practices. Multiple cropping with nitrogen-fixing legumes can improve soil quality by replenishing the soil with N. Increased N in the soil decreases the reliance on both organic and inorganic fertilisers, reducing the amount of money and labour farmers need to spend on inputs. At the same time, the natural capital of the soil is conserved. Increasing the availability of N in the soil is associated with improved crop growth, which can in turn improve the livelihood of the farmer. Further, incorporating nutrient dense crops such as legumes into the rotation can increase farming households access to nutritious foods, improving their nutrition security.
Multiple cropping of compatible crops can encourage biodiversity, by providing a habitat for a variety of insects and soil organisms that would not be thrive in a mono-crop environment. In turn, this may limit the number of outbreaks of pests by increasing the number and diversity of natural predator biodiversity. Additionally, reducing the homogeneity of the crop increases the barriers against biological dispersal of pest organisms as the pests may not be attracted to all crops within the system. However, certain insects, pests and diseases may spread easily from one crop to the next through crop residues.
Growing a diverse variety of crops through multiple cropping is also thought to be critical to nutrition particularly where households grow the majority of the food they eat. Micronutrient deficiencies, a sub-set of under nutrition, occur when the body lacks one or more micronutrients (such as iron, iodine, zinc, vitamin A or folate). It is estimated that more than 2 billion people, especially children, lack sufficient micronutrients. Growing highly nutritious and diverse crops, such as legumes, can increase the amount of nutrients a household receives. Multiple cropping with crops that are biofortified with micronutrients such as Vitamin A orange fleshed sweet potato can add further nutritive value per hectare.
Multiple cropping acts as an insurance against failure of crops in abnormal weather conditions.The risk of total crop failure due to uncertain monsoon is reduced if two crops of a different nature are grown simultaneously as a mixed crop. Further, crops of a particular species are more prone to a particular type of pest (weed, insects, or diseases) infestation. When different types of crops are grown together the chances of pest infestations are reduced or diluted. This further reduces the chances of total crop failure for farmers.
Care must be taken to plan the timing, spacing and mixture of crops so that the balance between competition and commensalism or mutualism is enhanced. Examples of planting strategies include combining a deep-rooted crop with a shallow-rooted crop, or planting a tall crop with a shorter crop that requires partial shade. Managing rotations requires more skill than managing a single crop and harvesting may be more complicated than for mono-crop systems. Successfully implementing multiple cropping may therefore require training and education that can be difficult for smallholder farmers to receive.
Farmers may be reluctant to try out new crops that they are not used to growing or eating, or for which no defined market exists. Multiple cropping with cash crops can be highly profitable offering additional income from the sale of additional crops. However, when diversifying the crops grown on a farm, it is vital to ensure that there is either market demand or household demand for the new crops introduced. Local people may be identified and trained as extension workers to change the attitudes of farmers.