Crop rotation

What

Crop rotation is the change of crops between harvests. This can be from one year to another or, depending on climate and crop type, several times over a season. In contrast, if the same crop is used all the time, no crop rotation is used. A crop rotation system can also include crops which main purpose is not to produce food or fiber directly, but to improve soil or reduce emission of climate gases, for example. Crop rotation systems include cover crops, green manure and intercropping. 

 

Where

Crop rotations of different kinds can be used in all ecosystems where crops are grown. What crops to use and how often to rotate is heavily dependent on regional and local climate, market, availability and farmer preference. 

Why

Crop rotations have the potential to counteract societal challenges mainly related to food security, biodiversity enhancement and disaster risk and preparedness. This is done through the following pathways:  

  • Reduced pest and weed build up for a certain crop 

  • Reduced erosion of soil between cultivation of food crops 

  • Reduced greenhouse gas emissions 

  • Improved nutrient availability in soils 

  • Improved soil biodiversity 

  • Increased diversity of resources for different organisms both below and above ground. 

  • Pull pollinators or natural enemies into the crops and/or push pest species from the crops 

How

Altering the crop rotation system needs a restructuring of the farming practices as well as agro-market structures, which can be both time consuming and costly, but it may have benefits in both the short and the long term. A new crop rotation system can increase the number of crops used on the farm, not only temporally but also spatially. This comes from the fact that not all fields on the farm are at the same stage in the crop rotation system, which means that neighboring fields usually differ in what is grown on them a single year. Growing cover crops between the harvest and the sowing next year, covers the soil and store nutrients that can be used more by the following crop. 

Cover crops and green manure

In agriculture, cover crops are grown to improve the soil. They are planted between the harvests of cash crops to cover what would otherwise be bare soil and protect it from erosion and nutrient loss. Cover crops are chosen to be easy to incorporate into the soil before the main crop is planted. Usually, annuals, which are taken away before they set seed.  

Green manure crops are used to bind nitrogen and add organic matter to the soil, improve soil life, and make other micro-nutrients available. The green manure crops usually consist of legume plants which improve soil when they decompose in the soil, thus releasing the nutrients. 

Intercropping 

Intercropping is not a crop rotation system per se, but makes use of increasing diversity of crops over space, and not time, as is the case for cover crops and green manure. For example, growing two different crops together can benefit their viability by keeping moisture in the soil or making use of the space more efficient, but also to increase variation in the field to strengthen the existence of natural enemies to pests. This is an extensively used method in, for example, agroforestry practices and in several agroecological practices. 

Intercropping works with both annual and perennial crops, or a mixture of annual and perennial crops, one example of intercropping is agroforestry. Agroforestry is one of the relatively rarely used NbS in western agriculture. One reason could be that harvesting may demand more manual labour as the different crops are grown close together and cannot be harvested in the same way with machinery. Globally the proportion of cropped land using inter-cropping varies widely from around 20% to as high as 94%. 

Inter-cropping combines two or more crops close to each other. This can be done in different ways.  For example, by having a mixture of two or more crops without any distinct separation within the field, or by growing different crops in each other row, or by growing two or more crops together for some part of the time, e.g. sowing a second crop a bit before the first is harvested. Intercropping can be planned to facilitate harvesting, for instance by planting different crops with enough distance between them to enable machine harvesting of each crop separately. 

Potential outcomes

Most farmers use some type of crop rotation already, but there are added benefits of increasing it further. Potential benefits of crop rotation of some sorts include the increase of soil biodiversity and soil organic matter and carbon content, this is especially true if legumes are included in the rotation.

Further benefits are reduction of soil erosion. Crop rotation also helps reduce the built-up pest load in the field and surrounding areas and can improve resource use as different crops have different needs of soil resources.

Inter-cropping can have benefits for the total yield and resilience to pest species and weather variability if chosen carefully. Perhaps the best-known example of this is the Central American milpa where maize, beans and squash are grown together. Here, the maize is offering support for the climbing beans, the beans fix nitrogen in the soil for next year, and the squash covers the ground and reduces weeds. This maximizes the use of space, and potentially resources, and can also suppress weeds and reduce water loss. In Europe the mix of, for example, oats and peas is a common way of producing fodder high in nutrients. 

The intercropping can also be used in a type of push-pull system where some plants attract pollinators and natural enemies, or repel and push away pest species, which in turn is beneficial for the main crop. 

ATTENTION

This NBS necessitates new cultivation practice which can be demanding for the farmer in terms of new knowledge and management. Sometimes it can require operation and purchases of new machinery as well as identification of new markets if it concerns crops that are harvested and not only, soil improving. 

Some cover crops are treated with herbicides before the main crop is planted, which can negatively affect either other living organisms, or if you make a mistake, damage the crop you plant afterwards. The cover crops are sometimes mandatory for farmers to use to reduce emissions of greenhouse gases from bare soil. 

How much do we know?

There are several studies on crop rotation but not all point in the same direction and the certainty of this NBS is of medium strength. How well it delivers in relation to the societal challenges heavily depends on the rest of the farming practices, the soil type and the local climate. 

Costs

There can be added cost to management in terms of both time, seed purchase and fuel for machinery. However, the main point of this NBS and sub NBS is not only to be beneficial to the environment but also to improve or maintain yield while decreasing external inputs including irrigation. 

References

Brooker, Rob W., Alison E. Bennett, Wen-Feng Cong, Tim J. Daniell, Timothy S. George, Paul D. Hallett, Cathy Hawes, et al. 2015. “Improving Intercropping: A Synthesis of Research in Agronomy, Plant Physiology and Ecology.” New Phytologist 206 (1). John Wiley & Sons, Ltd: 107–17. doi:10.1111/nph.13132. 

Liebman, Matt, and Elisabeth Dyck. 1993. “Crop Rotation and Intercropping Strategies for Weed Management.” Ecological Applications 3 (1): 92–122. doi:10.2307/1941795. 

Trenbath, B. R. 1993. “Intercropping for the Management of Pests and Diseases.” Field Crops Research, Intercropping-Bases of Productivity, 34 (3): 381–405. doi:10.1016/0378-4290(93)90123-5.