Crop Rotation, Intercropping, and Cover Crops

• It increases soil fertility: legumes (such as groundnuts and beans) fix nitrogen in the soil. When their green parts and roots rot, this nitrogen can be used by other crops such as maize. The result is higher, more stable yields, without the need to apply expensive inorganic fertilizer. 

• It helps control weeds, pests and diseases: planting the same crop season after season encourages certain weeds, insects and diseases. Planting different crops breaks their life cycle and prevents them from multiplying. 

• It produces different types of output: growing a mix of grain, beans, vegetables and fodder means a more varied diet and more types of produce to sell. 

• In some ways, crop rotation takes the place of ploughing the soil: it helps aerate the soil, recycles nutrients, and helps control weeds, pests and diseases. Intercropping, strip cropping and relay cropping bring many of the same advantages as rotation. 

Criteria for Crop Rotation:

A) Crop Selection

Before selecting the crops, it is necessary to answer the following question: 

• What to produce? Crops produce many different things: food, fodder, firewood, fence poles, thatch and medicines. Farmers grow some crops (such as cotton) only for cash. For other crops, such as cereals or vegetables, you may be able to sell what you do not use yourself. If your objective is marketing, make sure that there is a market of your main output or rotation crop. 

• Will it grow well? This depends on many factors: the amount of rain or moisture in the soil, the season (some crops and varieties do not grow well at certain times of year), the soil fertility, among others. 

• What are the roots like? Tall cereals (millet, maize, sorghum, etc.), finger millets and some legumes (e.g., pigeonpea and sunn hemp) have strong roots that penetrate deep into the soil – up to 1,2 m for tall cereals. Their roots improve the soil structure and porosity, so are a good choice if the soil is compacted. (Figure 6-2). 

• Does it improve the soil fertility? Legumes improve the soil fertility by fixing nitrogen from the air. They use part of it for their own needs, and leave the rest in the soil. Cereals and other plants can use this nitrogen if they are intercropped with the legume, or if they are grown as the next crop in the rotation. 

• Does it cover the soil well? Tall cereals do not cover the soil well because they have upright leaves and they are planted far apart. Short grasses (Brachiaria, Cenchrus, Andropogon) and many legumes (lablab, groundnut, cowpea, beans) cover the ground very quickly after they are planted. When their main use is indeed to provide cover, we call them cover crops. If their main use is to provide food, we call them food legumes (beans, groundnuts). 

• Does it work with other crops? Try to find combinations of crops that complement each other well (Table 6-1). For example, cereals grow well with legumes (either food legumes or cover crops): the cereals benefit from the nitrogen fixed by the legume. Two different legumes or two different cereals do not usually work well together. If you have problems with Striga in your field, you may want to grow trap crops such as Crotalaria or Tephrosia to encourage the Striga to germinate and die when they do not find any suitable plants (such as maize or sorghum) they can live off. It may be more difficult to find the right combination of crops for your situation. You and your neighbours can try out new combinations to see which ones work. Or you can check with extension workers, researchers or farmers in other villages to see what they suggest. 

b) Choosing the right varieties 

Farmers all know that not all sorghum is the same. Some varieties grow quickly and produce a yield in a short time. Others take longer until harvest. Some are taller than others, or produce more leaves. Some demand more or less nutrients, some are more tolerant to drought or Striga. The same is true for other crops. For example, some varieties of cowpeas can be harvested in 55 days; others take more than 100 days. Some climb, while others crawl on the ground. Choose a variety that has the characteristics you want. Make sure you get the right seed. If you find a variety that you like, consider producing your own seed to sow in the future. 

c) Choosing a crop rotation 

What crops should you plant next year, and the year after that? That depends on many factors, here are some considerations: 

• Knowing the family where your crops belong to helps you to decide what to plant on the next cropping season, by planting a crop that belongs to a different family to the previous one. The table below provides various crop families and their common names (Table 6-2): 

Make a list of the crops you want to grow, considering the following recommendations (Mohler and Johnson 2009): 

General Recommendations:

- Grow winter cover crops BEFORE late-planted crops to accumulate organic matter and nitrogen. 

- Grow winter-killed cover crops (oat-pea) BEFORE early season crops, so the seedbed will be easy to prepare. 

- NEVER grow any crop after itself. 

- Certain insect pests and diseases may spread easily from one crop to the next through the crop residues. Avoid crop combinations where this is a problem. 

- Markets do not always exist for new crops; however you may want to plant some of them as part of your rotation. However. If your objective is marketing, ensure that there is a market for your main output and rotation crops. 

- In addition it is important to check the source of seeds and price of the output before you decide which crops to plant. 

Nightshades (Tomatoes, Potatoes, Peppers, Eggplants):

- Grow tomatoes AFTER peas, lettuce, or spinach, because tomatoes need a considerable amount of nutrients. 

- Grow lettuce BEFORE potatoes, because it is a light feeder and an aboveground crop. 

- Grow legume cover crops BEFORE potatoes or corn, so that they can feed the crops. 

- Grow potatoes BEFORE crops that are poor competitors, because potato production involves aggressive cultivation and further working of the soil during harvest, both of which reduce weed pressure. 

- AVOID growing potatoes before corn, because both are heavy feeders. 

- BE CAUTIOUS when growing bell pepper before another vegetable crop, because of diseases. 

- AVOID planting potatoes after corn, because of wireworm problems. 

Grasses, Corn and Grains:

- Grow beans AFTER corn to rebuild nitrogen. 

- AVOID growing legumes before small grains to prevent lodging. 

Alliums:

- Use a summer fallow AFTER onions, because usually there are many weeds. 

Lettuce and Crops in the Beet and Spinach Family:

- Grow peas BEFORE fall greens, because there is time for double cropping, and fall greens benefit from the nitrogen fixed by the peas. 

- Grow a root crop like beets AFTER lettuce or cabbage 

B. Intercropping

Intercropping refers to the practice of growing two or more crops in close proximity: growing two or more cash crops together, growing a cash crop with a cover crop, or other non-cash crop that provide benefits to the primary crop (Mohler and Johnson 2009). 

However, this practice requires additional management to keep competition between intercropped species in balance. When two or more crops are growing together, each must have adequate space to maximize cooperation and minimize competition between them. To accomplish this, four things need to be considered: 

1) Spatial arrangement, 

2) Plant density, 

3) Maturity dates of the crops being grown, 

4) Plant architecture. 

There are at least four basic spatial arrangements used in intercropping. Most practical systems are variations of these: 

• Row intercropping—growing two or more crops at the same time with at least one crop planted in rows. This can be beneficial in situations when using tall crops to reduce drought or heat stress of shorter crops, by providing shade and reducing wind speed (Figure 6-3). 

• Strip intercropping—growing two or more crops together in strips wide enough to permit separate crop production using machines but close enough for the crops to interact, for example, intercropping beans and maize. Legumes have a nitrogen-fixing bacteria associated with their roots. Consequently they compete slightly with non-legumes for nutrients, and in some cases even supply nitrogen to adjacent plants (Figure 6-4). 

• Relay intercropping—planting a second crop into a standing crop at a time when the standing crop is at its reproductive stage but before harvesting (e.g transplanting lettuce next to tomatoes plants). The lettuce will use the space that is not yet occupied by the tomatoes and is harvested about the time the tomatoes are branching out to cover the width of the bed. 

• Mixed intercropping—growing two or more crops together in no distinct row arrangement (for further details of possible combination, please see Table 6-1). Some crops may also be sown as a border crop or as a trap crops at the hedges of the main crop to reduce pests. The pest, arriving in the field from the edges, encounter the trap crop (which is strongly preferred than the main crop) and stops. The trap crop may be sprayed with natural insecticide to control the pest, before it moves to the main crop (Figure 6-5).

A crop mixture with different growth forms or development may make cultivation and use of mulches more difficult and less effective. Therefore planting crop in alternate rows greatly simplifies management. 

Intercropping may also represent a problem for crop rotation. Knowing that one fundamental principle of crop rotation is the separation of plant families in time, replanting two families mixed in the same field may be difficult. However, a good planning could maintain a viable crop rotation. For example, suppose that a farm grows an area with tomato, squash, broccoli and lettuce. A simple rotation would put each of the crops in a different year, with a three year interval before a crop is repeated on the same bed in order to keep some diseases and pest under control. 

C. Cover Crops

Every plant which covers the soil and improves soil fertility can be a cover crop. It could be a leguminous plant with other beneficial effects, or it could be a weed characterised by its rapid growth and enormous production of biomass. The most important property of cover crops is their fast growth and the capacity of maintaining the soil permanently covered. 

The following characteristics make an ideal cover crop (Figure 6-6): 

• The seeds are cheap, easy to get, to harvest, to store and to propagate 

• Be of rapid rate of growth and be able to cover the soil in short time 

• Be resistant against pests and diseases 

• Produce large amounts of organic matter and dry material 

• Fix nitrogen from the air and provide it to the soil 

• Have a de-compacting root system and regenerate degraded soils 

• Easy to sow and to manage as single crop or associated with other crops 

• Can be used as fodder, grains as food grains 

The Example of Cowpea as a Cover Crop

Cowpea (Vigna unguiculata, French: Niébé) is an important grain legume throughout the tropics and subtropics. It has some properties which make it an ideal cover crop: 

• It is drought tolerant and can grow with very little water 

• It can fix nitrogen and grows even in very poor soils 

• It is shade-tolerant and therefore compatible as an intercrop 

• It yields eatable grains and can be used as an animal fodder rich in protein 

• It is quite resistant to pest attack 

Subsistence farmers in Sub-Saharan Africa usually intercrop cowpea in maize, sorghum, millet and cassava. 

Other legumes used as cover crops are alfalfa (Medicago sativa), crimson clover (Trifolium incarnatum), Faba beans (Vicia faba) and hairy vetch (Vicia vellosa). 

Some cover crops are used to improve the soil structure and to add organic matter to the soil; examples of non-legumes crops used for this purpose include barley (Hordeum vulgare), buckwheat (Fagopyron esculentum), oats (Avena sativa), annual rye (Lolium multiflorum), winter wheat (Triticum aestivum). 

D. Crop-Animal Association

This practice integrates crop and livestock systems. In this case, cropping provides animals with fodder from grass and nitrogen-binding legumes, leys (improved fallow with sown legumes, grasses or trees), weeds and crop residues. Animals graze under trees or on stubble, they provide draught and manure for crops, while they also serve as a savings account (FAO, 2001). 

An experimental farm in Thailand maintains pigs and chickens, as well as a vegetable garden and a fish pond. Animal wastes are used for fertilizer, fish feed and biogas generation. Crop and human wastes are also added to the biogas unit. Liquid effluent from the biogas generator is used in the fishpond and solid residues on the garden. Periodically, the locations of the garden and the pond are reversed, so residues from one serve as nutrients for the other (Based on BOSTID, 1981; FAO 2001). 

E. Designing Cropping Systems

Cropping systems should be designed in such a way that the soil is almost permanently covered with plant canopy. In arable crops, careful timing of sowing and planting can help to avoid uncovered soil being washed away during the rainy season. 

After the main crops are harvested, a green manure crop may be sown (Figure 6-7). On slopes, crops should be grown in lines across the slopes (along contour lines) rather than vertically. This can contribute enormously to reduce the speed of surface water, thus erosion. In crops which take some time to develop a protecting canopy, intercropping of fast growing species, such as beans or clover, can help to protect the soil in the initial stage of the main crop. 

In order to ensure a permanent plant cover it is important to consider the following aspects: 

• Timing of soil cultivation 

• Timing of planting or sowing 

• Producing seedlings and transplanting them 

• Mixed cultivation 

• Intercropping 

• Cover crops 

• Mulching 

• Timing of weeding 

• Sowing of a green manure crop in the off-season (Figure 6-8) 

• Expected effect on yields 

• Availability of suitable species 

• Costs of seeds 

• Availability of water 

• Availability of labour 

• Additional use of side-crops 

• Reduction of the risk 

• Food security 

Record Keeping is important!

A well-kept field record book is a great help in remembering which crop has in the past been grown in a particular plot within the field or farm. This is useful especially if the records also show past incidents of plant pests or diseases in each plot in the farm. 

For example, soil diseases and pests can build up during the life of a susceptible crop. If the same crop or a similar type belonging to the same family is grown in the same field, it will suffer from the accumulated pests and diseases from the previous crop(s) and may not grow well. This can be avoided if the soil is left fallow (not cropped) for a while, or a different crop is planted which is tolerant or resistant to the particular pest or disease. Better still is to plant a crop from a different family which will not share a same complex of pests and diseases. This will result in decline of soil problems and the original crop can be grown successfully again. 

REFERENCES 

BOSTID. 1981. Food, fuel and fertilizer from organic wastes. Report of an ad hoc panel of the Advisory Committee on Technology Innovation. Board on Science and Technology for International Development (BOSTID). Washington, DC, National Academy Press. pp. 154 

FAO. 2001. Mixed crop-livestock farming: A review of traditional technologies based on literature and field experiences. FAO Animal Production and Health Papers pp. 152 

IFOAM. 2003. Training Manual for Organic Agriculture in the Tropics. Edited by Frank Eyhorn, Marlene Heeb, Gilles Weidmann, p 124-129, 149-155, http://www.ifoam.bio/ 

IIRR and ACT. 2005. Conservation agriculture: A manual for farmers and extension workers in Africa. International Institute of Rural Reconstruction, Nairobi; African Conservation Tillage Network, Harare 

Kuepper G. and Dodson M. 2001. Companion planting: basic concepts & resources. Horticultural technical notes from the Appropiate Technology Transfer for Rural Areas (ATTRA). National Center for Appropiate Technology (NCAT): https://attra.ncat.org/ 

Mohler C.L., Johnson S.E. 2009. Crop Rotation on Organic Farming: A planning manual. Natural Resource, Agriculture, and Engineering Services (NRAES), Cooperative Extension, Ithaca, NY. 

SOURCES 

NRC 

ON TECA 

Crop Planning and Management in Organic Agriculture: http://teca.fao.org/read/8367