In this unit you would be studying about the historical development of crop production from the incipient stage to modern day crop production. The origin and distribution of indigenous and exotic field crops and its classifications are treated. This unit will give you a general overview of the historical development of crop production, its development and contribution in food and raw material supply, the place of Nigeria in world food production, constraints to crop production in Nigeria and measures that could help increase field crop production in the country are highlighted and discussed.
Historical Development of Crop Production
Early man lived on wild game, leaves, roots, seeds, berries, and fruits. As the population increased, the food supply was not always sufficiently stable or plentiful to supply his needs. This probably led to the practice of crop production. Therefore, crop production began at least nine thousand (9000) years ago when domestication of plants became essential to supplement natural supplies in certain localities. The art of crop production is older than civilization, and its essential features have remained almost unchanged since the dawn of history. These features are:
Gathering and preservation of seeds of the desired crop plants
Destroying other kinds of vegetation growing on the land
Stirring the soil to form a seedbed
Planting when the season and weather are right as shown by past experience
Protecting the crop from natural enemies
Gathering, processing and storing the product
The early husbandman cultivated a limited number of crops, the cereals being the first to be grown in most parts of the world.
The same crop was often produced continuously on a field until low yields necessitated a shift to new land. This is still common in parts of Africa. A modification of this practice was the introduction of bare fallow every two or three years. The primitive husbandman removed by hand the destructive insects in his fields and appeased the Gods or practiced mystic rites to drive away the evil spirits he believed to be the cause of plant diseases. With advancing civilization, materials such as Sulphur, brine, ashes, white-wash, soap and vinegar were applied to plants to suppress diseases or insects attack.
Cultivated plants are products of human achievement and discovery which has enable man to provide his food and fiber needs with progressively less labour.
The first successful domestication of plants by man has recently been suggested to have occurred in Thailand in Neolithic times.
The value of lime, marl, manures, and green manures for the maintenance of soil productivity was recognized 2000 years ago. Books on agriculture written by the Romans about the 1st century A.D. describe the growing of common crops including wheat, barley, clover, and alfalfa by procedures very similar to those in use today except that more of the work was done with hand and the implements then used were crude.
The old art of crop production still predominates in farm practice throughout the world. Plant pathologists and entomologists have found ways to control plant diseases and insect pests more effectively. Chemists and Agronomists have found supplements for manure and ashes formally used as fertilizers. Rotations perhaps are slightly improved. Many new crop hybrids and varieties (cultivars) have been developed. The control of weeds with herbicides was realized in the 20th century
Improved cultural methods, doubtless, followed observations made by primitive farmers. They discovered that crops yield better where manure, ashes or broken limestone had been dropped, or where weeds were not allowed to grow, or where soil is darker, deep, or well watered or where one crop followed certain other crops. Observations or empirical trials quickly revealed, roughly, the most favourable time, place, and manner of planting and cultivating various crops. These ideas were handed down through the generations.
Eventually, the exchange of ideas, observations, and experiences, through agricultural societies and rural papers and magazines, spread the knowledge of crops.
A VIDEO ABUOT CROP PRODUCTION AND MANAGEMENT
Origin of Cultivated Crops
All cultivated plants were domesticated from their wild species. However, the exact time and place of origin and the true ancestry of many crops are still as highly speculative as the origin of man. Man has domesticated some crop species that met his needs before the dawn of recorded history. Most of the domesticated crops were introduced into new areas far from their centres of origin by migrating human populations in prehistoric as well as in recorded times. As a result, both indigenous and introduced crops are grown everywhere in the world.
Centres of origin of Cultivated Crops
The centres of origin of both agriculture and culture were in populated areas favoured by a more equitable climate. Nicolai Ivanovic Vavilov (1926) concluded that a centre of origin was characterized by dominant alleles while towards the periphery of the centre, the frequency of recessive alleles increased and the genetic diversity decreased. He reported the following centres of origin:
Mediterranean Sea coastal and adjacent regions
Ethiopia or Abyssinia
South Mexico and Central America
South America (Peru, Ecuador, Bolivia, Chile etc.)
Contribution of the Different Centres
The following are the important crops that originated in the different centres. Some crops may have two centres of origin, (primary and secondary centres of origin).
China is one of the richest centres of crop origin contributing to many important crops such as Brassica campestris and related species,
Camellia sinenses, Colocasia esculenta, Corchorus sinensis, Glycine max, Panicum miliaceum, Raphanus sativus and Setaria italica. It is secondary center for Oriza sativa spp. japonica, Zea mays and other crops.
This region is important for such crops such as Cocos nucifera, Colocasia esculenta, Dioscoria spp., wild Oryza spp. and Saccharum officinalis
Important crops from this center include; Oryza sativa, Phaseolus mungo, Piper spp., Saccharum sinensis, Vigna sinensis and Cucurbita sativa
Central Asia centre
Among the important crops of this center include; Allium cepa, Daucus carota, lathyrus sativa, Spinacea oleracea and Vicia faba.
Near Eastern centre
This is the center of origin of Brassica olearacea, Hordeum Vulgare, Lens esculanta, Medicago spp., Secale spp., Triticum spp., Vicia sativa and Vitis vinifera
Many field crops have been domesticated in this region; Avena spp., Beta vulgaris, Brassica napus, B. oleracea, Lathyrus spp., Olea europaea, Raphanus sativus, Trifolium spp. and Vitis vinifera.
Ethiopian or African centre
Brassica juncea, Ceiba pentandra, Coffea spp., Cola spp., Cucumis spp., Gossypium spp., Hibiscus spp., Lablab purpureaus, Oryza spp., Pennisetum spp., Phoenix spp., Ricinus communis, Sesamum indicum, Setaria spp., Sorghum bicolor and Vigna unguiculata are all important crops of African centre of crops origin.
Central American and Mexican centre
Few important crops were domesticated in this region; Agaves pp., Capsicum spp., Gossypium spp., Ipomoea batatas, Phaseolus spp. and Zea mays.
South American centre
This centre account for most of the tuberous crops such as Solanum spp., Oxalis tuberose and Ullucus tuberous. Amaranthus spp., Arachis hypogaea, Capsicum spp., Lycopersicum spp., Lupinus spp., Manihot esculenta, Nicotina spp., Phaseolus spp., Solanum spp. and Theobromw cacao all have their origin from this centre. This centre also serves as secondary centre of diversity of Zea mays.
Spead of Cultivated Crops
The spread of crops from their centre of origin to other parts of the world was either by natural means or by agency of man.
Natural dispersal of crops
Coconuts may have floated across the Pacific Ocean from Asia to the western coast of Central America, and the capsules of sweet potatoes crossed the Pacific Ocean in the same way.
As people migrate, they take along with them cultivated plants to ensure a permanent food supply and support their culture.
Expansion of world trade
With the expansion of world trade, crops indigenous to the Americans such as: maize, groundnuts, sweet potatoes, potatoes, tomatoes and cassava were spread to other parts of the world. During the eighteenth and nineteenth centuries, the development of agricultural enterprises in the tropics was stimulated by the demand from Europe for agricultural raw materials for use in industry. As a result of these developments, many crops spread from one area to the other. For example: rubber from Brazil became popular in Malaysia, Sri Lanka and West Africa; American cotton became popular in the Old world and sugar cane became an economic crop of the new world.
International agricultural research collaboration
This has included the exchange of seeds or germ plasm between agricultural research institutions in different parts of the world in their programmes. Thus encouraging crop transfers across the globe
Indigenous Crops of Africa
Crops Introduced into Africa
There are many crops widely grown in Africa, that are introduced from other regions of the world. The most notable among them include the following:
This classification is based upon similarity of plant parts. Field crops belong to the spermatophyte division of the plant kingdom, in which reproduction is carried on by seeds. Within this division the common crop plants belong to the subdivision of angiosperms, which are characterized by having their ovules enclosed in an ovary wall. The angiosperms are divided into two classes, the monocotyledons and the dicotyledons. All the grasses, which include the cereals and sugarcane, are monocotyledonous plants. The legumes and other crop plants except the grasses are classified as dicotyledonous plants because the seeds have two cotyledons. These classes are subdivided into orders, families, genera, species, subspecies, and varieties.
The grass family
This includes about three fourth of the cultivated forage crops and all cereals crops. They are either annuals, or perennials. Grasses are almost all herbaceous plants, usually with hollow cylindrical stems closed at the nodes. The stems are made of nodes and internodes. The leaves are two-ranked and parallel-veined. The roots are fibrous. The small greenish flowers are collected in a compact or open inflorescence, which is terminal on the stem. The flowers are usually perfect, small, and with no distinct perianth. The grain or caryopsis may be free, as in wheat, or permanently enclosed in the floral bracts as in oats.
The legume family
Legumes may be annuals, biennials or perennials. Leaves are alternate on the stems, stipulate with netted veins, and mostly compound.
The flowers are almost always arranged in racemes as in the pea. The flowers of leguminous field crops are butterfly-like. The irregular flowers consist of five petals, a standard, two wings, and a keel that consists of two petals that are more or less united. The calyx is normally four- or five toothed. The fruit is a pod that contains one or several seeds. The root system is taproot. Often the roots have an abnormal growth called nodules caused by the activities of bacterium Rhizobium.
Other crop families
Among the other botanical families that contain crop plants are:
Cannabaceae (hops and hemp)
Chenopodiaceae (sugarbeets, mangels and wormseed)
Cruciferae (mustard, rape, and kale)
Solanaceae (potato and tobacco)
Compositae (sunflower ,safflower and Jerusalem artichoke )
Cereal or Grain crops
Cereals are grasses grown for their edible seeds, the term cereal being applied either to the grain or to the plant itself. Cereals include wheat, oats, barley, rye, rice, maize, sorghum, millets, etc.
These include pea nuts, field beans, cowpeas, soybeans, lima beans, mug beans, chickpeas, pigeon peas, broad beans and lentils. They all belong to the family Leguminosae and are grown for their edible seeds.
The oil crops include soyabean, peanuts (groundnuts), sunflower, safflower, sesame, castor bean, mustard, cotton seed, corn and grain sorghum, rape, flax and perilla, the seeds of which contain some useful oils.
Root and tuber crops
These include sugar beets, carrots, sweet potatoes, yams, cassava, potatoes and cocoyam.
These are grown for their fiber. They include cotton, jute, kenaf, hemp, ramie and sisal.
These are crops that are grown for their sweet juice from which sucrose is extracted and crystallized. They include sugarcane and sugar beet.
These are vegetable matters fresh or preserved that are utilized as feeds for animals. They include grasses, legumes, crucifers and other cultivated crops.
This group includes potatoes, tomatoes and onions.
Rubber crops/latex crops
These crops which include Para rubber, are grown for the milky sap, or latex which they produce.
These crops are also sources of stimulants. They include tea, coffee and cocoa.
These are crops planted to provide a protection to the soil against direct beating of rainfall. When crops are turned under while still green, they are termed green manure crops. Important green manure crops include alfalfa, soyabeans, cowpeas, rye, and buckwheat.
Catch crops are substitute crops planted too late for regular crops or after the regular crop have failed. Short season crops such as millet and buckwheat are often used as catch crops.
These are crops that are cut and fed green and may include legumes, grasses, kale, and maize.
Silage crops are those cut and preserved in succulent condition by partial fermentation. They include corn, sorghum, forage grasses and legumes.
These are crops that are grown with a crop such as alfalfa or red clover in order to secure a return from the land in the first year of a new seeding. Grain crops and flax are often used for this purpose.
These are crops planted to attract certain insects or parasites. Trap crops are plowed under or destroyed once they have served their purpose.
Trends in Crop Production Nationally and Globally
World cereal production in 1999 is forecast at 1 870 million tons (including milled rice). While on the supply side, the estimates are becoming firmer, the demand-related issues have yet to be determined. Global cereal utilization in 1999/2000 is forecast to rise only slightly, just less than one percent. Overall, the growth in direct food consumption of cereals is expected to keep pace with population increase. Nigeria with a total cereal production of18 million tones representing only 1% of the world cereal as reported by the Food and Agricultural Organisation of the United Nation (FAO) year book of 2002 and presented in tab.1.1 and fig. 1.1.
Constraints to Crop Production in Africa
Many factors have been cited as limiting the growth and development of crop production in tropical Africa. Here we concern ourselves more with social, economic and political constraints.
Poverty is indicated both by the few capital resources and the low cash incomes of African farming families. The small size of farm holdings (typically 1-5 ha.) represents very little collateral on a loan, and unless the family owns livestock there may be little prospect of raising money to buy equipment or meet emergencies. Income from the sale of farm produce is typically inadequate to meet essential family expenses such as taxes, school fees, medicine, clothing and house hold items. Rarely is enough left over for seed, fertilizer or chemicals, let alone for large capital items such as oxen, ploughs and water pumps. Thus the key components of Green Revolution Technology (seed, fertilizer, pesticides) are not generally accessible.
The adoption of improved varieties in Nigeria is minimal in the case of principal food crops. This is partly due to poor performance of the seed service agency in terms of production, quality assurance, and distribution of improved seeds.
The use of fertilizers and pesticide among the peasant farmers in Nigeria is low, this could be due to high cost or poor availability or both. The use of herbicide, insecticide and fungicides is minimal. Farmers generally lack spraying equipment and technical skills for the timely and effective application of pesticides, which if wrongly applied pose dangers to people, crops and wider environment. It is often not recognized that decisions concerning what, when and how much to spray are very complex, and sufficient guidance is rarely available to the farmer. Products are often sold without proper recommendations for use and threshold levels of insects infestation are rarely defined. The repeated application of the same chemicals may lead to rapid development of resistance to it. For this reasons pesticides represent a risk to small-scale farmers.
Labour constraints are particularly serious because of the low level of adoption of animal traction in the humid zones of Nigeria. The presence of tse-tse fly is one major reason for this but often it is poverty that prevents farmers from buying and using oxen and ploughs. Without animal traction, the area of land that can be cultivated by one adult is about 0.5 ha. Such small scale cultivation rarely provides a surplus of food or other crops for sale- a prerequisite for economic development. The use of oxen allows several hectares to be ploughed and perhaps weeded, opening up the possibility of marketing surpluses to fund the purchase of further equipment, new seeds and other inputs.
Measures of Improving Field Crop Production in Africa
Development of infrastructure
To achieve accelerated development in food crops production in Africa there is need to have a market orientation approach toward production process. Without selling their produce, farmers cannot usually raise enough cash necessary for farm maintenance and improvement, whether in form of a new plough, seed or fertilizer, extra labour or insecticide nor provide for his family. Markets and prices depend on access to sale points by traders and farmer. A good road transport system is vital in this respect.
The fewer and poorer the roads, the lower will be the gain of the farmer, because the cost of moving the farm products will be too high. Farming communities also need better water supplies. . Bringing good quality water nearer the farm reduces the time spent in collecting it. Provision of health care facilities closer to the farmers would enhance their health status and productivity. Development of infrastructure does not in itself produce more food but it strengthens and encourages the farming population and also helps to create an enabling environment for crop production to thrive and prosper.
The incentive approach
Government need to put in place incentives that would ensure stable prices high enough to stimulate and encourage farmers to produce more. Farm price support has proved very successful in the EU- to the point of over production. It is hoped that if implemented here, would produce similar results. Because inputs are very expensive for most peasant farmers, incentives are usually necessary if they are to be adopted on a significant scale. Incentives can take the form of direct subsidy, improved access to credit and granting ownership of land to peasant farmers. However subsidies need to be implemented with care to avoid corruption.
The institutional approach
Government agricultural development agencies are created to help the farmers through training and extension, to implement government policies and monitor performance. But unfortunately such government staffs are poorly motivated and ill- equipped. Extension services are frequently ineffective due to low morale among extension officers. This may be due to a top-down approach, under-funding, recommendation that is too complex or costly.
In addition there are severe logistical problems in disseminating information to the large numbers of small-scale farmers scattered over the vast areas in the Savanna. Most of the programs for accelerated food production programme are typically designed top-down approach instead of bottom-up planning. That is why most of these programmes remain out of touch with farmers’ real constraints and abilities.
A ‘them-and us’ mentality develops, so that soon, staff and beneficiaries have different attitudes to and expectation from the project. There is little identification by the people with the objectives of the programme, and no local enthusiasm to take it over and develop it. These are not inevitable results, but avoiding them needs great imagination, careful planning and inspired leadership.
Transfer of technology and green revolution
Transfer of technology could be described as an attempt to overcome local problems with exotic technology. The typical example of transfer of technology is the green revolution. More accurately referred to as the seed-fertilizer revolution, has centred around the simultaneous adoption of high-yielding varieties and greatly increased levels of chemical fertilization.
Pesticides, especially insecticides, have been required to counteract the greater susceptibility to pest and diseases associated with this combination, and improvements to irrigation and husbandry have frequently been added to the package. The seed-fertilizer revolution is based, then, on a package of inputs, most of which represent technology transferred from elsewhere but adapted to local condition through regional or national research programmes.
Very large increases in production have resulted from rapid adoption of this technology in a number of countries in Asia and Latin America but with limited successes in Africa south of the Sahara. This is partly due to scarce resources and poorly developed infrastructure to take full advantage of the new technology. To take advantage of this technology in Sub-Saharan Africa the following need to be put in place:
Appropriate plant breeding or selection from existing material, concentrating on yield stabilization.
Establishing efficient and effective seed multiplication system, especially for food crops.
Strengthening communication and infrastructure for marketing of crops and inputs.
Increased use of animal plough in order to overcome labour and timing constraints.
Involving the farmers more effectively in farming system research and programme formulation, implementation and appraisal.
Greater and consistent commitment to increased food production by national government.
Incentives for trained personnel to stay and work in their own countries.
Definition of Cropping System
The term cropping system is used to describe the pattern in which crops are grown in a given area over a period of time and includes the technical and managerial resources that are utilised. In short, when we talk about the cropping system of a given area, we are not only interested in how those crops are distributed on the field at any given time but also how this distribution changes over time. In addition, the level of management and amount of resource inputs are integral aspects of a cropping system.
Cropping systems are classified based on the following criteria
The distribution of crops in time; whether shifting cultivation, continuous cropping, monoculture, or crop rotation is practised.
The distribution of the crops in space on the field; whether intercropping or sole cropping is practised.
The level of management and resources utilised to produce the crop; whether production is intensive or extensive.
The type of crop grown; whether orchard, arable cropping, pasturing, forestry, etc. is practised.
In this system, the farm is not at a permanent location. Instead, a piece of land is cleared, farmed for a few years and then abandoned in preference for a new site. While the new site is being farmed, natural vegetation is allowed to grow on the old site. Eventually, after several years of bush fallows, the farmer returns to the original location. The practice of moving the home along with the farm is discontinued and in its place the practice of making home stationary is common in tropical Africa.
Common features of shifting cultivation
The farmer first selects a site which has been under bush fallow for several years.
Clears the vegetation by burning.
Crops are then grown on the field for one, two or three years, starting with crops with high nutrients requirement and ending with crops that has low nutrients requirement.
Low levels of technology, input and management.
Most of the operations are carried out using simple hand tools and the labour requirements are high while the yields are correspondingly low.
Factors necessitating shifting cultivation
Declining of soil fertility and increasing population.
Unusually high incidence of diseases and pests.
Social or religious customs may dictate the abandonment of site before its fertility level has become marginal.
Disadvantages of shifting cultivation
It tends to discourage high level of inputs.
Because the farms stays in one location only for a short while, there is no incentive to invest in permanent structures such as store sheds, irrigation and even certain pest control soil erosion or soil conservation measure that may have a long-term benefits.
It requires a great deal of land to maintain the system.
Low efficiency in land utilisation.
Low efficiency in labour utilisation.
In contrast to shifting cultivation, continuous cropping implies the cultivation of the same piece of land year after year. Fallowing may occur, but it never occurs more than a season or two. The absence of a protracted fallow periods means that other soil management practices must be employed in order to maintain high soil fertility.
Agricultural practices for maintaining soil fertility under continuous cropping
Application of fertilisers and other soil amendments in order to boost fertility.
Judicious selection of the crops and crop combinations to be grown.
Crop rotations and carefully planned intercrop combination are indispensable.
Introducing short term fallow periods in to the cropping cycle. A leguminous cover crop can be planted on the fallow land so as to aid the fixation of nitrogen by legumes during the fallow period and through increasing the soil organic matter content when the fallow crop is ploughed under
Advantages of Continuous Cropping
Land utilisation under continuous cropping is extremely efficient. A very high percentage of land is under crops at any given time.
It is possible and economically feasible, to erect permanent structures on the farm site.
The practice of growing different kinds of crops, one at a time, in a definite sequence on the same piece of land is referred to as crop rotation.
In designing a good crop rotation, the farmer must decide what crops to have in the rotation, in what sequence the crops should occur, and for how many years or season each cycle of the rotation must run.
A good rotation that provides for maintenance or improvement of soil productivity usually includes a legume crop to promote fixation of nitrogen, a grass or legume sod crop for maintenance of humus, a cultivated or intertilled crop for weed control and fertilisers. Perennial legumes and grasses may leave two to three tons of dry weight per acre of roots residues in the soil when plowed down.
Factors that affect crop rotation
The choice of a rotation for a particular farm depends upon the following:
Adaptation of the crops to a particular soil, climate, and economic conditions
Prevalence of weeds, plant diseases, and insect pests may also limit the kinds of crops that can be grown in a locality
Crops may be selected for rotation so as to spread labour throughout the year.
Factors to consider in deciding the sequence of crops (principles of crop rotation)
The target crop (the main crop) should be planted immediately after the legumes or fallow period. At this time the fertility of the soil is at its peak and the optimum realisable yield of the target crop is possible. Crops which are known to have a high demand for nutrients are also timed for the first season after the fallow.
Crops which are deep feeders should alternate with shallow feeders. In this way, nutrient removal occurs uniformly from the various soil layers rather than occurring in only one layer.
Crops that are botanically similar or are likely to be attacked by the same diseases and pests should not normally follow each other in the rotation. Yams, for example, should not follow cow-peas in rotation if the root-knot nematode is prevalent, as the nematodes left over from the cow peas crop will severely reduce yam yields. However, if the nematode problem does not exist in the area, yam could conveniently follow cow-peas.
The number of years for which each cycle of the rotation should run is determined by the number of crops in the rotation, the length of their growing seasons and how frequent the farmer can grow the target crop without running into problems of disease and soil fertility. For example, the time interval between the harvesting of the target crop and its being planted again on the same piece of land should be long enough to prevent the carryover of pathogens in crop residues from one cycle to the next.
Types of Crop Rotation
In planning crop rotation, the farmer may decide to consider his entire field as one plot. He then rotates the crops in sequence on the field. At any given time, there is only a crop on the field, and that crop would not return again until the next cycle some years later. This is commonly practiced in Northern Nigeria.
Example of a 3-Year Crop Rotation as Practiced in Savanna Zone of Nigeria
This system, however, has certain disadvantages:
The growing of one crop means that the demand for labour occurs in peaks. Labour demand is more evenly spread if many crops are grown simultaneously.
The risk of crop failure is ever present, and the risk is greater where only one crop is grown.
Since each crop occurs on the farm only once every several years, specialized facilities for the target crop, can only be utilized once in several years, a situation which is definitely inefficient.
Most farmers who practice crop rotation find it more convenient to divide their field into as many plots as there are years in the rotation. The farmer then starts with a different crop on each plot and progress through the rotation. In this scheme, all the crops are present on the farm at any given time.
Example of such type of rotation is given below.
Example of a 3-Year Crop Rotation Found in
Advantages of Crop Rotation
It is an effective means of controlling diseases and pests. The pathogens and pests of a particular crop are more likely to die off when their host crop is followed by a completely different nonhost crop. Many insects are destructive to only one kind of crop. The life cycle is broken when crops grown are un favourable to the development of the insect pest. Cotton root-knot can be reduced by the growth of immune crops in the rotation.
Crop rotation is the most effective practical method for controlling many farm weeds. Some weeds are particularly adapted to cultivated crops, the absence of such host crop in the field for many years due to rotation, effectively control the weeds. Rotation may include smother crop as a means of controlling certain weeds.
The type of crop rotation where the field is divided into several plots, offers the farmer some insurance against crop failure, and enables him to spread out his labour needs.
Crop rotation is an effective means of reducing erosion in comparison with continuous cropping. Grass legumes mixtures in a rotation have been very effective in the reduction of erosion.
This is the practice of incessantly cultivating the same type of crop on the same piece of land year after year. For example, sugar cane farming in Bachita, Kwara State, Nigeria.
Disadvantages of mono culture
In mono culture diseases and pests of the particular crop always have their host present, and therefore have the opportunity to build up over the years.
Mono culture encourages rapid depletion of soil nutrients and destruction of the soil structures.
The risk of crop failure is great and ever present.
Advantages of mono culture
The main advantage of mono culture is that it permits maximum concentration of production effort on a single target crop.
The practice of growing one crop variety in pure stands on a field is referred to as sole cropping. In this practice, only one crop variety occupies the land at any one time. The alternative practice of growing two or more crops simultaneously on the same field is called inter-cropping. The various crops in the intercrop do not necessarily have to be sown or harvested at the same time; the main requirement is that they are on the field at the same time for a significant part of their growing periods.
Types of inter-cropping
Row inter-cropping: This is when the various crops are grown in separate rows.
Mixed cropping: This is when the various crops are grown intermingled more or less at random with each other.
Relay inter-cropping: This is when a second crop variety is sown between the stands of an existing sole crop just before the first crop is harvested. As such, both the first and second crops spend most of their field lives as sole crop, and grow together on the field for only a brief period.
Factors that determine the crops combination and spatial arrangement
Tillage practices: When ridges have been made, the spatial arrangement of the various crops may be determined by particular needs of each crop. For example, yam which requires a deep layer of tilled soil, is planted at the top of the mounds, while rice because of its high moisture requirement, is planted in the lower ground between the mounds. Other crops such as maize, pumpkins and melons are planted at intermediate positions between the rice and the yams.
The crop the farmer considers as target crop and which one is considered a subsidiary influence the proportion of the crops.
Nature of the crops themselves: A few strands of pumpkin or melon occupy a lot of land and have high economic yield, whereas rice strands, for example, would have to be much more numerous to be meaningful.
Evaluating yield from inter-cropping
The relative yield of each component crop in an inter-cropping situation is the yield of that component in the inter-cropping situation divided by what that crop would have yielded as a sole crop, covering the same area as the inter-crop and managed at the same level. Suppose, for example, that a field with a crop combination of maize and cow-peas yields 1.5 tonnes/hectare of maize and 0.25 tonnes/ha. of cow-peas.
If the expected sole crop yield of maize is 2.0 t/ha and that of cow-peas is 0.5 t/ha. Then the relative yield of maize is 1.5/2.0 = 0.75 and the relative yield of cow-peas is 0.25/0.50 = 0.50. The sum of the relative yields of the various component crops in the inter-crop is sometimes called the relative yield total. A little reflection of how many times the land area used for inter-crop would be required to produced the same yields of the component crops when they are grown as sole crops.
The relative yield total is therefore more conventionally referred to as land equivalent ratio (LER). Mathematically: LER = relative yield of crop A and relative yield of crop B + relative yield of crop n In the maize/cow-peas combination considered above the LER = 0.75 + 0.25 = 1.25. An LER greater than 1.0 implies that for that particular crop combination, inter-cropping yielded more than growing the same number of stands of each crop as sole crops. An LER of less than 1.0 implies that the inter-cropping was less beneficial than sole cropping.
LER can also be calculated based on the monetary value of the yield obtained from the various inter-crop and sole crop situation and make comparisons on this basis. Calculating LER by comparing the total energy value of the yield in kilo calories in various situations and compared the values to determine which arrangement was most beneficial. Advantages of inter-cropping
In a carefully planned inter-cropping the LER is usually greater than one. This means that there is yield advantage in growing crops together than growing each one separately. This advantage may rise from several sources.
The crops may complement one another in their use of field time. The periods of their peak demands for light, water, nutrients and other resources may differ, so that in general there is a more efficient utilization of the resources available. For example, during the two or three month that yam takes to sprout and establish adequately on the field, a quick inter-crop of maize or melon would beneficially utilize the field resources during this period.
The component crop may complement each other in their use of space. For example, an inter-crop of a deep rooted crop can exploit various horizons of the soil.
An inter-crop may be able to utilize resources which the main crop may not be able to utilize or which may even be disadvantageous to it.
Certain crops may exert specific beneficial effect on others. For example, plantains inter-cropped with young cocoa seedlings provide shade for the seedlings. Similarly, in an inter-crop of a legume with a cereal crop, the cereal would benefit from the nitrogen fixed by the legumes.
By having many crops growing simultaneously on the field the farmer is more or less buffered against failure of one of the crop.
Inter-cropping allows for a more uniform distribution of labour throughout the year.
When one component of an inter-crop combination fails, the other combinations are able to utilize the resources that would have been available to the failed crop and so yield better than they would have done otherwise. In other words there is yield stability.
The spread of diseases and pests is less rapid than in sole cropping. This is probably because the mean distances between the plants of the same component crops are greater. In many instances, the other component crops are not susceptible to the particular disease or pest afflicting one component and may act as physical barriers to the spread of diseases and pest.
Disadvantages of inter-cropping
Since many crops exist together on the field, it is not possible to tailor production practices to the needs of any particular crop.
Control of pests and diseases is particularly difficult because pesticides which have been developed to control a disease on one particular component crop may have deleterious effect on other crops in the combination.
It is difficult to mechanize operations such as planting, weeding and harvesting.
Cropping Systems in Africa
Shifting cultivation and inter-cropping are the predominant practice among the peasant farmers of Tropical Africa while the large scale farmers of East and Southern Africa as well as irrigated agriculture of the Nile valley in Egypt practice sole cropping with crop rotation. In each of these cases continuous cropping is the rule rather than shifting cultivation. Plantation agriculture mainly practices sole cropping such as oil palm, cocoa, rubber, coffee and tea.
In some instance, they may be inter-cropped with food crops when the plantation is still young. For arable cash crop such as tobacco and groundnuts, sole cropping is the rule but the sole crop may be subjected to shifting cultivation, or to continuous cropping with rotation.