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Paper 13: Root and Tuber crops: Concepts and methods recommended by FAO and operational issues

N Keita
Statistician, ESSS/FAO


The paper provides an overview of the concepts and methods recommended by FAO for the establishment of statistics on root and tuber crops. It presents FAO classification of the seven root and tuber crops defined by the Organization, with their common and Latin names.

The paper also reviews the definitions and various concepts and of area, yield and production used in agricultural statistics as well as the related operational issues in the presence of some agricultural practices in traditional farming systems in Africa: continuous planting and harvesting, mixed and associated cropping.

The paper discusses the issues raised in estimating area, yield and production in the case of root and tuber crops with particular emphasis on the “cassava problem”.

Finally the paper proposes some recommendations and elements of discussion, including: the need to address each root and tuber crop separately as the problems for estimating area, yield and production are different from one crop to another; the need to use stratification in agricultural surveys regarding root crops, using as stratifying variables, the production patterns, the varieties of crops, the need to conduct further studies and field tests to validate experiences undertaken in some countries. This could be done through a regional research project (3 years with a technical assistance component and pilot countries components)


Le papier fournit une vue d'ensemble des concepts et des méthodes recommandées par le FAO pour de l'établissement des statistiques sur des plantes à racine et de tubercule. Il présente la classification de la FAO pour les sept plantes à racine et de tubercule définies par cette Organisation, avec leurs noms communs et en Latin.

Le papier passe également en revue les définitions et divers concepts de superficie, de rendement et de production utilisés dans des statistiques agricoles ainsi que les problèmes opérationnels qui résultent de certaines pratiques culturales dans les systèmes agricoles traditionnels en Afrique: plantation et récoltes continues, cultures mixtes et associées etc.

Le papier discute les difficultés rencontrées pour l'estimation des superficies, rendement et production de dans le cas des plantes à racine et tubercules avec un accent particulier sur le problème du manioc.

Enfin le papier propose quelques recommandations et éléments de discussion, incluant: la nécessité de traiter chaque plante à racine et tubercule de manière spécifique, étant donné que les difficultés pour estimer les superficies, rendement et production sont différentes d'une culture à l'autre; la nécessité d'utiliser les techniques de stratification dans des enquêtes agricoles concernant des plantes à racines, en utilisant en tant que variables de stratification, les systèmes de production, les variétés des cultures, la nécessité d'effectuer d'autres études et test sur le terrain pour valider des expériences entreprises dans quelques pays. Ceci pourrait être fait par un projet de recherche régional (3 ans avec une composante assistance technique et des composantes dans quelques pays)

1. FAO Classification of root and tuber crops

FAO defines roots and tubers as plants yielding starchy roots, tubers, rhizomes, corms and stems. They are used mainly for human food (as such or in processed form), for animal feed and for manufacturing starch, alcohol and fermented beverages including beer. Apart from their high water content, (70-80 percent), these crops contain mainly carbohydrates (largely starches that account for 16-24 percent of their total weight) with very little protein and fat (0-2 percent each).

Under the heading of root and tuber crop, FAO classifies seven primary crops. The code, name and the botanical name of each of the crops follow.

0116 POTATOES (Irish potato): Solanum tuberosum

A seasonal crop grown mainly in temperate zones all over the world, but primarily in northern hemisphere.

0122 SWEET POTATOES: Ipomoea batatas

A seasonal crop grown in tropical and sub-tropical regions.

0125 CASSAVA (manioc, mandioca, yuca): Manihot esculenta, utilissima and dulcis

A semi-permanent crop grown in tropical and sub-tropical regions.

0135 YAUTIA ("Chou caraibes"): Xanthosoma

Grown mainly in the Caribbean and used for food.

0136 TARO (Cocoyam, old cocoyam, colocasse): Colocasia esculenta

Taro is grown throughout the tropics for food.

0137 YAM: Dioscorea

Grown throughout the tropics for food.


Five of the above crops are significant in Africa in terms of production and consumption by the populations in the continent: Irish potatoes, Sweet potatoes, Cassava, Taro and Yam.

2. Concepts and definitions of crop area, yield and production used in agricultural statistics

2.1 Crop area

2.1.1 Definition of area: horizontal projection of a particular extent of earth's surface

2.1.2 Various concepts of crop area in agricultural statistics

The concepts of crop area to be used in agricultural statistics are closely related to the use of data. The following concepts may be used according to FAO:

Two other concepts of area are to be distinguished:

2.1.3 Highlight of some conceptual and operational issues for estimating area relevant for root crops

a) Continuing planting/harvesting: repeated planting and harvesting of crops at intervals of time in an agricultural year. This can take different forms:

b) mixed or associated cropping: two or more different temporary and/or permanent crops simultaneously growing on the same field or plot. The following distinction is to be made:

In Africa, successive and mixed cropping involving root and tuber crops are frequently used in traditional agriculture. In some cases the number of crops in a mixture is so high that some arbitrary rules may be used for ignoring minor crops:

Estimating area for each crop, once the total area of a field is known, particularly in the context of mixed cropping raises difficult operational problem, which various proposals are made.

2.1.4 Estimating crop area in mixed cropping

Various methods are being used in countries, which vary from simple methods to more complicated methods (often recommended but rarely used)

Other more complex methods of dividing the area of a plot between crops in a mixture include the concepts of imputed and allocated area:

Imputed area: area, which would have been occupied by a crop in pure stand. The sum of the imputed area in the mixture may be different from the physical area of the plot. [Ai= A (ci/Ci); Ai being the imputed area, A, the total area of the plot, ci, the density of the crop in the mixture and Ci the density of the crop in pure stand)]. The criteria for imputation should be highly correlated with area or production and may include: the amount of seeds, the density, the volume of production, the commercial value, etc.

Allocate area: fraction of the physical field area in which the particular crop is cultivated. The sum of areas for each crop in the mixture, equals to the physical area of the field. [A'i=Ai(A/SUMAi)]

2.2 Crop Yield/Production

2.2.1 Definition of yield and production: the yield is the average amount obtained per unit of area. The production is the total amount produced.

2.2.2 Various concepts of production used in agricultural statistics

Various concepts of yield and production may be used depending on the use of data

The concept of yield and production must be consistent with the corresponding concept of area. The issues discussed regarding the estimation of area have direct implication on the estimation of production, when this is estimated by P=AxY.

3. Some issues for estimating area, yield and production of root and tuber crops

The estimation of area, yield and production of root and agricultural crops in Africa in the context of traditional agriculture, raises specific problems related to the agronomic characteristics of the crops, the cropping patterns used by farmers, the destination of the production, and the consumption patterns of local populations.

The difficulties are variable from one product to another and across countries.

Production/yield data for root and tuber crops must be reported in terms of clean weight, i.e. free of earth and mud.

Irish potatoes

This is a seasonal crop usually cultivated in pure stand. Obtaining statistics on this crop does not raise problems different from other crops. Usual methods used in agricultural statistics for estimating area, yield and production may apply without much changes.

Sweet potatoes

Not enough methodological documentation exists on the estimation of statistical data on this product.


Most of the world production is in Africa, in limited number of countries. Among the issues related to this crop are the distinction between varieties and the cropping pattern and harvest used by farmers.


Not enough methodological documentation exists on the estimation of statistical data on this product.


Cassava is a semi-permanent crop (with crop cycle which may go beyond an agricultural year), which may also be cultivated as mixed crop with continuing planting, continuous and incomplete harvesting. Therefore, obtaining reliable statistical data on this crop raises important challenges for agricultural statisticians. The “cassava problem” has been widely discussed among agricultural statisticians with little methodological progress so far.

A useful distinction is to be made between situations where cassava is mainly used as cash crop and cassava as “ reserve” crop.

Cassava is grown in some situations as a cash crop and the ground is completely cleaned when it reaches maturity, which may be from 9 months after cultivation onwards. It may also be planted as reserve crop. Under these conditions, the practice in some places is to harvest what is required, choosing the younger plants, which are likely to be more palatable and allow the rest to go to waste.

Under these conditions there are two definitions possible of production:

It should be noted that while the concept is reasonably clear- cut in the first case, concept the latter is much less so since the weight of produce, which is regarded as being available will depend on the date at which the assessment is made. As one passes from early days of growth of the tuber the weight available will increase until a stage is reached where parts must be regarded as inedible even under conditions of grave food shortage. After a certain point these must be written off for food purposes and the quantity effectively available will begin to fall.

Therefore a key issue is what definition of production is desirable?

Production for market for which the crop is completely cleared presents no more difficulty than the production of any other cash crop.

If potential production is required then no special difficulty than work is involved for organizing the survey at a suitable period after planting (12 months period)

If actual production is required the problem may be of a different order. Usually it is not possible to estimate harvested area because users pick a root here and there from amongst the growing crop as seems best to them. Under these conditions a reasonable approach could be to include the production of cassava within the recording programme for a farm output survey.

4. Discussion elements for follow-up action


H. F. Hunt (1969): Agricultural Statistics for Developing Countries

FAO, Statistics Division (1982): Estimation of crop areas and yields in agricultural statistics. Rome, 1982

FAO, Statistics Division: World Census of Agriculture 1960, 1970, 1980, 2000.

United Nations Economic Commission for Europe- Statistical Office of the European Community- FAO-OECD (1995). Conference of European Statisticians: Handbook of concepts and definitions used in international collection of food and agricultural statistics U. N., New York and Geneva, 1995

FAO (1991): Racines, Tubercules, Plantains et Bananes dans la nutrition humaine

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