Paper 15: Estimation of crop losses due to different causes in root and tuber crops: The case of Malawi

By
V. S. Sandifolo
IITA/SARRNET, Chitedze Research Station
email: [email protected]
Box, 30258, Lilongwe 3, Malawi

Summary

Root crops, especially cassava and sweetpotato, are important food crops and contribute significantly to the food availability to the people of Malawi. Recurrent adverse weather conditions have contributed to the increasing negative trend that these crops going through. The crop estimates conducted by the ministry of agriculture and irrigation do provide an indication of the availability of food per season. However, these figures are normally not accurate as crops losses are not seriously incorporated into the crop estimation process. This results in over-estimation in some seasons and under estimation in others. Major causes of crop losses in roots and tubers are pests such as cassava mealybug, sweetpotato weevil, termites and green mite. Disease such as the cassava mosaic disease, cassava brown streak and cassava bacterial blight also do increase crop field losses while rotting, larger grain borer and cylas weevils are major causes of post-harvest losses of stored products. Crop losses vary from season to season and from one location to the other. As such use of uniform figures across the seasons and location does not reflect accurate food availability. Crude methods of estimating crop losses are available although data from these methods are not used in the estimation of national estimates and food availability. Studies should be conducted to determine quantitative crop loss (pre- and post-harvest) estimates for each administrative or agro-ecological unit to improve estimation of food availability.

Résumé

Les plantes à racines et tubercules, particulièrement, le manioc et la patate douce contribuent de manière significative à la disponibilité de nourriture au peuple du Malawi. La détérioration continue des conditions climatiques contribue aux tendances à la croissance de la production et de la consommation de ces cultures.

Les évaluations de récolte conduites par le ministère de l'agriculture et de l'irrigation fournissent une indication de la disponibilité alimentaire par saison. Cependant, ces données ne sont généralement pas assez fiables car les pertes à la récolte ne sont pas sérieusement incorporées dans le processus d'évaluation de récolte. Ceci résulte en une surestimation ou sous-estimation de la production suivant les saisons. Les causes principales des pertes à la récolte des racines et tubercules sont des parasites tels que la cochenille du manioc, le charançon de la patate douce, les termites et les acariens verts. La maladie telle que la mosaïque du manioc, la striure brune du manioc et la rouille bactérienne du manioc augmentent également les pertes au niveau des champs. Les pourritures, térébrants et charançons (Cylas spp.) sont les causes principales des pertes après récolte des produits stockés.

Les pertes à la récolte varient d'une saison à l'autre et d'un endroit à un autre. Par conséquent, l'utilisation d'un taux uniforme pour toutes les saisons et tous les endroits ne permet pas d'avoir une estimation correcte de la disponibilité alimentaire. Des méthodes d'approximation des pertes à la récolte sont disponibles bien que les données issues de ces méthodes ne soient utilisées dans l'évaluation des disponibilités alimentaires nationales. Des études devraient être entreprises pour déterminer des évaluations quantitatives de perte à la récolte (avant et après la récolte) pour chaque unité administrative ou agro-écologique afin d'améliorer l'évaluation de la disponibilité alimentaire.

Introduction

Root crops form an important part of the diet for the people of Malawi. Cassava alone is a staple food crop for about 35% of the population especially for those living along the shores of Lake Malawi. Other important root crops grown include sweetpotato, yams European potatoes and to a lesser extent cocoyams. Apart from cassava which is produced in some parts as a staple food crop the rest are produced as snack crops. Cassava and sweetpotato have off late been promoted to major crops due to recurrent droughts affecting the country because of their ability to tolerate drought, wide adaptability to environments and ability to utilize mineral reserves efficiently of infertile soils. However, promotion of these crops intensifies when crises are looming. Yams, coco yams and taro have remained very minor crops produced by few farmers in selected locations. Cocoyams are at times planted around homesteads as ornamental crops.

Apart from being used as food crops markets for both fresh and processed products have emerged. A big fresh market has emerged for fresh cassava and sweetpotato roots in rural, urban and peri-urban centers. Local industries such as the textile, timber, confectionery and the food have started using cassava as a raw material for industrial production. These industries are now substituting imported modified corn-starch and wheat flour for cassava flour. Thus root crops have started playing a role in conserving the scarce foreign reserves for the country.

The crop estimation process

The government of Malawi through the ministry of Agriculture and Irrigation conducts annual surveys to determine the levels of crop and livestock production. The principle aim is to obtain a general estimate of the amount of food that will be available from July of one year to June of the next. The surveys are done through a methodology that involves all staff of the extension service across the country. The country is divided into 8 agricultural development division which are subdivided into rural development project. The rural development project are also subdivided into extension planning areas which are in-turn split into sections (Figure 1). Field assistants who work at the section level collect field data from farmers and report it to Development Officers at EPA level who aggregate the data and in-turn report to the project officers (PJO) at RDP level. The PJOs also aggregate the data and report to ADD level where there are evaluation and horticultural and crops officers who are responsible for supervision of the whole excesize. Data from ADDs is aggregated at national level to form the national crop estimates under the facilitation of the planning division of the ministry of agriculture and irrigation.

The process of crop estimation is done through a series of 4 rounds of estimates per year. The first round is done well in advance of the season and is meant to collect the intentions of the farmers for that season. The second round estimate is intended to collect field measurements to confirm the intentions of the farmers. The third round estimate is intended to do the actual measurement of the produce to quantify how much has been produced. The fourth round is intended to capture produce from winter production. Winter production is normally practiced by farmers who use irrigation, have access to areas with residual moisture especially along rivers and dambos.

Figure 1: Structure for the extension department of the ministry of agriculture and irrigation for conducting crop estimates.

Estimation of root and tuber crops

Cassava, sweetpotato and European potatoes are the only roots and tubers regarded as important crops. As such estimates are done on these crops. Estimation is done together with other crops such as maize, groundnuts tobacco etc. Cassava and sweetpotato production have shown an increasing trend for the past 7 years due to efforts by government and other partners, promoting them as food security crops (Table 1).

Table 1: Cassava and sweetpotato production trend from 1994 to 2001

Causes of crop losses for root crops (cassava and sweetpotato)

Crop losses in cassava and sweetpotato do occur both during the process of production in the field and after harvest. Both losses affect the availability of the crop to people. However, effects of these losses have not been incorporated fully into the crop estimation process. This has resulted in overestimating the availability of these crops at times. This scenario is also common for other major crops such as maize, tobacco, groundnuts etc.

Causes of crop losses in the field

Cassava and sweetpotato production is often affected by pests, diseases, floods and livestock damage. The major causes of yield losses in Malawi are the cassava mosaic disease and cassava brown streak which are prevalent in the traditional cassava growing belt along the shores of lake Malawi. This problem is aggravated by use of local varieties most of which are highly susceptible to the diseases and lack of knowledge on the part of farmers on disease identification and management. Yield losses are dependent on the severity of the attack and time at which the crop is infected. In severe cases yield losses of upto 90% are incurred in some varieties. The cassava brown streak disease apart from reducing yield per unit area also affects the quality of tubers. Cassava bacterial blight has also been reported in some selected locations especially where overcast weather is experienced throughout the season.

Apart from the field losses caused by diseases, cassava production is also affected by pests. The major pests are the cassava mealy bug, cassava green mite, termites and the elegant grasshoppers in some seasons. The mealy bug resurgence made farmers abandon cassava production in the mid-80s as most the cassava growing areas were attacked. The epidemic was put under control through biological control measures. However current observations indicate that the pest is coming back.

Sweetpotato production do get affected by pests and diseases also but to a lesser extent. The major pest are the sweetpotato weevil (Cylas puncticollis)which attacks the crop especially if it is harvested late and also after harvest. Disease infestation is not a very serious problem although altenaria leaf spot and the sweetpotato virus complex has been observed in some fields.

Another source of crop losses in the field for root crops are floods in certain seasons, livestock damage and monkeys in specific locations. The country has in some season experienced floods in certain areas. In such seasons huge areas of land become affected and yield from these plots are lost. Damage from monkeys is restricted to specific areas. However heavy yield losses are incurred where sweet varieties of cassava are grown.

Crop losses due to post-harvest factors

Crop losses also occur after harvest due to many causes such as pests and diseases. Some of the major causes of post-harvest losses to cassava and sweetpotato are larger grain borer for processed dry products such chips and slices, sweetpotato weevil, moulds, rotting and rodents. Post-harvest losses affect the food availability considerably if control measures are not applied. Malawi has registered significant losses due to post-harvest losses in previous season and this has contributed to short falls in food availability (Table 2).

Table 2: Domestic food crop production in maize equivalent minus seed and post-harvest losses in the 2000/2001 season.

Source: Ministry of agriculture and irrigation

Estimation of field crop losses for roots and tubers

Attempts to estimate crop losses have resulted in use of various methods by different groups. For field losses several techniques are used and these involve visual observations, data collection and modelling. Estimation of cassava and sweetpotato losses involve measurement of area that shows symptoms of the disease or presence of the pest. Counting of the number of plants attacked by the pest and/or diseases are done. This measurement or the number of affected plants is calculated as a percentage of the total area planted to the crop. National crop losses are calculated through extrapolation from the individual plot figures.

Estimation of crop losses after harvest

Calculation of crop losses is normally done at research stations and the figures derived are adopted for calculation of national figures. Eg for calculation of losses in sweetpotato due to the sweetpotato weevil and processed cassava due to larger grain borer would be done by counting the number of clean and damaged tubers and weigh them separately. % weight loss would be calculated as follows

Where:

U = Weight of clean tubers/chips
Nu = no. of clean tubers/chips
D = weight of damaged tubers/chips and
Nd =No. of damaged tubers/chips

Despite the fact that crop losses differ from season to season uniform figures are normally used across the seasons. This results in over estimating food availability in some season while underestimating in others. Crop losses for cassava are tagged at 10% each for pre-harvest and post-harvest for cassava and 15% for sweetpotato.

Conclusion/recommendations

Root crops especially cassava and sweetpotato contribute significantly to the food availability for the people of Malawi. Crop estimates conducted by the ministry of agriculture and irrigation indicate that production of the two crops is on the increase due to promotion by government and partners as food security crops. The recurrent erratic weather patterns for the past seasons have assisted in the promotion of the two crops. Estimation of crop losses is normally not done for specific season and administrative or agro-ecological units. Major causes of crop losses for root and tuber crops are pests such mealybug, green mite, elegant grasshoppers and termites mosaic disease, brown streak and bacterial blight for cassava while sweetpotato is mostly affected by the sweetpotato weevil. Estimation of crop losses has to be looked at seriously if the country will generate realistic crop production figures. Uniform figures of 20% for cassava and 15% for sweetpotato are used across season regardless of factors that may contribute to crop losses. This results in the country having erratic figures as a result of not taking crop losses into consideration especially when estimating food availability.

Crude methods of estimating crop losses both in the field and after harvest exist but are not put into practice. However, there is need to conduct studies to determine quantitative crop loss (pre- and post-harvest) for each administrative and/or agro-ecology units for improved estimation of food availability.

References

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Akoroda M.O, 1999. Study of the contribution of cassava and sweetpotato to total food availability in Malawi. Consultancy report

Teng, P. S 1987. Quantifying the relationship between disease intensity and yield loss. Crop loss assessment and pest management. Pp105-113

Moyo,c.c, I. R. M Benesi and V. Sandifolo, 1998. Current status of cassava and sweetpotato production and utilization in Malawi. In food security and crop diversification in SADC countries: the role of cassava and sweetpotato. Proceedings of the scientific workshop of SARRNET.pp 51-68

Daudi A. T. 1999. Cultural management of sweetpotato weevil (Cylas puncticolllis). In Integrated crop management research in Malawi: developing technologies with farmers. Pp41-47