Organisation: Food and Agriculture Organization of the United Nations (FAO), Rome, Italy
Author: Carlos Gómez
Edited by AGST/FAO: Danilo Mejía, PhD, FAO (Technical), Emanuela Parrucci (HTML transfer)

CHAPTER XXXII - COWPEA: Post-Harvest Operations

2 Post-Production Operations

2.1 Pre-harvest Operations

2.2 Harvesting

2.3 Transport

2.4 Threshing

2.5 Drying, cleaning and packaging 

2.6 Storage

2.7 Processing

2  Post-Production Operations

2.1 Pre-harvest Operations

Losses between maturity and harvest of beans and cowpeas are caused by:

• pod shattering: spillage of seeds from drying pods that split can be a problem, but losses are not usually serious unless harvest is delayed;

• bruchid weevils: these insects are not only serious storage pests of pulse crops but also they can fly to the fields to infest cowpeas by laying eggs in cracks or cuts in the pods;

• seed deterioration: this can be a serious problem in cowpeas and can occur soon after maturity if rainfall continues. Studies by IITA have found that cowpea seed quality and germination decline rapidly when harvest is delayed. In tests under wet conditions, seed germination fell to 50 percent or lower within three weeks after maturity, and pre-harvest fungicide sprays were of little benefit in preventing this. (****)

If the leaves are still green at the time pods mature, Gramaxone may be applied as a harvest aid.

2.2 Harvesting

In the United States, cowpea can be harvested at three different stages of maturity: a) green snaps, b) green-mature, and c) dry. Depending on temperature and fresh-market demand, peas are ready for harvest 16 to 17 days after bloom (60 to 90 days after planting). Harvest date for green snap pods is normally specified by the processor. Most domestic cowpea production is mechanically harvested, however, hand harvested cowpeas suffer less damage and the harvest season may continue over a 1 to 3 week period. (Davis, W. et al., 1991)

Determining Maturity

The pods begin to turn yellow during the final stages of growth, become brown and rather brittle once maturity is reached. Determinate bush varieties and some indeterminate types have fairly even pod maturity, and the plants have usually lost most of their leaves by the time the pods have ripened. (****)

Fig. 16. Cowpea and maturing pods

Most indeterminate vining types mature much less uniformly, and a good number of pods may ripen while most of the leaves are still green. Seed moisture content is around 30-40 percent physiologic maturity. Pods are ready to be harvested as they turn light straw in colour and the seeds within turn brown or mottled in colour. At this stage the moisture content of seeds will be about 18 percent. (Fig. 16).

After the air dry pods during 1-2 sunny days, pods become brittle and easily to break with pliable bamboo sticks. At threshing, the seed moisture content should be about 12 percent (*)

When to Harvest

Indeterminate varieties with an uneven maturity are usually harvested in several pickings, while determinate bush types are harvested all at once when most of the pods are dry.

In Australia the ideal time to cut a cowpea crop for hay is at peak flowering, which occurs 70-90 days after sowing. Quality of hay declines as the crop matures; hay yields are generally 3 000-5 000m kg/ha. As a cover crop, cowpea can be incorporated at any time when sufficient green material is available, but is best done at the time of peak flowering. Grain or seed crops should be ready to harvest 120-150 days after sowing. (Cameron, A.G., 1999)

If the crop is grown for seed, harvest would be when 75-80 percent of pod is dry. When it is grown for hay, cutting has to be when 25 percent of pods are coloured. (Rij, N.V., 1999). Harvesting should be carried out before the crop is too dry to avoid damaging the seed. In mechanized harvest, drum speed must be low (250-300 rpm) to avoid seed damages. (Cameron, A.G., 1999)

Method of Harvesting

The harvesting process will be determined by cultivar choice. Ranking types are harvest in windrows and threshed; determined types are harvested by pulling and threshing the same day. Hand harvesting is recommended for small areas. (Rij, N.V., 1999). If the harvest is mechanized and combine is used, a low drum speed is required to avoid splitting and cracking of seeds or grains.

The following methods apply to bush or semi-vine varieties with uniform maturity:

• by hand: the mature plants are pulled from the ground and placed in piles for drying. Pulling is best done in the early morning when the pods are moist to prevent shattering;

• mechanized: two basic methods are used. The plants are cut or "glided" out of the ground using a tractor with front-mounted horizontal blades with blunt cutting edges or rotating disks operated slightly below the soil surface. Several rows are combined into one windrow using a side-delivery rake, which can be rear-mounted behind the cutters. The windrows are dried for 5-10 days before threshing with tractor-drawn or self-propelled threshers;

• direct harvesting: it is popular in the United States and Canada using grain combines with modifications. (****)

For the whole seed market in the United States, quality of seed is important, so care in harvest and post-harvest handling may be important to avoid cracked or split seed. Handling the product at higher moisture reduces splitting of the seeds. If the leaves are still green at the time pods mature, Gramoxone may be applied as a harvest aid.

Cowpea grown as a dried pea product can be direct combined using a platform head or a row crop head. Adjustments to combine settings, and possibly screen/sieve sizes, should be made for the cowpea seed. (Quinn, J., 1999).

Grading

Seed must be graded at 10 percent moisture content using 4.8 mm diameter round perforated sieve and 4.0 mm diameter for small seeded varieties. (*)

2.3 Transport

The unit of shipping grains differs by the nature of the road and whether the truck is rented or shipment is by transport operator.

In Burkina Faso for example, the cost of shipping a tonne of cowpea on a rented truck on an unpaved road is $0.12 km^-1 and about 25 percent more if transported by transportation agents. In Ghana, Togo and Benin the unit cost of shipping a tonne on unpaved road are $0.11, $0.15 and 0.32 km^-1 respectively (Langyintou, A.S. et al., 2003).

Fig. 17. Farmers transporting cowpea forage in African Savannah

In developing countries, draught animals like horses, donkeys, oxen or even camels in desert areas as is shown in Fig. 17 are still a viable alternative suitable to small-scale farmers because of its low costs and local use transporting inputs and produces from field to farm and vice versa.

2.4 Threshing

Cowpea can be threshed manually by beating the plants or bagged pods with sticks once they are dry enough. Whatever the method used, cowpea seed can be easily injured if threshed too roughly or when too dry. Injured seed when planted will produce weak, stunted plants and other abnormalities. (**** )

 

Countries	Senegal
Commodities	cereals - cowpeas - grains - maize - millet - primary cereal grains - sorghum
Topics	Primary processing of food commodities (e.g. cleaning, drying, milling, etc.)
	Rec. ID 49095 - Inputer none selected - Input Date 31/12/1987 - Last Updated

Fig. 18. Cereal thresher/threshing machine (URPATA/SAHEL)

There are different threshing machines developed by african research institutions. Many of them are used to thresh various commodities such as maize, cowpea, sorghum, millet and other grains. (Fig. 18)

2.5 Drying, cleaning and packaging

In Africa, different multipurpose machines have been developed to cleaning, drying and milling of food commodities. Fig. 19 gives an idea about a design multi-crop thresher machine.

Countries	Ghana
Commodities	cowpeas - millet - rice - sorghum - soya, beans
Topics	Primary processing of food commodities (e.g. cleaning, drying, milling, etc.)
Human Presence	none
	Rec. ID 49047 - Inputer none selected - Input Date 31/12/1987 - Last Updated

Fig. 19. Multi-crop thresher (GRATIS - Tamale ITTU)

Fig. 20. Transport of cowpea bags in an african local market.

In the United States, cowpea pods are packed, 25 pounds net in mesh bags (not burlap sacks) and dry cowpea seed is cleaned, graded, fumigated and packed in small plastic bags for sale to users. (Davis, W. et al., 1991.)

The graded seeds after the removal of the broken and immature seeds should be dried to 7 to 8 percent of moisture content. Gada cloth bags or gunny bags are suitable for short-term storage and gauge thick polythene bags are appropriate for long-term storage. (*).

Fig. 20 shows labor force transporting 100 kg bags containing dried and cleaned cowpea grain to be sold in an african local market

In industrialized countries, when sold for the processing market, cowpeas are frequently sold at harvest by the truckload; at around 17 percent moisture is accepted for delivery. The product may benefit from a coarse cleaning process after harvesting to remove foreign material.

It should then be delivered quickly (one day or less) to prevent quality degradation. Cowpeas are checked for discoloured seeds, as well as foreign material and the payment adjusted accordingly. Product may be rejected if there are too many discoloured, broken or cracked seeds. (Quinn, J., 1999).

2.6 Storage

Harvested green cowpeas will "heat" resulting in spoilage unless kept cool. Post-harvest facilities have to provide shade and adequate ventilation on the way to the cooler. Cowpeas cooled below 45^o F may show chilling injury. (Davis, W et al., 1991)

In the United States is recommended the grain be stored short term at around 12 percent moisture or less, with 8 to 9 percent recommended for long-term storage. Some buyers will want the seed cleaned and bagged, while others will take the grain in bulk form and clean it themselves. For some markets, the cowpeas must be harvested at a higher moisture, such as 18 percent and trucked directly from the field to the processor ( Quinn, J., 1999)

An ISRA survey conducted in June 1996 indicates that the metal drum storage technology is used by most of the farm households (over 80 percent) and that it is used for the quasi-totality of the cowpea stored (95 percent).

As with almost every agricultural technology, farmers have introduced their own modifications of the drum storage method. In particular, many of them put insecticide in the drum. The insecticide may be a form of insurance that protects stored cowpeas even if the drum has unknown air leaks and/or it may limit reinfestation if the drum is opened regularly to obtain cowpea for family use. (Bean/Cowpea CRSP West Africa, Fy 96 Annual report October, 1995-April, 1997)

In Africa, cowpea storage is done in a variety of traditional structures. Figs. 21 through 24 shows some of them:

Photo
Geografic Datas	Countries Burkina Faso - Etnic group Gourmantché / Mossi - Geographic Area West Sahelian Africa - Socio-cultural Area Gourmantche area - Agro-climatic Area guinean
Building Datas	Single structure. Framework : Branches - Body : Rammed earth, pisé (new evolution) - Platform : Wood - Supports : Stone/Wood - Roof : Palm
Location	Within the concession	Storage Tradition	Suspended storage
Preserving Technique	Ventilated storage	Life Time	none selected
Commodities	cereals and grains - cowpeas - grains - maize - millet - sorghum
Reference	Le stockage non étatique des grains dans les pays sahéliens -R. Audette et M.Grolleaud - Paris - Avril 1984 Rapport terminal de mission CEAO, CILSS et FAO - Rome, 1987
Author	R. Audette , M.Grolleaud, A. Diop
	Rec. ID 28 - Inputer unknown - Input Date 27/2/2000 - Last Updated

Fig. 21. Bwa - Bwa traditional storage

Geografic Datas	Countries Mali - Etnic group Dogon - Geographic Area West Sahelian Africa - Socio-cultural Area Dogon area, Bandiagara - Agro-climatic Area guinean
Building Datas	Single structure. Body : Rammed earth, pisé (building is long and not easy) - Platform : Wood/Rammed earth, pisé - Supports : Stone/Rammed earth, pisé - Roof : Rammed earth, pisé
Location	Within the concession	Storage Tradition	Suspended storage
Preserving Technique	Confined storage	Life Time	12 year
Commodities	cereals - cereals and grains - cowpeas - grains - millet - rice
Product Conditioning	threshed or trodden grain	Preserving Quality	Good
[contenance]	Stored Weight (grains) 5 - 10 tonnes	Storage Duration	none selected
Reference	Dossier Tecnique traditionnel de conservation des céréales locales, Projet de Recensement des Technologies Nouvelles au Mali, January 1985
Author	CINAM, ZOLAD , Montpellier
	Rec. ID 8 - Inputer unknown - Input Date 27/2/2000 - Last Updated

Fig. 22. Dogon - Dogon traditional storage

Photo Gallery
Geografic Datas	Countries Niger - Etnic group Haoussa - Geographic Area West Sahelian Africa - Socio-cultural Area Tahoua region - Agro-climatic Area guinean
Building Datas	Body : Rammed earth, pisé (also used as roof)
Commodities	cereals - cereals and grains - cowpeas - millet - rice
Reference	Le stockage non étatique des grains dans les pays sahéliens -R. Audette et M.Grolleaud - Paris - Avril 1984 Rapport terminal de mission CEAO, CILSS et FAO - Rome, 1987
Author	R. Audette, M.Grolleaud, A. Diop
	Rec. ID 16 - Inputer unknown - Input Date 27/2/2000 - Last Updated

Fig. 23. Haoussa - Storage from ethnic group Haoussa

Fig 24. Traditional Djerma storage called Barma

2.7 Processing

In Africa, particularly in Ghana, traditional milling and other processing practices are time and labour intensive, cumbersome and expose the product to losses and adulteration. Innovative technologies include decortication fermentation, extrusion and improved domestic processing. New cowpea-based product includes weaning mixes and blending, new formulation and fortification. (Nyankori, J., 2002 )

No industrial processing entities currently exist in Niger for processing of dried peas to meal or flour. The only processing activities based on cowpea are artisanal operations, and are small in scale, producing cowpea fritters using cowpea semolina, which are generally eaten with rice. Modern commercial cowpea operations utilise adequate techniques for storage, based on drying followed by insecticide treatment where storage is for over 6 months. (European Union, 2002)

Summarising, Table 15 establishes a comparison between traditional and mechanized cowpea post-harvest operations.

Table 15. Comparison of traditional and mechanized cowpea post-harest operations

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