Monoculture cropping, by its nature, exposes communities and makes them vulnerable to the dire consequences of crop failure. Drought, floods and disease can cause crop wipe-out and spell bankruptcy and hunger for farmers and their families and reduced food security for the country.

As one Zambian agriculturalist puts it: “Monoculture means mono-diet, and we should be getting away from this kind of scenario.”

Small-scale commercial farmers are better advised to plant a variety of crops for the markets and for their family tables.

The yam, a staple food of West Africa, is a recommended option for a smallholder planning to put diversity into his cultivated hectares. A tuber crop, it is one of the few staples native to Africa where it has been cultivated for about 50 000 years. The yam is also considered to be the least perishable of all the tuber crops.

FOOD VALUE OF YAM

A cylinder-shaped tuber, with an attractive vine, yam is a quality food with a high protein content; its complex carbohydrates and low glycaemic index, help to regulate blood sugar.

Vitamin A in yams keeps mucous membranes and the skin healthy, maintains night vision and contributes to healthy bone development.

Dr Joseph Mercola (foodfacts.mercola.com) lists yam as having 27% of the recommended daily intake of vitamin C for fighting off colds and flu and keeping the immune function strong. Other nutrients are potassium, manganese, metabolic B vitamins, thiamin (vitamin B1), riboflavin, folic acid, pantothenic acid and niacin. The yam contains beneficial minerals; copper (which produces healthy red blood cells), calcium, potassium (supporting optimal cell and body fluids), iron, manganese and phosphorus.

Unless they’re peeled and cooked, some yams may contain toxins, so avoid eating them raw.

GROWING YAMS

Depending on the species, the yam has a growing season of between six and ten months, which generally corresponds to the wet season. The arrival of the dry season signals the beginning of dormancy which may last from two to four months.

At the beginning of the dry season vegetative growth stops and leaves begin to yellow as they translocate nutrients to the tuber which goes dormant.
The yam is now ready to harvest, a delicate operation which is traditionally done with a digging stick and a great deal of care. A damaged, cut or bruised tuber is not good for storage or for retaining as seed. These bruised or blemished yams are better eaten immediately.

CROP REQUIREMENTS

Yams do best in temperatures ranging from 25°C to 30°C. Although they can tolerate drought, yams need water during the growing period when the tuber bulks up. If it gets too dry during the growing period, the farmer must irrigate.

Yams are fairly heavy feeders and need fertiliser six to eight weeks after planting, 15cm to 20cm from the plant.

Shorter daylight periods favour tuber formation and longer daylight favours vine growth.

Like other tuber crops, yams are ridged into deep ploughed, loose soil. Space the ridges about a meter apart and contour them if there is any slope in the land. Contouring will minimise soil erosion and runoff, so don’t be tempted to leave this step out.

Because yams are vines they need trellising.

SEED PREPARATION

One of the problems with yam cultivation has been access to good seed. Subsistence farmers who grow yams have traditionally kept back a third of their crop for the next planting, which is not ideal.

Farmer constraints and shortages mean that, quite often, the smallest and worst quality tubers are held back for the new season’s planting. This kind of practice, while it may be expedient, is against the principles of plant breeding and means there is no genetic gain for the farmer. In fact, quite the opposite because bad quality seed means bad quality harvest.

The preparation of mini-setts solves this problem. Dr Chris Okonkwo of the International Institute of Tropical Agriculture says the first critical step for mini setts is to select a healthy mother tuber of about 1kg, that has broken dormancy. The setts are cut across the tuber, with a clean, sharp knife, into slices about 6cm thick; each section may be cut into four pieces roughly the size of a matchbox, weighing between 25g and 50g.

Dip the setts into a solution of fungicide/pesticide or, traditionally, into ash, and leave them to air dry. Yam is susceptible to the yam mosaic virus, anthracnose and to nematodes, so the fungicide/pesticide option is a safer route to disease protection. But some advisors prefer the ash coating because it is chemical free.

After this, the setts can be planted directly into the prepared land, or they can be pre-sprouted before planting. The grower may want to use pre-sprouting beds, which must be shaded, or for smaller volumes containers will do. The seed will sprout in two to three weeks.

Although yam has a long growing season it is well adapted and has in-ground storage capability which gives flexibility to the farmer in the harvesting season. This nutritionally rich staple is popular in local markets and therefore less exposed to global market shocks. This makes for a more stable food system and source of income.

References:
http://africayam.org/
https://www.afdb.org/fileadmin/uploads/afdb/Documents/Events/DakAgri2015/Root_and_Tuber_Crops__Cassava__Yam__Potato_and_Sweet_Potato_.pdf
Root Crop Digest PRIS Vol. 2 No. 4
Propogating yam through mini setts IITA communication

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