Yams

The traded yam is a tuber, an underground stem organ, and not a root — the distinction it shares with the potato and not with cassava or sweet potato. Several species are grown and traded, and they are not commercially interchangeable: white yam is prized for pounding and holds a premium in West African markets, water yam is more productive and stores differently, and yellow yam occupies its own niche. Buyers deal in species and local variety names, not in an undifferentiated "yam".

Because the unit is large and the value per tuber is high, yams are traded by the piece and by the heap far more than by weight. Tubers are assessed individually — for size, shape, wholeness of the skin, and freedom from rot at the cut head — in a way that would be impractical for a bulk commodity.

Yam production has two outputs from the same crop. Ware yam is the tuber sold for eating and is what this entry describes. Seed yam is planting material: small whole tubers or cut setts reserved from the harvest or grown deliberately for the purpose, traded as its own category and valued on sprouting capacity and health rather than on eating quality.

The two compete for the same harvest, which is a defining economic feature of the crop: every tuber held back for planting is one not sold for food, and the share of a harvest committed to seed is substantial. This is a real trade split, comparable in kind to the ware and seed division in potatoes, though it operates through farmer and market channels rather than through a formal certification scheme in most producing areas.

Harvest wounds — especially at the head, where the tuber is severed — are the main route by which rots enter, so yams are cured after lifting to let the wound tissue suberise and a protective layer form. Cured tubers then enter storage during their dormant period, and dormancy is what makes long holding possible: while it lasts, the tuber does not sprout and its losses are limited to respiration and moisture.

Traditional yam barns — shaded, well-ventilated structures where tubers are tied individually to a frame — remain the characteristic storage system, and their logic is sound: shade limits heat load, airflow removes respiration heat and moisture, and individual placement lets a spoiling tuber be spotted and removed before it affects its neighbours. Losses over a storage season come from rots, from weevils and other storage insects, from continued respiration, and eventually from sprouting once dormancy breaks.

Quality judgements centre on tuber size, shape regularity, skin integrity, and the condition of the head. Yam nematode damage, showing as dry rot beneath the skin, and wet rots spreading from wounds are the defects that most often destroy value, and both may be advanced before they are obvious from outside. Cracked, forked, and mechanically damaged tubers are discounted because they neither store nor display well.

• Dry rot associated with yam nematode
• Wet and soft rots entering through wounds and the cut head
• Storage insect and weevil damage
• Mechanical damage, cracks, and broken tubers
• Sprouting once dormancy breaks
• Forked or misshapen tubers

Trade is overwhelmingly domestic and regional within the West African yam belt, with a smaller export flow serving diaspora markets. Beyond fresh consumption, tubers are dried and milled into yam flour, which is a stable and transportable form, and out-of-specification tubers find food and feed uses locally.