The Four Categories of Crop

Crops can be classified into four categories defined by the techniques used in their breeding:

Clonal crops are vegetatively propagated, usually because seed propagation leads to an immediate loss of valuable agricultural qualities. Examples include aroids, bananas, berry fruits, cassava, citrus, dates, figs, ginger, grapes, hops, potatoes, rubber, stone and pome fruits, sugarcane, sweet potatoes, turmeric, and yams.

Seed-propagated, open- pollinated crops are usually cultivated as hybrid varieties or improved populations called 'synthetic varieties'. Hybrid varieties are produced by crossing inbred lines in order to exploit hybrid vigour. These crops include the cucumber family, maize, millets, the onion family, rye, and sorghum.

Seed-propagated, self-pollinated crops are usually cultivated as pure lines. A pure line is the result of four to six generations of self-pollination and selection. These crops require late selection. They include barley, oats, many grain legumes, rice, and wheat.

Tree crops differ mainly in that their generation time is long. For example, Brazil nut trees (Bertholletia excelsa) may not flower until they are twenty years old, and this makes their breeding a very long-term affair. The most promising approach with many tree crops, including plantation forest species, is to select within existing populations (see 7.14) but, generally, each species must be treated on its own merits.

Because tree crops are so long-lived, as well as being expensive to breed and to replace, the danger of clones should be noted. Clones are a wonderful way of propagating selections made within existing populations, but it is dangerous to rely too heavily on a single clone, or even a few clones because a newly introduced parasite could be devastating. This comment applies to most crops, including those cultivated as pure lines. We must always remember the ecological adage that diversity produces stability, and that uniformity produces instability. It is acceptable to cultivate crops as homogeneous populations but, within a crop species, there should be as many cultivars as possible. This is one of the advantages of a multiplicity of plant breeding clubs (see 11.9).

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