collection of plant types, each type having proven to be adapted to the growing
conditions in a particular locality or climatic region.
Method of Composite Seed Production
Pairs of individual plants which normally self-pollinate, such as maize, are artificially
crossed (or made to pollinate) by a plant breeder. The plants that result from this cross,
which now contain genes from two parents, are then also artificially crossed together.
The same process is repeated the following year, and sometimes also the year after that.
This results in plants that contain genes from eight or more of the original parents.
These plants are then grown in the area for which they are intended, and allowed to self-
pollinate. This population of plants is then grown on for a number of years in a certain
type of environment. Those plants that are best adapted to that environment will be
selected, initially by the seed producer/plant breeder and later on by food growers
themselves.
These “survivor plants” gradually evolve to dominate the population. The result is a
series of landraces—a population of naturally selected plants—that are adapted to grow
in certain conditions. These plants become increasingly well adapted every year by a
process of progressive selection, and a certain proportion are always likely to survive
difficult conditions.
The fact that composite seed can be grown on by farmers from year to year is also
obviously a major advantage over hybrid seed.
Influence of the Food Grower on Composites
A composite population of plants, often known as a composite variety, has a
continuously changing genetic makeup, and natural selection is the principal force
which acts to produce genetic change.
Food growers are the secondary force, who every year select from their composite
plant population those plants which are best adapted to the conditions on their farm. If
they have a good eye for selecting plants, their composite crop should become
increasingly well adapted to the local environment.
Leguminous plants (the “legumes” or “pulses”) normally grow better if they have root
nodules which are present and functioning ie pink in colour. The nodules utilise
Rhizobia bacteria to “fix” atmospheric nitrogen, which becomes available to plants.
A healthy, well-nodulated crop growing in a soil with low nitrogen content can fix
200–300 kg/ha of nitrogen, which represents a large—and, in effect, free—source of
fertiliser. In some cases even the protein content of the grain may be increased.
When a new plant species, or a new variety of an existing species, is introduced to
an area it is important that the appropriate Rhizobia bacteria are present in the soil to
allow nitrogen fixation to occur in the root nodules.
Some of the nitrogen that is fixed in the nodules is used by the host plant, and the
rest is released into the soil when the nodules are old and disintegrate, ready for the next