Principles of Plant Genetics and Breeding

448 CHAPTER 24
1 Site (location) selection. The location should be
such that the seed can be produced in isolation
to reduce contamination from natural outcrossing.
Even though drier climates are desired for seed production,
it is best to produce seed in regions of crop
adaptation to reduce genetic shifts. The site should
be well drained and of good fertility. Cropping history
is critical, since a piece of land that previously
carried a different cultivar of the crop may produce
volunteer plants in the field that could be a source of
admixture.
2 Field preparation. Apart from developing a good
seed bed, the key in field preparation is to control
weeds to avoid contamination of the harvested commercial
seed with seed that reduces the quality of the
product.
3 Management. This includes additional weed control,
disease and pest control, fertilization, and irrigation
as needed for optimal seed yield and healthy,
disease-free products.
4 Harvesting. The seed should be harvested at the
proper stage of maturity for the species and the
right moisture content, without being in jeopardy
of lodging and shattering, where applicable. It is
best to harvest at a moisture content that requires
no post-harvest drying. Sorghum and wheat can be
safely stored at 13% or less seed moisture, whereas
sunflower is best stored at 9.5–13% seed moisture.
Where artificial drying is needed, seed may be dried at
40°C for 6–8 hours.
5 Drying and storage. It is best for seed to dry in the
field to the desired moisture content. However, when
needed, seed should be dried after harvesting to the
level prescribed for the species.
6 Conditioning. Conditioning is done to remove inert
material and weed seeds. Also, seed is sampled and
analyzed (seed testing) to provide the data needed for
producing a seed tag to accompany the product.
Foundation fields require isolation to avoid contamination.
Off-types should be rogued out before pollination,
as previously indicated. Harvesting should be
done carefully to avoid mixing of seed of different
cultivars. Harvesting equipment should be thoroughly
cleaned to remove any seed from the previous harvesting
operation.
Seed conditioning entails cleaning, sorting (by size),
seed treatment with pesticides, and packaging (bagging).
Seed is cleaned to remove inert material, weed
seeds, broken seed, and other undesirable materials
(plant debris). The grain is sorted into size classes
where applicable. Fungicides may be used to dress
seeds to protect them from soil-borne pathogens.
Clean, treated seed is packaged for the market. When
needed, seed is stored in cold dry place (e.g., relative
humidity of 50% and temperature of 10°C).
7 Packaging. Clean seed is bagged and tagged for
marketing.
Isolation is a key provision in the field production of
crop seed, to prevent mechanical admixture of seed,
especially when several cultivars of the same species or
related species are being simultaneously produced. In
the production of seed from self-pollinated species,
cultivars only need to be separated by about 3–6 m of
uncropped or mowed strips. On the other hand, spacing
between different cultivars of cross-pollinated species
is usually in excess of 200 m, reaching over 1,000 m
in certain cases. Techniques used to reduce mixing
from cross-pollination include the use of windbreaks
between cultivars, and laying plots such that the prevailing
wind effect is minimized. Because insects tend
to first visit flowers at the edge of the field, crop fields
should be square in shape for insect-pollinated species.
Furthermore, the border plants may be discarded at
harvesting.
Production of hybrid seed
Hybrid seed production was discussed in detail in
Chapter 18. Most commercial hybrids are single crosses
(A × B). The success of commercial hybrid seed production
is the availability of adequate foundation seed.
Foundation seed is derived from crossing inbred lines.
Cytoplasmic male-sterility (CMS) techniques may be
incorporated to eliminate the need for emasculation. In
corn, artificial emasculation by detassling may be used.
It is critical that adequate pollen be available for maximum
seed set. Pollen shed varies from one growing
environment to another. Corn producers often use a
planting pattern consisting of a ratio such as 1 : 4 of
parent row to seed parent rows, or a ratio of 1 :2:1:4
of pollinator to seed producing rows. The female : male
ratio for sorghum ranges from 3 : 1 to 6 : 1, while
sunflower producers use ratios of 2 : 1 to 7 : 1 to optimize
seed set. Producing hybrids of insect-pollinated
species may require the aid of artificial colonies of bees
for effective pollination. Seed set may also be optimized
by synchronizing the flowering of the parents in a cross.
It is important for the parent lines to be genetically pure
to reduce the need for roguing, which can be expensive
if the production field is large.