Principles of Plant Genetics and Breeding

486 CHAPTER 28
grown at below sea level to 4,000 m. Corn is adapted to
warm temperatures.
History of breeding in the USA
Corn is believed to have been domesticated more than
7,000 years ago. Deliberate attempts at improving corn
began with the development of open-pollinated varieties
that are still common in developing countries.
Even though the first corn hybrid in the United States
was developed in the early 1920s, most producers continued
to use open-pollinated cultivars until the 1940s.
Producers in the short season northern regions preferred
flint (northern flint) corn varieties. However, early
production acreage was dominated by the Corn Belt
dent varieties. However, growers in the mid-maturity
production areas preferred the southern dent openpollinated
corn races for their superior high yield potential.
Natural crossing between the northern flint and
southern dent varieties produced “hybrids” (intervarietal
hybrids) with superior performance. Farmers were
able to select and develop numerous open-pollinated
varieties, one of the most successful being “Minnesota
13”, which was later widely used in early hybrid variety
development.
The pioneering work of G. H. Shull of the Carnegie
Institution of Washington started the move toward the
development and spread of hybrid corn. His landmark
publication “A pure-line method in corn breeding” laid
the foundation for corn hybrid breeding. Later, E. M.
East, while providing collaborative evidence for Shull’s
work, at the same time discouraged the commercial
development and use of hybrids. This was because the
first hybrids were inferior and, further, the single-cross
hybrid produced seed on an inbred line, making it an
expensive undertaking. The first commercial hybrid, the
“Copper Cross”, was a single-cross variety. This first
commercial variety was produced in Iowa under contract
by H. A. Wallace in 1923. Wallace founded the Hi-
Bred Corn Company, which later became the Pioneer
Hi-Bred Company. Commercial hybrid production
became feasible when in 1918 D. F. Jones proposed the
use of the double-cross hybrid, which was a product of
two single crosses. This meant that a hybrid seed was
produced on a relatively high-yielding single-cross hybrid
female plant. Double-cross hybrids dominated corn
production until the 1960s. The dominance of hybrid
varieties was due to their superior characteristics, especially
greater uniformity, higher yield, tolerance to biotic
and abiotic stresses, and amenability to mechanization.
Plant breeders later (after the 1960s) developed superior
inbred lines with high yield potential and standability.
Single-cross hybrids became more productive and
more uniform, eventually replacing the double crosses.
Initially, corn breeding was primarily conducted in the
public arena. However, private companies began developing
superior lines in the 1960s for hybrid breeding,
eventually dominating the commercial seed corn market.
Corn yield has characteristically increased from
about 64 bushels/acre with double crosses in 1959 to
about 129 bushels/acre using single crosses.
It has become clear to breeders that most of the corn
hybrids in North America originate from a few inbred
lines developed in the public arena, severely narrowing
the genetic base of corn and making them vulnerable.
There is an effort by breeders in both private and public
sectors to enhance the genetic base of corn through the
germplasm enhancement of maize. In the latter parts of
the 20th century, the use of biotechnology became a
major feature of maize breeding, resulting in transgenic
products, such as Bt corn. Nutritional augmentation
breeding has also produced products such as high-lysine
corn and quality protein maize (QPM).
Types of corn
Corn belongs to the family Poaceae, other members
of this family being Zea, Euchlaena, Tripsacum, Coix,
Chionachne, Polytoca, Scherachne, and Triobachne. Of
these, the closest relatives to corn are Euchlaena
(teosinte) and Trypsacum. Intergeneric crosses between
Zea and Euchlaena, and Tripsacum, Saccharum, and
Coix have been successfully attempted, the products
usually being sterile. Zea × Euchlaena (corn × teosinte)
crosses are, however, often successful with fertile
offspring.
Corn may be grouped into seven types on the basis
of endosperm and glume characteristics – dent, flint,
flour, pop, sweet, waxy, and pod corns. Of these, five are
commercially produced (dent, flint, flour, sweet, and
waxy corns).
1 Dent corn (Z. mays indentata). Dent corn is the
most widely cultivated type in the US. It is characterized
by a depression (dent) in the crown caused by
the rapid drying and shrinkage of the soft starch at the
crown. Of the multiple colors available, the yellow or
white kernels dominate commercial production.
2 Flint corn (Z. mays indurate). Flint corn is predominantly
comprised of corneous or hard starch that