ARS plant physiologist Mark Westgate examines different hybrid corn varieties
planted together for synchrony between pollen shedding and silking.
percent of our corn acreage now is planted to hybrid corn. We produce at least
20 percent more corn on 25 percent fewer acres than in 1930, when seed of
hybrid corn became available in quantity to American farmers. From the time of
the first settlers, who obtained corn from the Indians, farmers selected seed
ears from the standing stalk or from the crib. They entered the best ears of
these open-pollinated varieties in fairs and shows, and people thought the
prizewinners would pass on their superiority to their progeny and so improve
yields. It did not turn out that way.
G.H. Shull, a geneticist at Cold Spring,
Harbor, N.Y., started experiments in 1906 on inheritance in corn. From them
came important observations on the reduction in vigor on inbreeding and the
restoration of vigor on crossing. They provided the basis for hybrid corn.
Studies of inbreeding were made at other experiment stations. The general
opinion was that hybrid corn was not feasible because of the poor vigor of the
Open-pollinated varieties are maintained by
mass selection. Windborne pollen effects fertilization, and there is no control
of the male parentage. Inbred lines are developed by a combination of
inbreeding and selection. Inbreeding involves the transfer of pollen from an
individual plant to the silks of the same plant. This process is repeated for
several generations until the strain becomes stable, or true breeding.
Selection is practiced in each generation to maintain only the superior types.
Cross-breeding involves the crossing of selected parents. Single crosses are
produced by crossing two inbred lines. Double crosses are produced by crossing
two different single crosses.
D.F. Jones, who was in charge of research on
corn at the Connecticut Agricultural Experiment Station, suggested in 1918 the
double-cross hybrids involving four inbred parents, which partly removed the
limitation imposed by poor vigor of the inbred parents. The use of this system
of crossing male hybrid corn commercially feasible. The first commercial
double-cross hybrid, Burr-Leaming, was released and recommended by the
Connecticut station in 1921.
Researchers analyze hybrid corn samples.
Many of the
State and Federal inbreeding and hybridization programs were started in the
early twenties. Basic genetic theory was inadequate to serve as a guide. New
procedures were required. Many persons contributed to this phase of the
development. Hundreds of inbred lines were isolated, and these were evaluated
in thousands of crosses.
When the best of them became commercially
available, some farmers were reluctant to adopt them, but demonstration
plantings and field observations proved the worth of the hybrids. The demand
for hybrid seed in 1935 in the Corn Belt exceeded production, and the hybrid
seed industry developed rapidly.
The production of hybrid seed requires
careful control of the parents. During the experimental phases of developing
inbred lines and hybrids, this control is accomplished by covering the ear
shoots and tassels with bags and transferring pollen of the desired type by
hand. In commercial seed production, control is achieved by the isolation of
the seed fields and by the removal of tassels, before shedding of the pollen
begins, from the rows to be used as female parents. Detasseling originally was
done by crews walking through the field pulling tassels before the pollen was
shed. Machines were developed later to carry the workers through the seed
Still later, cytoplasmic-sterile stocks and
fertility-restoration genes were discovered and incorporated into the more
widely used inbred parents. Stocks carrying cytoplasmic sterility shed no
pollen. Cytoplasmic sterility is transmitted only through the female parent and
results in the absence of fertile pollen. In effect, the strain is male
sterile. Fertilization restoration is controlled by genes having the ability to
restore male fertility to otherwise cytoplasmic male-sterile lines. The proper
manipulation of these two traits completely avoids the necessity for
detasseling but assures full fertility in the farmers field.
Besides an increase in production, other
benefits have been achieved by the use of hybrid seed. Hybrids make more
efficient use of applied fertilizer. Progress has been made in developing
hybrids resistant to some insects and diseases; the result is a product of
higher quality and a more stable yearly production. Because of their greater
uniformity in maturity and resistance to lodging, the hybrids have helped make
large-scale mechanization possible.