Agriculture Reference
In-Depth Information
genetic advances and yield increases achieved by corn
breeders were markedly lower than those realised by
other small grain cereal breeders developing wheat and
barley cultivars. Indeed the pedigree selection schemes
used by corn breeders, although considered suitable at
that time, were essentially not effective. The knowl-
edge that hybrid progeny produced by inter-mating two
inbred lines often showed heterosis (i.e. produced yields
greater than the better parent) suggested that hybrid
cultivars could be exploited by corn breeders on an agri-
cultural scale by manually detasseling female parents
to produce a population that was entirely comprised
of F
1
hybrid seed that could be sold for commercial
production.
The first suggestion of using controlled crosses was
made by W.J. Beal in the late nineteenth century
based specifically on the ideas of Darwin on inbreed-
ing and outbreeding. These ideas were then refined
by G.H. Shull in 1909, on the basis of genetic stud-
ies, who put forward the idea of a hybrid cross being
produced by first developing a series of inbred or near-
inbred breeding lines and inter-mating these inbred
production systems in the 1960's, rendered detasseling
of females in hybrid corn production obsolete.
From the onset of hybrid corn breeding it was real-
ized that there was a limit to production based on the
inbred lines available, and a large effort was put into
breeding superior inbred lines to use in hybrid com-
binations. Introduction of efficient and effective CMS
hybrid production systems combined with the devel-
opment of more productive inbred parents lead to the
return of single cross hybrids, which now predominant
corn production in the USA, and many other developed
countries. The relatively high cost of hybrid corn and
other hybrid crop seeds does, however, limit the use of
hybrid cultivars in many developing countries.
The rapid increase in popularity and success achieved
in hybrid maize could not have occurred without two
very important factors. The first is that many coun-
tries, including until recently the United States, do
(or did) not have any Plant Variety Rights legislation
or other means that breeders could use to protect pro-
prietary ownership of the cultivars they bred. There was,
therefore, little incentive for private companies to spend
time and effort in developing clonal, open-pollinating
or inbred cultivars as individual farmers could increase
seed stocks themselves, or they could be increased
and sold by other seed companies. Hybrid varieties
offered the potential for seed/breeding companies to
have an in-built economic protection. The developing
companies guarded all the stocks of the parental lines
and only sold hybrid seed to farmers. These hybrids,
although uniform at the F
1
stage, would segregate if
seed were retained and re-planted (i.e. they would be
F
2
progenies). Secondly, the introduction of hybrid
maize occurred simultaneously with the transition from
traditional to intensive technology-based agricultural
systems. The new hybrids were indeed higher yield-
ing, but were also more adapted to the increased plant
populations and rising fertility levels of the times.
There are hardly any agricultural crops where hybrid
production has not at least been considered, although
hybrids are used in still relatively few crop species.
The reasons behind this are first that not all crops
show the same degree of heterosis found in maize,
and second that it is not feasible in many crop species
to find a commercial seed production system that is
economically viable in producing commercial hybrid
seed. Indeed if maize had not had separate male
and female reproductive organs and hence allowed
easy female emasculation through detasselling, hybrid
×
inbred crosses (single cross hybrid) and using the hybrid
seed for production. These hybrid progeny were indeed
high yielding and showed a high degree of crop uni-
formity. His basic concept, however, was not adopted
at that time because the most productive inbred lines
had very poor seed yields (most likely due to inbreeding
depression) and consequently, hybrid seed production
was very expensive.
In the interim the traditional open pollinated corn
cultivars were quickly superseded by double cross
hybrids ([Parent A
×
Parent D]) suggested by D.F. Jones in 1918. Double
cross hybrids were not as high yielding or as uni-
form compared to the single cross hybrids proposed
by Shull. However, hybrid seed production was less
expensive than single cross hybrids and as a result dou-
ble cross hybrids completely dominated the USA corn
production by the 1940's.
Initially all commercial hybrid corn seed was pro-
duced by detasseling
female
plants and growing them
in close proximity to non-detasseled
males
and only
harvesting seed only from the
female
plants. When
this method of hybrid corn seed production was
most prominent it was estimated that more than
125 000 people were employed in detasseling oper-
ations. Increased labour costs combined with devel-
opments in using cytoplasmic male sterility (CMS)
×
×
Parent B]
[Parent C
