The Royal Society Report - Push-Pull

Case study 3.2. The development of hybrid maize
Hybrid vigour
While testing his theory on the origin of species, Darwin compared inbred and cross-pollinated (hybrid) maize and
found that the hybrids were taller than the inbred plants and were more tolerant of cooler growing conditions. This
‘hybrid vigour’ (heterosis) was further studied by William Beal at Michigan State College, who observed increased grain
yields in hybrids of different varieties.
Single and double crosses
In the early 1900s, experiments were conducted in which plants were self-pollinated for several generations to
produce pure-breeding lines, which were then crossed to produce hybrids. The resulting high-yielding hybrids could
be produced every year. These hybrid seeds could easily be produced by removing the tassels from one block of
inbred maize plants to allow pollination by an adjacent block of a second inbred line. This is more easily done on a
large scale with maize than other cereal crops as maize is wind pollinated and the male and female flowers are on
separate organs.
However, as seed yields of the inbred parents were low with this method, the cost of hybrid seed was too high for
farmers. When an additional, ‘double cross’ was performed (by crossing two of the single cross hybrids to produce the
seed sold to farmers), yields were better than open pollinated varieties although not as good as the best single crosses.
Seed production from ‘double crosses’ between high-yielding single cross hybrid parents became routine in the 1930s.
Because of the doubling of yields, adoption of hybrid maize increased from 0 to 50% of Iowa’s corn acreage in just
six years following its release in 1932.
Commercial development
Farmers could either grow hybrid maize by purchasing the single cross parent seed and performing the cross on their
farm, or by purchasing ready to plant hybrid seed from farmer cooperatives or commercial seed companies. The latter
emerged as the preferred choice.
Although hybrid maize was first developed in the 1930s, the basis of hybrid vigour is still unknown. Further
improvements in yield have largely resulted from improvements in the yield of the inbred lines. By the 1960s, the inbred
lines were high yielding enough to use as seed parent and produce single cross hybrids for sale (which had a higher
yield and were cheaper to produce than the best double crosses). Yields are now 4–5 times greater than those achieved
with self-pollinated varieties in the 1920s. The aim of commercial seed companies is to increase yields again from about
150 to 300 bushels per acre by 2030.
Source: Duvick (2001).
increased biomass production and light energy harvesting
(Kebeish et al. 2007).
Transfer of C4 metabolism into rice could achieve a yield
increase of up to 50% (Hibberd et al. 2008), but the real
gain could be a substantial increase in water use efficiency,
a character normally associated with C4 plants.
Engineering a bona fide C4 metabolism into a C3 plant may
take at least 10 years but the metabolic shunt method
could be achieved sooner.
3.3.2 Abiotic stress
Abiotic stress describes the impact of non-living factors
such as drought, salinity, heat and toxic heavy metals.
Genetic improvement and modified management of crops
both have a role to play in dealing with abiotic stress.
3.3.2.1 Crop management strategies to mitigate the
effects of abiotic stress
There are diverse crop management strategies to mitigate
the effects of abiotic stress. Some of these strategies have
been derived empirically. The use of seed mixtures has
been trialled to increase the robustness of yield against
environmental stresses (see Case study 3.3). For example,
genotypes of beans selected for high capacity to acquire
phosphorus often have shallow roots (Lynch 2007). This
can cause problems for crops in water-scarce
environments, where deep roots can be advantageous for
water scavenging. Mixtures of genotypes can be planted to
buffer the crop yield against combinations of stresses. In
such mixtures, it is possible that shallow rooted genotypes
may also benefit from the extraction of water by deep
rooters in the community (Caldwell & Richards 1989).
Development of these techniques requires an
understanding of the different crop ideotypes that are
helpful to combat different environmental stresses.
Turner (2004) has shown how wheat yields in Western
Australia have increased by around 3-fold in 70 years, as
rainfall has decreased. This has been achieved largely by
changing the planting date of the crop to cover the ground
while there is water available in the soil. This greatly
reduces unproductive water loss via soil evaporation.
Other options which would not require major scientific
advances for their initial implementation, where
26 I October 2009 I Reaping the Benefits The Royal Society