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First Innovation: High-volume Crossbreeding A
serious problem in Mexico that caused enormous
fluctuations in yields was epidemics of wheat rust,
Professor Stakman’s shifty enemy. Stem rust often
blasted the wheat plants before harvest and turned the
fields to sickly gray instead of a field of golden
grain. Tragically, stem rust generally was deadliest in
exactly those areas where wheat was potentially most
productive. Two other kinds of rust—brown leaf rust and
yellow stripe rust—were seldom as devastating as stem
rust.
The
native wheats were susceptible to many races of the
stem-rust organism. In three years, from 1939 to 1942,
stem rust had slashed Mexico’s national wheat harvest in
half. Losses were greatest in Sonora, the most important
wheat production region. Much of the former wheat land
had been given over to flax, cotton, and other crops,
which fared only somewhat better.
The
objective of Borlaug’s first innovation, then, was to
breed varieties of wheat that were resistant to stem
rust. His approach was to crossbreed hundreds and
hundreds of different lines in hopes of finding a few,
or even one that was resistant to prevalent rust races
and yielded well.
Most
plant breeders made only a few crosses or a few dozen
crosses each season. Each of the many individual plants
that resulted had to be observed throughout the growing
season and seeds from the “best” individuals harvested
and planted the next year, when more selections were
made, a process that was then repeated for six to seven
years. Norm couldn’t wait that long. He had to speed the
process. He collected thousands of varieties from widely
varying wheat-producing areas throughout the world. He
and his Mexican apostles began crossing them. Borlaug
says, “Crossbreeding is a hit-or-miss process. It’s time
consuming and mind-warpingly tedious. There’s only one
chance in thousands of ever finding what you want, and
actually no guarantee of success at all.”
 Crossbreeding
by hand is a delicate operation, performed with
surgical-type tweezers. The breeder must remove the male
stamen, which contains the immature pollen, from each
bisexual wheat flower. Otherwise, the plant will
pollinate itself. The emasculated wheat head is then
covered with a small glassine bag to prevent promiscuous
out-crossing with wind-blown pollen. After two days the
pistil (ovary) of the emasculated flower is pollinated
(fertilized) with pollen of the other parental variety.
Norm
was willing to take on the immense amount of work this
entailed. From daylight ’til sundown, he was bent over
in the experimental wheat trials, making notes and
recording differences in the varieties in resistance to
rust disease. At the time of crossbreeding, he sometimes
slept out at the field station in his sleeping bag and
cooked his food over an open campfire, in effect
reverting to his days as a forester. He went back to his
hotel in Mexico City for a bath and hot meal only
occasionally, when he could hitch a ride in the
one-vehicle fleet that the Office of Special Studies had
at that time.
The
tedious work started to pay off. It resulted in the
production of rust-resistant lines adapted to conditions
in Mexico. Yields from the improved varieties ranged
from 20 percent to more than 40 percent higher than the
yields of those they replaced. Borlaug never wasted time
searching for the perfect variety, but after adequate
testing released for commercial use the best available
at that point in time.
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