Environmental Engineering Reference
In-Depth Information
the proportion of photosynthate channeled into harvested tissues (Donald and
Hamblin 1976; Gifford and Evans 1981; Hay 1995).
These efforts have succeeded in pushing modern harvest ratio close to or even
above 0.5 (and for rice close to 0.6). Obviously, with an HI of 0.5, phytomass is
equally divided between crop and residual mass, while in traditional cultivars, whose
HIs were often as low as 0.25 and rarely higher than 0.35, residual phytomass
weighed 1.8-3 times as much as the harvested tissues.
Modern short-stalked wheat cultivars incorporated a semidwarf Norin-10 gene
and were introduced by the International Maize and Wheat Improvement Center
during the 1960s (Dalrymple 1986). During the same time IRRI released its IR8
variety (HI 0.53), which yielded as much as 10 t/ha. Corn yield began to improve
already before World War II with the introduction of hybrids seeds in the United
States starting during the 1930s, and by the early 1960s all U.S. corn was hybrid
(Smil, Nachman, and Long 1983). Roughly 30 years after the introduction of high-
yielding varieties came the products of the latest great breeding wave aimed at
guaranteed higher yields, genetically engineered (transgenic crops), starting with
corn incorporating a bacterial toxin, then both an herbicide-tolerant corn and soy-
beans (Fernandez-Cornejo and Caswell 2006). Because this innovation met with
considerable consumer and regulatory resistance, it has not been rapidly extended
to the two staple food crops, wheat and rice.
All of the far-reaching changes that affected cropping during the twentieth century
can best be demonstrated using the statistics of major Western nations and Japan
and combining J. L. Buck's pre-1930 studies with nationwide statistics for China,
which begin only in 1949. Between 1950 and 2000, the average yields of America's
three most important crops (when ranked by their value in the year 2000), corn,
soybeans, and wheat, rose respectively 3.5, 1.7, and 2.5 times, to about 8.5 t/ha,
2.5 t/ha, and 2.8 t/ha (USDA 2000). During the same period China's rice, wheat,
and corn yields increased respectively 3, 5.4, and 1.7 times, while Japanese rice
yields doubled between 1950 and 2000, rising from 3.2 t/ha in 1950 to 6.4 t/ha (SB
2010). Without these improvements the area needed to grow staple food crops for
expanding and better-fed populations would have to have been substantially larger.
Dietary Transitions and Meat Production
The transition from foraging to cropping, from a mobile to a sedentary existence,
increased the average population densities (eventually by as many as four orders of
