Environmental Engineering Reference
In-Depth Information
aboveground residues (whether legume and oilseed crops or the vines and stems
of tuber and root crops) have been generally much smaller. The HI values of tradi-
tional cultivars were most often below 0.3. Traditional wheats had an HI as low as
0.25-0.30, producing up to three times as much residual phytomass as grain (Singh
and Stoskopf 1971). In contrast, Mexican semidwarf cultivars introduced during
the 1960s measured no more than 75 cm, and their HI was around 0.35; by the
late 1970s many short-stalked wheat cultivars had an HI close to 0.5, producing as
much grain as straw (Smil 1987). Typical HI averages are now between 0.40 and
0.47 for semidwarf wheats.
Similarly, traditional barley cultivars had HI values below 0.35, while the
improved varieties grow to just 70-80 cm and their HI range is between 0.45 and
0.50 (Riggs et al. 1981). Traditional low-yielding, long-stalked rices had HI values
between 0.12 and 0.20, medium-yielding cultivars could reach 0.3-0.4, and only
the best cultivars grown under optimal conditions had an HI in excess of 0.4. The
i rst short-stalked
Oryza japonica
varieties were bred in Japan early in the twentieth
century; a similar
indica
cultivar (TN1) was released in Taiwan in 1956. A major
breakthrough came only in 1966, when the International Rice Research Institute
introduced IR8, the i rst in a sequence of new cultivars that transformed Asian
rice farming (Donald and Hamblin 1984; Smil 1987). Under optimal conditions
these plants have an HI between 0.5 and 0.6, and their typical HI is between 0.45
and 0.50. Soybeans have an HI around 0.5, up from 0.4 or less for traditional
cultivars (Weilenmann de Tau and Luquez 2000). Sweet potato is the only major
crop whose HI is now often at or even slightly above 0.60. The HI values of
vegetables range from less than 0.3 for a poor crop of tomatoes to more than 0.7
for red beets.
The overall effects of a higher HI can be shown by using a national example.
The average yield of French winter wheat rose from 1.3 t/ha in 1900 to 7.5 t/ha in
the year 2000, and during the same time the total aboveground phytomass increased
from 4.3 t/ha to 15 t/ha. With an unchanged HI of 0.3, the grain yield would have
reached 4.5 t/ha in the year 2000, a gain of 3.2 t/ha compared to the 1900 harvest,
while with the HI rising to 0.5, this increased phytomass productivity brought a
gain of 6.2 t/ha. The difference between the two gains (6.2 - 3.2) is attributable to
the HI gain, and in this case it accounted for nearly half (48%) of the higher grain
yield. Many HI values have been published for major i eld crops, but there are no
comprehensive national data sets of typical rates, and estimating past global aver-
ages is even more uncertain. As a result, the actual production of crop residues could
have departed by as much as
±
20% from the rough pre-1950 estimates, and the
