Environmental Engineering Reference
In-Depth Information
was storable for years. Tubers are also a poor nutritional
choice because of their low protein content and cannot
be the only staple. Leguminous grains have commonly
twice as much protein as cereals, but their yields are
much lower. The dominance of cereals is thus an ener-
getic as well as a nutritional imperative: they combine
fairly high yields, good nutritional value (high in filling
carbohydrates, moderately rich in proteins), relatively
high energy density at maturity (roughly five times that
of tubers), and low moisture content suitable for long-
term storage.
The dominance of a particular species is largely a mat-
ter of environmental limits and taste preferences (all cere-
als have remarkably similar energy content, about 15
MJ/kg). Wheat spread to all continents because it does
well in semideserts as well as in rainy temperate zones, at
elevations ranging from sea level to 3 km, and in well-
drained soils (Briggle 1980). In contrast, rice, originally
a semiaquatic plant of tropical lowlands, grows in fields
flooded with water until just before harvest. Its cultiva-
tion has also spread far beyond the original South Asian
core, but the best yields have always been in rainy tropi-
cal and subtropical regions. Corn yields are best in
regions with warm and rainy growing seasons, but it,
too, prefers well-drained soils. Farmers had to set aside a
portion of seed for the next year's planting; with low
yields it was often as much as one-third or even one-half
of medieval crops.
Manual grain harvesting (in double-cropping areas
followed closely by the planting of a new crop) was the
most time-consuming task in traditional farming. In
South China, 94%-98% of all available labor had to be
engaged between March and September (Buck 1937).
In parts of India with a very short rainy season, the two
peak summer months required more than 110% or even
120% of actually available labor, and a similar situation
existed in other parts of monsoonal Asia (Clark and Has-
well 1970). This need could be met only if all members
of a farming family worked arduously long hours or by
relying on migratory labor. These peak labor demands
were among the most important energetic bottlenecks
of traditional farming.
6.3 Muscles, Implements, Machines
People can pull primitive, surface-scratching ards, but
pulling moldboard plows to open up new farmland or to
deep-till heavy clay soils of temperate latitudes could be
only an in extremis possibility (F. Bray 1984). Such plow-
ing has always required traction and endurance much su-
perior to those routinely deliverable even by a gang of
strong men. Thousands of years of draft animal breeding
resulted in a profusion of physiques and performance
capacities (Rouse 1970; Cockrill 1974; Clutton-Brock
1992; Budiansky 1997). Indian bullocks weigh usually
less than 300 kg, Italian Romagnola or Chianina draft
cattle at least 650-700 kg. North China's Sanhe horses
average only 350 kg, the heaviest European breeds (Per-
cherons, Clydesdales) about 1 t (fig. 6.3). And water buf-
faloes range from 250 kg to 700 kg. With sustainable
pulls at about 15% of body weight for horses and 10%
for other species, typical drafts range between 20 kg and
80 kg.
Figure 6.3 superimposes the performance fields of
common working species on power isolines, showing
most horses below 1 hp and donkeys often no better
than humans. Brief exertions are substantially higher.
Maximum 2-h pulls during German drawbar tests were
260-290 kg for heavy horses and 170-190 kg for small
