Environmental Engineering Reference
In-Depth Information
Box 11.2
Crop Residues
A rough global approximation for the recent production of crop residues can be done
by assuming an HI of 0.4 for cereals, pulses, and oilseeds, 0.5 for tubers, 0.6 for sugar
crops, and 0.1 for fruits and vegetables. Errors can be minimized by using different
specii c HI averages for major producing regions, or at least for the continents: the
differences are largest for food and feed grain crops, with low-yielding African wheat,
corn, sorghum, and barley having HI values of just 0.25-0.3, compared to HI values
of 0.45-0.50 for comparable crops grown in Europe and the United States. The vari-
ability of environmental and agronomic factors precludes any accurate calculation
of annual crop residue production, and well-documented l uctuations in cereal straw
output make a particularly large difference. For example, using an average nationwide
HI of 0.42 instead of 0.45 in calculating the total harvest of the U.S. cereal phytomass
results in about 50 Mt of additional straw, a difference larger than all the residues
produced annually by all of the country's tuber, oil, and sugar crops!
My historical reconstructions show that in 1900, crop residues accounted for nearly
75% of the total harvest of 1.5 Gt, and that share was still about 70% 2.7 Gt in 1950,
but it fell to 65% by 1975 (about 3.2 Gt out of the total harvest of 4.9 Gt) and to
58% by the year 2000. Two accounts for the mid-1990s used disaggregated HI data
for major crops or crop categories and ended up with very similar results. I concluded
that crop residues incorporated 3.5-4 (most likely about 3.75) Gt of dry phytomass,
or 55%-60% of the total agricultural harvest; cereals (stems, leaves, and sheaths)
accounted for two-thirds of all residual biomass, and sugarcane tops and leaves were
the second largest contributor (Smil 1999b). Wirsenius (2000) ended up with 3.46 Gt,
including 2.7 Gt in cereal straws and corn stover, 400 Mt in oil crop residues, and
about 200 Mt in sugarcane and beet residues.
most likely error is at least 10% even for the latest estimates. Only approximations
can be offered as to the postharvest fate of crop residues (box 11.2).
Given the enormous uncertainties in estimating the annual production of forage
crops grown on agricultural land, I will not offer detailed outlines of alternative
extrapolations and will simply assume an annual output on the order of 250 Mt
of fresh silage and just over 1 Gt of hay. These rates would translate (with typical
moisture contents of respectively 75% and 10%) into roughly 1.2 Gt of dry phy-
tomass in the year 2000, making the total harvest of about 7.6 Gt (2.7 Gt in i eld
crops, 3.75 Gt in their residues, 1.2 Gt in forages) harvested from i elds and per-
manent plantations. This i gure represents an annual aggregate; the peak global
preharvest phytomass is considerably lower, owing to differences in harvest times.
In northern temperate latitudes, harvests extend over a period of about six months,
starting in mid-May for winter wheat, peaking in August and September for major
