Environmental Engineering Reference
In-Depth Information
phytomass is harvested as cereal and leguminous straws and as sugarcane stalks;
chopped corn stover and tuber and vegetable vines are usually left behind and are
recycled back into the soil.
While crops are harvested for their carbohydrate, lipid, or protein content, the
composition of crop residues is dominated by cellulose, hemicellulose, and lignin
(Barreveld 1989). Cellulose accounts for 30%-50% of residual phytomass, but it
can make up as much as 61% of rice stumps (Mukhopadhyay and Nandi 1979).
Hemicellulose accounts for up 25%-30% of residual dry phytomass, while lignin's
share is 10%-20%. When converting crop residues to carbon equivalents, 45% has
been the most often used generic value: although very close to the shares prevalent
in most cereal straws it is still a simplii cation, mainly because of differences in
mineral content. Shares of alkaline elements (Ca, Mg, K) are not that different
among crop residues, but oats have only about 1% of silica, winter wheat more
than 3%, and some rice cultivars more than 13%, and their total ash content may
reach about 20% (Antongiovanni and Sargentini 1991). The carbon content of rice
straw and corn stover is thus no higher than 40%, and that of wheat is about 45%
(Demirba¸ 2003; Sawyer and Mallarino 2007).
Harvested residues have low density, with cereal straws rating just 50-100
kg/m
3
(compared to more than 500 kg/m
3
for wood); uncompressed corn stover
weighs just between 21 and 111 kg/m
3
, and sugarcane bagasse leaving the i nal mill
averages about 120 kg/m
3
(Smil 1999b; Lam et al. 2008). Crop residues traditionally
had a wide variety of competing uses. They were a major (even the only) source of
household energy (inefi ciently burned in open i res or simple stoves) and a common
construction material (particularly in straw-clay mixtures for bricks and roof thatch-
ing), as well as the main source of bedding for domestic animals and indispensable
feed for domesticated ruminants. Recycling of crop residues was an important
source of soil organic matter and plant nutrients, and straw was also an excellent
substrate for cultivating mushrooms and a raw material for making paper. While
some of these uses have virtually disappeared (thatching, fuel) in modern settings,
some remain as important as ever, none more so than feeding ruminants.
Unlike other domesticated animals, ruminants can digest cellulose because the
microorganisms in their rumen produce the requisite enzymes; indeed, to maintain
normal rumen activity, at least one-seventh of their normal dry-weight diet should
be in cellulosic roughages (NRC 1996). Some residue feeding is still done as tradi-
tional stubble-grazing of harvested grain i elds, but in all modern economies most
feed straw is consumed as a part of chopped residue mixes (Bath et al. 1997). In
modern feeding straw is often made more palatable and more nutritious by alkali
