Sunn hemp (Crotalaria
juncea L.) is one of the earliest and most distinctly named
fibers of India.
It has great potential as an annually renewable, multi-purpose
fiber crop. It is the most important species of the
Crotalaria genus, which is comprised of over 350 species
located in the tropics and subtropics of both hemispheres.Past
research efforts have shown that the soft, lignified fibers
produced in the stem of sunn hemp could be utilized in the
manufacturing of pulp and paper, and more recent efforts have
indicated that other potential products can be developed from
these fibers. Additional characteristics that enhance the value
potential of sunn hemp as a non wood, fiber crop are low
nitrogen fertilization requirements, the ability to fix nitrogen
and to grow in marginal soils, drought resistance, and
resistance to root-knot nematodes.
The bast fibers are utilized for the manufacture of cordage and
high quality paper. In addition to the uses as a green manure
crop and as a fiber source, research is being conducted in Texas
to determine if the shorter core fibers of sunn hemp can be
utilized in the manufacture of soil-less potting media for
commercial nursery application.
is generally considered to have originated in India; Most of the
present-day sunn hemp production is located in India,
Bangladesh, and Brazil, where it has been cultivated since
prehistoric times. It is grown as a green manure crop, a fodder
crop, or for the bast fibers.
Sunn hemp is a short-day, erect shrubby annual, generally 1 to 4
m in height. The stems are cylindrical and ribbed. Branching in
the upper portion is minimized with dense plantings. The simple,
elliptic to oblong shaped leaves, are spirally arranged on the
stem. The root system is characterized by a long, strong
taproot, well developed lateral roots, and much branched and
lobed nodules, up to 2.5 cm in length. Extensive
cross-pollination occurs in sunn hemp and self-pollination takes
place after the stigmatic surface has been insect or
mechanically stimulated. Seeds are small, flattened,
kidney-shaped, and contain approximately 35% protein. Due to
cultivar and environment, seed weight is highly variable,
ranging from 18 to 30 seed per gram.
Cultivation and Management
Good land preparation should be made before planting sunn hemp
for fiber production. Although reports on fertilization
requirements vary, additions of P are generally recommended for
low phosphorous soils. Sunn hemp is fast growing and generally
suppresses weed populations due to dense canopy shading.
However, early season weed control has been shown to improve
yields when sunn hemp is grown for fiber. No herbicides are
currently registered for use in sunn hemp production.
Planting dates differ among locations; however, adequate soil
moisture and frost-free, warm weather conditions will provide
rapid emergence and the highest yields.
As one of the most widely grown green manure crops throughout
the tropics, sunn hemp is often grown in rotation with several
different crop species. The stems of sunn hemp are composed of
two fibers, the bast and woody core. The bast fibers, which are
located in the outer bark, are much longer than the core fibers,
but the two fiber widths are very similar. The proportion of
bark in the total stalk, by dry weight, generally ranges from
15% to 20%.
It produced high organic matter yields, was able to fix
nitrogen, and could reduce the build-up of root-knot nematode
populations However, the difficulty in producing seed caused
many farmers to abandon the growing of this crop. Interest in
growing sunn hemp was renewed during World War II, as C.
juncea was added to the list of critical war materials in
1942 because of its potential use as cordage fiber, especially
for Marine Oakum. Sunn hemp may be superior to kenaf for some
fiber properties, including bast fiber length (3.79 vs. 2.62 mm)
and width (24.3 vs. 19.7 µm).
Sunn hemp possessed the following properties that made it an
excellent candidate for papermaking:
(1) Good yields of bleachable sulfate pulps,
(2) Pulp strength properties that are equal to or greater than
those of mixed southern hardwood pulp, and
(3) Bast fiber length to width ratio that is greater than those
of wood fibers.