Introduction:
The Cassava plant (Manihot esculenta)
The crop is known in the New World as manioc or yucca it is a
herbaceous shrub or mini tree which grows up to 4 metres high
with thin hand-like leaves. It was introduced to Gambia
by the Portuguese between the 17th and 18th centuries.
It is one of the highest starch yielding plants grown in Gambia
and on average Gambians consumption levels is about 100kg per
person per annum in 2002 (FAO). This tuber is
poisonous
unless processed in a certain way, so it is best to seek advice
before attempting to make your own nyambe nyebe! (beans &
cassavas in palm oil). The root of
the cassava plant, a large thick-skinned tuber like the potato
when boiled, that is eaten in many tropical countries and is the
source of tapioca.
Toxicity Levels:
The crop's tubers must be processed very carefully as they include
a glucoside, linimarin, which is acted upon by an enzyme to liberate
prussic acid. The peeled roots contain much less prussic acid
than unpeeled roots because most of the prussic acid is in the
skin.
 There are 2 main
varieties:
1. Bitter varieties with roots containing 0.02-0.03% prussic acid.
2. Sweet varieties with roots containing less than 0.01% prussic
acid. These can be used raw for feeding.
The prussic acid content depends not only on the variety, but
also, and possibly even more, on soil conditions. Usually the
bitter varieties have longer and thicker roots than the sweet
varieties, but there is no simple safe method to judge the level
of prussic acid in the roots.
The toxic elements can also be removed by cooking or by drying
slices of the roots for about two weeks.
Cassava root meal is not attacked by insects; the same is true,
of feed concentrates to which 15% cassava root meal has been added.
Uses of Cassava:
 Both
fresh and dried cassava roots and peels are consumed by ruminants
such as cows and goats in different forms (sliced, chopped, ground).
Dried cassava roots have given satisfactory, results as the principal
energy source for dairy cattle, intensive beef fattening and lamb
growth. Cassava can replace almost all of the grain in the diets
with little reduction in performance. Inclusion levels of up to
65%, preferably pelleted, do not seem to affect health, carcass
quality or overall performance when the diets are carefully balanced.
Palatability can be enhanced by the addition of molasses if pelleting
is not possible. The whole cassava plant (including root and aerial
part) can be chopped and ensiled in simple pit silos for dry-season
feeding at the village level. Simple
equipment is required both for harvest and preparation of the
silage. The silage is fairly well balanced for ruminants.
Complete replacement of grain by cassava root meal in layer feed
has yielded similar egg production.
Leaves:
It is possible to obtain from cassava more than 6 tons of crude
protein per hectare a year with the proper agronomic practices
directed toward foliage harvesting. Cassava leaf and stem meal
has been used at the 35% level in dairy cow concentrates to advantage.
Cassava bushes can be harvested as forage when they are three
to four months old. They are cut about 40 cm from the ground and
chopped in small pieces by hand or in a stationary forage chopper.
The forage has been used to provide by-pass protein to ruminants
fed urea and molasses. The intake of cassava forage was about
5 kg per day, and about two months of adaptation was required
before full production was obtained. In poultry rations the replacement
of as little as 5% of the lucerne meal with cassava leaf meal
significantly reduced broiler weight gains; however, the inclusion
of methionine and vegetable oil additives in rations consisting
of up to 20% cassava leaf meal practically eliminated the depression
in response.
Pomace:
Often called cassava meal, pomace is the residue from the extraction
of starch from cassava roots. Cassava roots yield approximately
equal amounts of starch and pomace, which have less feed value
than cassava root meal but can be included in rations for cattle.
Starch and pomace are extensively used for pigs in Southeast Asia,
where they are regarded as a valuable feed. Up to 10% has been
used in poultry rations via suppliers.
Cassava Crop Production Methods:
The first system is one in which the farming family's household
is the major source of labour used for food crop and livestock
production.
In this crop system, cassava
is commonly intercropped (the practice of growing 2 or more crops
on the same field) with early maturing annuals such as maize,
and vegetables like okra,
and bitter leaf. The crops
intercropped with cassava in different parts of Africa very with
regions of growth and food preferences.
In The Gambia, rice is the dominant intercrop.
Protected trees, and perennials such as bananas and plantains,
are also regularly grown in patches or as individual areas in
cassava fields. While cassava and related crops are the main food
crops, trees provide building materials and firewood.
Cassava leaves are cooked and eaten by people and also fed to
livestock, or used as poultry feed. These animals provide the
household with meat and eggs for consumption and as a cash crop.
Animal wastes are used for soil fertilisation.
Cassava is highly compatible for use in intercropping with these
annual crops because of its lengthy early growth, especially between
planting and 6 to 8 weeks of growth. The annuals which grow faster
during the initial growth phases rarely compete adversely with
cassava and have been shown to smother weeds.
In times of war, drought or low national incomes, cassava consumption
increases relative to alternative food staples such as rice, maize,
yam, and wheat. This is why it is sometimes known as a crisis
crop.
Cassava in certain forms is a low income consumers' staple. Although
an individual may not increase the quantity of cassava consumed
in a year, as national income declines, annual average cassava
consumption per person increases because more people begin to
substitute cassava for more expensive alternative food staples.
A Gambian farmer's capacity to react to falling fallow periods
due to demographic, market, agricultural pests such as the cassava
mealy bug or Phanacoccus manihoti, diseases and other pressures
by replacing more susceptible crops with cassava is limited by
its long cropping cycle. Cassava can be harvested from 6 months
after planting, but most available local varieties do not achieve
maximum yield before 22 months. Shortening fallow periods require
varieties selected for efficient nutrient absorption and for better
ability to be intercropped with beans or other soil fertility
preservation techniques.
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