Background of Study
Cowpea is an erect to sub-erect, or prostrate, climbing,
glabrous, annual herb. The stems are somewhat square and ripped, often with violent
nodes (Ton and Koop, 2008). It belongs to the family Fabaceae, it is one of the
most ancient crops known to man. Cowpea is botanically known as Vigna
unguiculata L., (Walp.) which in older references may be identified as Vigna
sinensis (L.)(James, 1999). It is widely grown in Africa (with Nigeria and
Niger Republic predominating), Brazil, West Indies, India, United States,
Burma, Sri Lanka, Yugoslavia and Australia. The history of cowpea dates to
ancient West African cereal farming, five to six thousand years ago, where it
was closely associated with the cultivation of sorghum and pearl millet(Ton and
Cowpea is a heat-loving, drought tolerant crop with high
protein content and lower soil fertility requirements than many other crops
(Coetzee, 1995). The seed coat can be smooth or wrinkled and of various colours
including white, cream, green, buff, red, brown and black. Seed may also be
speckled, mottled or blotchy. Many are also referred to as “eyed” (black-eye,
pink-eye, purple hull, etc.) where the white coloured hilum is surrounded by
another colour (Davis et al., 1991).
Cowpea can be used at all stages of growth as a vegetable
crop. The tender green leaves are an important food source in Africa and are
prepared as herb, like spinach. Immature snapped pods are used in the same way
as snap beans, often being mixed with other foods. Green cowpea seeds are
boiled as a fresh vegetable, or may be canned or frozen. Dry mature seeds are
also suitable for boiling and canning (Davis et al., 1991).
Cowpea has been identified as an ideal summer cover crop for
many areas which can both produce abundant biomass and fix substantial
quantities of atmospheric nitrogen (Creamer and Baldwin, 2000). It was observed
by (Singn and Rai, 1988) that less time was required for germination initiation
of medium seeds and small seeds than big seeds of cowpea. However, germination percentage
has a distinct advantage of larger over smaller seeds. Larger seeds showed
early vigorous seedling growth as compared to smaller seeds) (Singn and Rai,
1988). Small seeds were reported to germinate earlier as well as higher
germination index than large seeds in Turkish cultivars of chickpea (Cicer
arietinum L.) under interactive study of seed size and NaCl (Kaya et al.,
2008). In many cases one observes small or tiny seeds of cowpea which were
sorted and thrown away germinated and produced fruits. It is therefore the
objective of this study to investigate the effects of seed size and sowing
depth on the germination and seedling development of some local cowpea
varieties in Akwa Ibom, Nigeria.
1.1.1 The Importance of Cowpea
Cowpea (Vigna unguiculata L., (Walp.)) is a major staple
food crop grown in sub Saharan Africa, especially in the dry savannah regions
of West Africa. It is known to be the most economically significant African
traditional legume (Valenzuela and Smith, 2002; Langyintuo et al., 2003).
Cowpea plays a critical role in the lives of millions of people in Africa and
other parts of the world. According to IITA (2007), about 7.6 million tonnes of
cowpea are produced annually on about 12.8 million hectares of land worldwide.
Cowpea contains 20-25 % of protein, about twice the protein content of most
cereals. The leaves, immature pods and seeds are all used as food, and this is
an indication that when the crop is given a careful attention, it would be able
to support 850 million people in the world, and the high incidence of
undernourishment in sub-Saharan Africa would reduce (FAO, 2006).
1.1.2 Climatic and Soil requirements
Cowpea can be grown under rain-fed conditions as well as by
using irrigation or residual moisture along river or lake flood plains during
the dry season, provided that the range of minimum and maximum temperatures is
between 28 and 30°C (night and day) during the growing season. Cowpea performs
well in agro-ecological zones where the rainfall range is between 500 and 1200 mm/year.
However, with the development of extra-early and early maturing cowpea
varieties, the crop can thrive in the Sahel where the rainfall is less than 500
mm/ year. It is tolerant of drought and well adapted to sandy and poor soils.
However, best yields are obtained in well-drained sandy loam to clay loam soils
with the pH between 6 and 7.
1.2 Statement of Problem
There is limited documentation on the effect of seed size
and depth of sowing on cowpea vigour, germination, establishment, growth and
yield. This makes it a challenge to predict the best outcomes possible when
propagating the seeds of cowpea (Vigna unguiculata L., (Walp.)), hence making
it difficult to achieve maximum yield and production.
1.3 Justification of Study
The effect of seed size and sowing depth on germination of
cowpea would yield very vital data which is useful and helps to provide
agricultural extension delivery to farmers for increased crop productivity and
poverty alleviation. If the right combination of seed size and sowing depth are
known, it becomes possible to achieve greater yield which is beneficial for
both subsistence and commercial Agriculture.
1.4 Aims and Objectives
The aim of this study was to compare the effect of different
seed sizes and depths of sowing on the germination and early growth of cowpea
and the specific objectives of this study included to:
determine the effect of seed sizes on the growth performance
of cowpea (Vigna unguiculata)
determine the physicochemical properties of the experimental
determine the effect of sowing depths on the growth
performance of Vigna unguiculata and
determine the optimum seed size and sowing depth for higher
productivity and yield of Vigna unguiculata.