Studies were conducted in the greenhouse of the Department of Biological Sciences to determine the effect of seed sizes and sowing depth on the germination and early growth of cowpea (Vigna unguiculata L., (walp)). The seeds were sorted into two categories which were larger and smaller and planted at sowing depths of 5cm and 10 cm. The results of the experiment showed a germination percentage of 89%. There was a significant effect of the seed sizes at different sowing depths (5cm and 10cm) on shoot lengths given a p-value of 0.0029. There was also a significant effect of the seed sizes at different sowing depths (5cm and 10cm) on leaves lengths given a p-value of 0.0092. On the shoot analysis, there was no significant effect on the seed sizes at different sowing depths (5cm and 10 cm) on root lengths since p > 0.05 and for the root weights (fresh and dry), there was no significant difference for the seed sizes sown at different sowing depths (5cm and 10cm) since p > 0.05. The study therefore revealed that smaller seeds sown at 5cm depth produced the best germination and growth performances in Vigna unguiculata and is recommended to farmers for planting for optimum growth performance and yield.
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 Koop, 2008).
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 soil
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.