ABSTRACT
The low productivity of the cowpea (Vigna unguiculata L. Walp.) leafy vegetables in smallholder farms in Nigeria is partly attributed to moisture stress, declining soil fertility and poor leaf harvesting practices. To increase the productivity and utilization of these crops requires development of suitable agronomic practices such as appropriate watering regimes, effective nutrient management, and best harvesting practices. Pot and field experiments were conducted at the University of Nairobi's Kabete field station, to determine the effects of water stress, nutrient management and leaf harvesting method on growth, yield and nutritional quality of African nightshade and cowpea. In the first objective, individual plants were grown in10 liter polythene pots containing 10 kg of soil (a mixture of sand, topsoil and manure in the ratio of 2:4:1) each and watered daily for two weeks with tap water, to maintain soil at pot capacity, until the start of the treatments. The treatments, comprising four watering regimes namely 100% pot capacity (PC), 80% PC, 60% PC and 40% PC, were laid out in a randomized complete block design (RCBD) and replicated three times. In the second objective, the fertilizer treatments comprising 200 kg/ha Di-ammonium phosphate (DAP) fertilizer, 10 t/ha farmyard manure (FYM), 10 t/ha chicken manure (CM), 100 kg/ha DAP + 5 t/ha FYM, 100 kg/ha DAP + 5 t/ha CM and no- fertilizer (control) were tested against two harvesting methods (piecemeal and wholesome harvesting) in a randomized complete block design, replicated three times. Growth and yield data collected included leaf number, leaf area, plant height, number of branches, leaf fresh weight, leaf dry weight, leaf yield, number of pods/fruits, total grain weight, seed weight and grain yield. Chlorophyll concentration was determined at weekly intervals for four weeks. Data on leaf vitamin A and C, total anti-oxidant activity and phenolic content were also collected. All data collected were subjected to analysis of variance (ANOVA) and means separated, where the F-test was significant, using the least significant difference test at p≤0.05.
Plant height, leaf area, number of branches per plant, number of leaves per plant, total grain weight, leaf yield, number of pods or fruits per plant and chlorophyll concentration, vitamin A and vitamin C significantly decreased with reduction in soil moisture levels in both cowpea and African nightshade. Reduction of moisture level from 100% pot capacity to 40% pot capacity led to decreases in chlorophyll, leaf yield, vitamin A and vitamin C by 21.1, 65.9, 78.1 and 81.6 %, respectively, in cowpea and 52.5, 85.3, 52% and 55.8%, respectively, in African nightshade. In cowpea, reduction in soil moisture levels significantly decreased 100-seed weight, and grain yield. Phenolic content and total anti-oxidant activity significantly (p≤0.05) increased with reduction in soil moisture levels in both crops. Reduction of moisture from 100% PC to 40% PC led to increases in phenolics and total antioxidant by 29.4 and 18.7%, respectively, in cowpea and 34.5 and 45%, respectively, in African nightshade. Fertilizer application significantly increased plant height, number of leaves per plant and leaf yield of cowpea and African nightshade. Compared to the no-fertilizer control, application of DAP, DAP + CM, DAP + FYM, CM and FYM increased leaf yield by 68.6, 58.3, 56.6, 52.2 and 42.6%, respectively, in cowpea and 74.9, 63.7, 60.8, 56.4 and 41.1%, respectively, in African nightshade. Fertilizer application had no significant effect on vitamin A and C, phenolics and antioxidant activity in both crops. Compared to wholesome harvesting, piecemeal harvesting significantly increased the number of leaves and leaf yield by 54.1 and 43.9%, respectively, in cowpea and 51.2 and 49.3%, respectively, in African nightshade.
Water stress reduced growth, yield and nutritional quality of cowpea and African nightshade, but increased phenolic content and total antioxidant activity in both crops. Fertilizer application increased the growth and leaf yield but had no influence on leaf vitamin A, leaf vitamin C, phenolics and antioxidant activity in both cowpea and African nightshade. Relative to wholesome harvesting, piecemeal harvesting enhanced growth and leaf yield of cowpea and African nightshade but reduced the leaf nutritional quality in both crops
CHAPTER ONE
INTRODUCTION
1.1 Background Information
African leafy vegetables (ALVs) constitute a significant source of food in both rural and urban areas. Their production and consumption are, however, constrained by declining yields. Callas (1994) noted that there were more than 45,000 species of plants in Sub- Saharan Africa (SSA) and about 1000 of these species were consumed as green leafy vegetables. African leafy vegetables are vegetables which have been consumed over a long period in Africa until they form part of the cultures and traditions of communities (Maundu, 1999); they could have originated in that area or were introduced (Abukutsa- Onyango, 2007). These vegetables include: African nightshades (Solanum spp), Amaranth (Amaranthus spp), cowpea leaves (Vigna unguiculata L.), spider plant (Cleome gynandra L.), African kales (Brassica oleracea), sweet potato leaves (Ipomeas spp), cassava leaves (Manihot esculenta L.), jute mallow (Corchorus spp) and pumpkin leaves (Curcurbita spp) (Lebotse and Lyatuu, 2010). The leafy parts comprising flowers, young fruits and young succulent stems are used as vegetables (Chelang‟a et al., 2013).
There exist approximated 6,376 important indigenous African plants exist, 397 of which are vegetables (PROTA, 2004). Africa‟s annual per capita production of vegetables estimated at 50 kg is lower than the rest of the world, and is declining (Kamga et al., 2013). Onim and Mwaniki (2008) found large differences in the amount of vegetables eaten in different Sub-Saharan African countries. Vegetable consumption is very low in some countries such as Ethiopia (20 kg/person/year), Malawi, Tanzania and urban Guinea (40 kg/person/year each) and Ghana (50 kg/person/year) compared to Nigeria with an average vegetable consumption of 147 kg/person/year in urban areas and 73 kg/person/year in rural areas. In Nigeria, there are about 210 species of indigenous plants used as leafy vegetables (IPGRI, 2006). A study done in western Nigeria, (Abukutsa, 2007) reported that leafy vegetable cultivation continued to face challenges of optimal production. Their cultivation is limited to subsistence levels, hence their potential for commercial production has not been tapped into adequately yet the region continues to face high poverty levels.
Most people in SSA include ALVs in their diets, though the consumption patterns differ from region to region among households. For instance, in South Africa, the consumption is different and varies with poverty status, degree of urbanization, season of the year and distance to fresh market (Van Rensburg et al., 2007). In Nigeria, the consumption of ALVs increases during the rainy season, when ALVs are normally plenty and cheap. Besides, ethnicity also influences preference and consumption of ALVs (Kimiywe et al., 2007). Intake of ALVs, especially spider plant and African nightshade, which are the most affordable and available sources of micronutrients, help avert health problems, high mortality and low economic productivity (WHO-FAO, 2013). African leafy vegetables have been reported to provide important sources of both micronutrients and non-nutrient bio-active phyto-chemicals that have been related to protection against cardiovascular and other degenerative diseases (Akhtar et al., 2012). These vegetables are very important component of human diet as they give indispensable micronutrients that encourage proper development of the human body and good health (Abukutsa-Onyango, 2007a).
The contribution of these African leaf vegetables in the Nigerian domestic market is reported to have risen from 4.3% in 2011 to 5% in 2013 (Abukutsa, 2013). Acreage of production also increased from 31,864 ha to 40,000 ha in the same time frame, leading to production increase per unit area from 1 t/ha to 4.5 t/ha (HCDA, 2014), way beneath the optimal range of 20-40 t/ha (Abukutsa-Onyango, 2003). The ALVs have a significant potential as income earners (Onyango, 2002a). According to Chelang‟a et al., (2013), ALVs would fetch a higher price at supermarkets than in open air markets (informal markets) given that formal markets are patronized by many nutritionally vigilant clientele with higher incomes and the supermarkets hire contemporary retail technology in relation to storage, display and packaging.
The ALVs are better adapted to widespread environmental conditions and constitute food that is readily accessible and affordable to the resource challenged consumers (Keantinge et al., 2011; Ojiewo et al., 2012). Even though vegetables are commonly cultivated in the country, there are many production challenges that lead to reduced productivity. The major limiting factors include: low soil fertility (inadequate N, P, K, Ca. Mg, S and organic matter), plant nutrient imbalances, low soil moisture content, particularly in drought periods, poor harvesting methods and the use of unsuitable agronomic practices (poor land preparation, late and unsuitable weeding and inappropriate plant population densities and fertilizer application rates). Other constraints include the perception of these traditional vegetables as crops for the poor, lack of partnership and networking, low capacity within institutions, poor policies, undeveloped value chains and markets, and low research attention (Eyzaguire et al., 2006).
1.2 Statement of the Problem and Justification
Although the African nightshade and cowpea are increasingly becoming important vegetables in Nigeria, their yields in smallholder production systems are far below the crops‟ potential partly due to inadequate soil fertility, low soil moisture and poor harvest practices. For example, growers of African nightshades obtain yields of about 1.5-3.0 t/ha (MOALD and M, 1995) compared to the potential yields of 20-30 t/ha (Chweya and Mnzava, 1997). Continuous cultivation with limited external fertilizer inputs leads to nutrient mining and reduced crop productivity of ALVS. Compared to inorganic fertilizers, the use of organic fertilizers which comprise farmyard, compost and green manures are cheap and has been regarded to have long term benefits that include release of nutrients to plants slowly and for a prolonged period of time in addition to improving the soil physical properties.
Organic fertilizers also sustain cropping systems through better nutrient recycling (Gulshan et al., 2013). However, the amount of farmyard manure and plant biomass (other than crop residues) readily obtainable to African farmers as organic input is usually inadequate (Tittonel and Giller, 2012). Besides, they have low nutrient content and release nutrients slowly hence they cannot be used during periods of peak crop nutrient demand. However, inorganic fertilizers which release nutrients fast and in large quantities are expensive and therefore out of reach of poor farmers.
Most smallholder farmers involved in ALVs production depend on rain-fed agriculture which limits production to the rainy seasons. However, irrigation during the off-season presents an opportunity for the farmers to benefit from the low-supply versus high demand scenario with better returns and good nutritional quality of these crops. There is need to establish the impact of water stress on leaf yield and nutritional quality of ALVs. There are various harvesting options used in ALVs production. Some of the harvesting practices such as wholesome and piecemeal harvesting may affect the productivity and nutritional quality of ALVs. Thus, call for studies to establish optimal water, nutrient management and best harvest practices to realize the yield potential without compromising the quality attributes of ALVs.
African leafy vegetables have been regarded as „insignificant crops‟ and excluded from research attention in favor of major food and cash crops such as maize and tea even though they have short production cycles, high yields with strong nutritional quality. Limited studies have been carried out to determine the impact of fertilizers (organic and inorganic), moisture stress and harvesting method on yield and nutritional quality of cowpea and African nightshade.
1.3 Objectives
The main objective of this study was to determine the leaf yield and nutritional quality of cowpea and African nightshade grown under different fertilizer application, harvesting methods and water stress levels to alleviate food insecurity, malnutrition and poverty in Nigeria.
The specific objectives of the study were:
i. To evaluate the effect of inorganic and organic fertilizers on growth, yield and nutritional quality of African nightshade and cowpea.
ii. To evaluate the effect of harvesting method on the yield and nutritional quality of African nightshade and cowpea.
iii. To determine the effect of water stress on growth, yield and nutritional quality of African nightshade and cowpea.
1.4 Null Hypothesis
i. Organic and inorganic fertilizers enhance the growth, yield and nutritional quality of African nightshade and cowpea.
ii. Piecemeal harvesting enhances the yield and nutritional quality of African nightshade and cowpea.
iii. Water stress reduces the growth, yield and nutritional quality of African nightshade and cowpea.
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