PHYSIOLOGICAL RESPONSES OF COWPEA (VIGNA UNGUICULATA L. WALP) VARIETIES TO RHIZOBIA INOCULATION, PHOSPHORUS APPLICATION AND SEQUENTIAL CROPPING SYSTEM IN MINNA, NIGERIA

ABSTRACT

Cowpea is the most important grain legume in Nigeria but the yield obtained on farmers’ fields is far below the potential yield of the crop. This study aimed at exploiting rhizobia inoculation, phosphorus application, varietal differences and sequential cropping system to improve the productivity and profitability of cowpea per unit area in Minna, Nigeria. A glasshouse and two field experiments were conducted between 2015 and 2017. The glasshouse experiment was a factorial combination of four nitrogen sources (uninoculated, inoculated with USDA 3451 and USDA 3384 rhizobia strains and 90 kg N ha-1), soils collected from 20 locations in the Nigerian savannas and two varieties of cowpea (IT93K-452-1 and IT99K-573-1-1) replicated three times and laid in a completely randomized design. The first of the field experiments conducted on three farmers’ fields in Minna was a factorial combination of three nitrogen sources (uninoculated, inoculated with USDA 3451 and USDA 3384 rhizobia strains), three phosphorus rates (control, 20 kg P ha-1 and 40 kg P ha-1) and three varieties (IT93K-452-1, IT99K-573-1-1 and TVX-3236) in the first year. In the second and third year of planting, the strains were replaced with BR 3262 and BR 3267 and 90 kg N ha-1 was added as part of the treatments. The treatments were laid in randomized complete block design. The second field experiment evaluated the performance of six cowpea varieties (IT93K-452-1, IT99K-573-1-1, TVX-3236, Kanannado, Oloyin and IT90K-76) in cowpea sequential cropping system. Data were collected on growth, yield, nodulation and physiological parameters. The results revealed that the cowpea varieties successfully formed symbiosis with the introduced rhizobia strains in all the locations. Rhizobia inoculation increased nodulation in 11 out of the 20 locations with percentage increase ranging from 4 to 43%. Plants fertilized with 90 kg N ha-1  had significantly higher biomass yield than the inoculated and uninoculated plants which had similar biomass yield. Phosphorus significantly (P≤0.05) increased the photosynthetic activities, nodulation, N-fixation, growth and yield of the cowpea varieties in the three years with application of 20 kg P ha-1 increasing grain yield by 49-95% over the control. Significant interactions exists between rhizobia inoculation, phosphorus and varieties on some growth and yield attributes. IT99K-573-1-1 maintained the highest productivity among the varieties however, TVX-3236 appeared to be more P efficient having significantly higher growth rate, nodule weight and grain yield at lower P rates. Number of pods per plant had the highest correlation coefficient with grain yield in the three years (r=0.79).  Crop growth rate (CGR), leaf area index (LAI) and quantum yield of photosystem II (Phi 2) explained 67.29% of the variation in grain yield (R2= 67.29%) and these three physiological parameters could significantly predict the grain yield. All the varieties were successfully planted in two sequence in each growing season except Kanannado and there was significant variation in the growth and yield attributes of the cowpea varieties in sequential cropping system. IT93K-452-1, IT99K-573-1-1, TVX-3236 and IT90K-76 all had significantly higher (P≤0.05) grain yield and profitability with gross margin ranging between ₦407,910 and ₦1,131, 967 than Oloyin and Kanannado varieties which had gross margin range of -₦123,065 and ₦88,931. Similarly, double cropping of these four varieties increased the gross margin by an average of 115% in 2016 and 140% in 2017 over the traditional system of planting once. It can be concluded from these results that N limits the productivity of cowpea in some soils of Nigerian savannas and rhizobia inoculation with highly effective strains than what was used in this study can overcome this limitation. CGR, LAI and Phi 2 could be exploited to increase the grain yield of cowpea through agronomic practices and plant breeding programs. Application of 20 kg P ha-1 is sufficient for the optimum performance of cowpea in the study area and double cropping of improved, early and medium maturing varieties of cowpea per season is capable of increasing the productivity and profitability of cowpea in the study area.

CHAPTER ONE

1.0 INTRODUCTION

1.1 Background to the Study

Cowpea, Vigna unguiculata L. Walp, is an important grain legume. It is a food and fodder crop grown in the semi-arid tropics of Africa, Asia, Europe, United States, Central and South America (IITA, 2009). The grain contains about 25% protein (Ogbonnaya et al., 2003). Cowpea has been for many years a crop that combats food insecurity. It also provides cash income for subsistence farmers, retailers and food vendors in rural and urban areas. The haulm is quality fodder for livestock. In addition, the crop is capable of fixing nitrogen from the atmosphere for its growth demand (Asiwe et al., 2009), thereby reducing nitrogen fertilizer demand and cost of production. It is an important companion crop in most cereal-legume cropping systems because of the benefit from the residual nitrogen originating from the decay of its leaf litter, roots and root nodules (Okereke et al., 2006).

Inoculation is the practice of applying microorganisms to agricultural or chemical systems to bring about desirable transformations such as enhanced nitrogen (N2) fixation, increased phosphorus uptake, more rapid biodegradation and improved disease resistance. The most important microorganism used as inoculant in legume crop production are rhizobia. Rhizobia inoculation is the process of introducing the commercially prepared rhizobia into the seed or soil to enhance nitrogen fixation. Cowpea establishes symbiotic association with rhizobia forming root nodules. It is inside these nodules that the nitrogenase enzyme in the rhizobium reduces N2  into ammonia (NH3) through the glutamate synthase/glutamine oxoglutarate aminotransferase (GS/GOGAT) pathway. This leads to exchange of nitrogenous solutes with host plant for photosynthate (Pule- Meulenberg, 2010) thereby making N available for the use of the host plant; but the amount of N2 fixed is not in all cases enough due to the presence of ineffective or low numbers of indigenous rhizobia (Ulzen, 2014).

Nitrogen  and  phosphorus  are  the  most  important  mineral  nutrients  limiting  increased  crop productivity in the Nigeria Savanna. Among these, phosphorus is the most critical nutrient needed by legumes for growth and optimum symbiotic relationship. It is important to cowpea performance as it increases seed yields, nodulation and nitrogen fixation (Magani and Kuchinda, 2009). Sequential cropping refers to growing two or more crops in sequence on the same field per year; the succeeding crop is planted after the preceding crop has been harvested. This is achievable in some cowpea varieties if planted early. It is one of the alternatives of increasing the productivity of crop from a unit of land. Its adoption may substitute for expanding cropland acreage which may not always be available and sustainable due to increase in population and urbanization.

1.2 Statements of the Research Problem

Over the years, food requirements have increased while land availability has become less due to increasing population and urban development. Thus, the only way to increase agricultural production is to increase the yield of individual crop per unit area of land.  Low crop productivity is a general problem facing most farming systems in sub-Saharan Africa. These low yields are pronounced in grain legumes and are often associated with declining soil fertility and reduced nitrogen fixation (Mfilinge et al., 2014). Nitrogen fixation in cowpea is inferior to that of other grain legumes such as groundnut, soybean and pigeon pea, perhaps because of the inadequate effectiveness or efficacy of the indigenous rhizobia to supply the nitrogen required (Fening and Danso, 2001). The amount of N2  fixed by cowpea is usually not adequate to meet the plant’s demand for nitrogen thus affecting its yield (Ulzen, 2014).

Cowpea yields in Nigeria remain one of the lowest among all food legume crops with an average of 450 kg ha-1 (Omotosho, 2014). This is very low compared to the grain yield of over 2600 kg ha-1 reported in South Africa (Pule-Meulenberg et al., 2010). Low soil fertility is among the factors responsible for the low yield experienced in cowpea as most tropical soils are deficient in essential nutrients particularly N and P (Abayomi et al., 2008). Nigeria is the largest producer and consumer of cowpea worldwide (IITA, 2009). She accounts for about 45% of the global cowpea production and over 55% of the production in Africa. All the cowpea produced in the country is consumed and the shortfall is augmented with imports from neighbouring countries, mainly Niger Republic (Coulibaly and Lowenberg-DeBoer, 2013). Nigeria is projected to remain net-importer of cowpea through 2020 (Olufajo et al., 2014).  The traditional cropping systems used in the production of cowpea in Nigeria in which cowpea is planted as a minor crop in intercrop or relay cropping is not delivering the potential yield of the crop.

1.3 Justification for the Study

Cowpea is an important grain legume in Nigeria and in many other parts of the world. It is estimated that cowpea supplies about 40% of the dietary protein requirement of most of the people in Nigeria (Kamai et al., 2014). The current demand for cowpea grains is nearly 3.2 million tons and this is expected to grow at 3.6% per year (Olufajo et al., 2014). Therefore, there is need to work on improving the yield to meet the growing demand. Rhizobia inoculation has been reported to improve the yield of legumes and enhance soil fertility but there is a dearth of information on the responses of cowpea to rhizobia inoculation. Ulzen (2014) indicated that legumes grown without rhizobia inoculation may be retarded in growth with consequent low yield. Improvement in cowpea nodulation, nitrogen fixation and grain yield have been reported in few studies as a result of rhizobia inoculation and fertilizer application (Taiwo and Oladapo, 2000; Sarker et al., 2001; Fening and Danso, 2001; Abayomi et al., 2008; Nyoki, 2013; Omotosho, 2014) but very few have been documented in Nigeria.  It is important to promote inoculant use in African agriculture, especially among resource-poor farmers who cannot afford expensive mineral fertilizers (Ndakidemi et al., 2006). Some African countries such as Rwanda, Malawi, Egypt and Zimbabwe have turned to efficient exploitation of biological nitrogen fixation (BNF) by legumes in their farming system in an attempt to cut down on fertilizer expenses (Otieno et al., 2007). Effective symbiotic relationship depends on the cultivar and the rhizobium strain (Fall et al., 2003). Hence, the need for the use of different varieties and different rhizobia strains. Use of improved crop cultivars and alteration in sowing dates of crops to accommodate more than one harvest in a growing season is an option to increase crop production from a unit of land, improve cash flow, spread risk and achieve more efficient use of equipment and greater return from investment (Prasa and Singh, 1997; Asante et al., 2001; Mahama et al., 2013). Many work have been done on cowpea-cereal in sequential cropping system but there is a dearth of information on cowpea in sequential cropping.

Increasing crop yield requires input from many areas of agriculture including physiology. The study of the processes that determine yield in crop (physiology) is very important for sustainable high yield realization in all crops. One of the most important approach of crop productivity improvement is effective evaluation of physiological traits. Hence, the need to study the physiological responses of selected varieties of cowpea to rhizobia inoculation, phosporus rates and cowpea sequential cropping system

1.4 Aim and Objectives

The aim of the study was to determine the physiological responses of cowpea varieties to rhizobia inoculation, phosphorus application and sequential cropping system in Minna, Nigeria.

The objectives of this study were to determine:

i.   if cowpea growth in savanna soils is limited by Nitrogen (N)

ii.  the effect of rhizobia inoculation on the photosynthetic activities, nodulation, N-fixation, growth and yield of selected cowpea varieties

iii. the effect of increasing phosphorus (P) rates on the photosynthetic activities, nodulation, N-fixation, growth and yield of selected cowpea varieties

iv. the inter-relationships between yield, growth and physiological characters of cowpea

v.   the inter-relationship between environmental factors and photosynthetic efficiency of cowpea

vi. the performance of some selected cowpea varieties in cowpea sequential cropping system vii. the profitability of the cowpea varieties in cowpea sequential cropping system

viii. and to identify how many times  selected cowpea  varieties can be grown in sequence within a cropping season in the southern guinea savanna of Nigeria.

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