Cowpea (Vigna unguiculata) is a tropical, annual herbaceous legume that belongs to the family Papilionacea (Fabaceae), order Leguminosae and genus Vigna [1]. It is an important component of agricultural food crops consumed by smallholder farming households. Cowpea plays an important role in achieving food security [2]. It is highly nutritive, having dry matter content of 91% with Iron, Zinc, and Calcium up to 363, 32.2 and 1112.9 mg·kg⁻¹ respectively, and globulin proteins [3]. The crop is capable of bridging malnutrition and hunger periods since it matures fast and its leaves and grains are edible.

It is estimated that about 14 million hectares of land are globally under cowpea annually [2]. In Africa, the average production area annually under cowpea is over 12.5 million hectares [4]. In sub-Saharan Africa (SSA), the total area harvested for all food legume crops totaled 20 million hectares by 2006-2008 of which 54% was under cowpea [2]. In Uganda, 11,000 MT from 23,800 ha was harvested in the same period. From this production, 51.5% was consumed at production household level, 20.7% sold while 18.5% was stored for seed [5]. Cowpea is consumed by about 200 million people in Africa making it a very important legume [6].

On-farm yields of cowpea on smallholder farmlands have remained low [6] [7]. For example, between 50 kg·ha⁻¹ and 500 kg·ha⁻¹ attained on-farm is in marked contrast to over 2000 kg·ha⁻¹ obtainable at research stations [2] [7] [8]. The low yields of cowpea are due to low soil fertility with P being the most limiting nutrient [4] [8] [9], which is exacerbated by low adoption of improved cowpea varieties and technologies [4] [10] [11]. Around 10 kg·ha⁻¹ of starter N and 13 to 30 kg·ha⁻¹ P were reported as a requirement in low fertility soils for some varieties to ably fix atmospheric N and better yields in other parts of Africa, Rhodesia [12]. Implying that there is a need for inorganic P fertilizers to be incorporated for soils with low P quantities that lose more P than it is replenished [8]. Thus, the cultivation of cowpeas will require P for efficient nodulation and thus nitrogen fixation in the soil [13].

West Nile loamy farmland agro-ecological zone also known as Annual Cropping and Cattle West Nile System located in northern Uganda is a key zone for cowpea. This zone is dominated by Oxisols as mentioned in Arua State of Environment Report of 2015 and characterized as old and highly weathered soils which are generally low in fertility [14]. Low fertility soils have also been reported to have insufficient rhizobia cells for improvement of N fixation [15]. Increase in N for plant uptake with rhizobia inoculation could be expected to increase vegetative growth in cowpea [16]. But however, the increase does not directly translate in grain yield in cowpea [16]. Phosphorus fertilizer application in P deficient soil increases yield in crops [16] [17] and however, the increase varies with varieties as for cowpeas [18]. Information on the combined effect of inoculation and P fertilizer on performance of cowpea varieties is limited. A full understanding of the effects of inoculating rhizobia with inorganic P fertilizer application on yield of cowpea is needed.

To address the low yields of cowpea, the National Semi-Arid Resource Research Institute (NaSARRI) of the National Agricultural Research Organization (NARO) has released several cowpea varieties with high yields ranging between 1500 and 2500 kg·ha⁻¹ [19]. Some of the released varieties include SECOW 2W, SECOW 3B, SECOW 1T, SECOW 5T, and SECOW 4W. However, these varieties do not realize their full potential on-farm due to poor agronomic practices [7]. Even where commercial farmers have attempted to use P fertilizers, there is a paucity of appropriate rates to be used. Besides, the performance of these improved varieties when subjected to different rates of inorganic P and inoculation across different major cowpea growing agro-ecological zones in Uganda is unknown.

This study, therefore, evaluated the yield capacity of Agondire, a local variety, and SECOW 2W, an improved variety after inoculation and P fertilizer application at different rates in Oxisols of northern Uganda. This study aimed at providing information on enhancing cowpea productivity to both farmers and research stakeholders including policy makers and cowpea breeding programs in Uganda. Soil characterization considering chemical properties was done prior to experimentation. The assessments were done to identify suitable sites for experimentation. In this study, Leaf Area Index (LAI), Above Ground Biomass (AGB) and grain yield parameters were considered.

Preliminary soil characterization in this study crucially led to proper experimental site selection in northern Uganda. The sites had slightly acidic soils which favor most nutrient availability [12] and crop growth [24]. However, the soils had low soil organic matter and low fertility confirming to the existence of Oxisols. For all major nutrients, N (Kjhedhal) and P (Bray 1) were limiting below the critical minimum (15 mg kg⁻¹ available P) required for cropping in East Africa [11]. However, cowpea has been reported to grow well in low fertility soils with limiting nutrients when compared to other crops [21].

Leaf area index (LAI) was averagely small across all sites. Likely, hot weather in the sites during 2015/2016 planting seasons could have been the reason for small LAI observed in both varieties. According to [25], LAI is greatly affected by hot weather environments. Due to differences originating from varietal phenotypic traits, we mainly considered LAI differences within each variety than between them. The observations were however enhanced by P fertilizer application and inoculation effects being fundamental in this study. Agondire is characteristically a spreading variety with wide branches and hence anchor many leaves on them which positively contributed to larger LAI. On the other hand, SECOW 2W is an erect variety with non-spreading branches that anchor fewer leaves. Higher LAI for Agondire contributed to a larger AGB at flowering and harvest stages. LAI significantly contributes to increased biomass of cowpea [9]. Interestingly, SECOW 2W highly responds to rhizobia inoculation than Agondire and hence recorded higher LAI under rhizobia inoculation. It is expected for N fixation increment with varietal positive responses to rhizobia inoculation which also increases vegetative expansion [16] [26].

Generally, higher yields were recorded in this study than the range often got on-farm (50 - 500 kg·ha⁻¹). This could ably be attributed to inoculation and P application [26]. Importantly, though, the yields from all sites were below the attainable on research stations. Both varieties were affected by environmental stresses of unreliable rainfall and high temperatures to over 29˚C recorded in some days in the planting seasons of 2015/2016. These caused severe water stresses in Arua [5] for the crop. Elsewhere, a significant reduction in yield was reported in cowpea grown in stressing environments like hot weather and inadequate rainfall [27].

The higher grain yield for SECOW 2W is due to its intrinsic high yield ability than Agondire. Higher grain yield with inoculation for both varieties were attributed to the interplay of parameters like LAI and AGB which increased with inoculation. Yield components such as LAI and dry matter greatly affect grain yield in cowpea [28]. The positive effect of P on biomass and yield in this study confirms that P remains a limiting nutrient in most soils in sub-Saharan Africa (SSA) [8] [28]. Phosphorus is an essential nutrient highly needed by legumes like cowpea. It influences flowering, improvement of ADP and ATP energy functions [16] [21] and cowpea yield [8]. The availability of P for uptake also increases photosynthesis [16] [28] and thus boosting yields of cowpea growth and its production in the West Nile Farming system. Earlier studies have reported a significant and positive increase in grain yield of cowpea with an increase in P fertilizer application rate [29]. We also remarkably consider that this study was relevant as contradicting results of P fertilizer effect on yield have been reported [30].

Application of P fertilizers together with inoculation increased AGB and grain yield of cowpea, which is consistent with results reported by [8] [17]. They emphasized that the combination of P application with right strains of rhizobia inoculation increases yield in cowpea. Unlike SECOW 2W, Agondire variety AGB and grain yield increased even at the highest P rate (40 kg P ha⁻¹). It was determined that the local variety requires more P units than SECOW 2W. For this result, the differences in morphological structures of the two varieties could also be at play. Grain yield drop observed at higher P rates (40 kg P ha⁻¹) for SECOW 2W explain other factors that limit cowpea productivity. However, higher amounts of applied P surpassing a variety’s requirements could also lead to reduced seed development and subsequently its grain yield [31].

Outstandingly, internal nitrogen utilization efficiency determined for both varieties were (26.9 kg·kg⁻¹) and (18.2 kg·kg⁻¹) for SECOW 2W and Agondire respectively. This explains that SECOW 2W is more efficient in converting nutrients into yield than Agondire and hence required lesser P units for yield recovery. This is similar to [8], where improved varieties tested had better nutrient utilization efficiency than the local spreading varieties. This could further be explained by the early maturing variety (80 - 95 days) nature of SECOW 2W, which is consistent with [21], who reported that quick maturing cowpea varieties are efficient in utilizing available nutrients even at low nutrient levels.

We conclude that there are marked varietal differences in yield of cowpea. Application of rhizobia inoculation with P fertilizer application increases cowpea yield in northern Uganda. This study also concludes that there are other factors affecting yielding of cowpea in this region. These could include the environmental stresses like unreliable rainfall and hot weather.

SECOW 2W attains its maximum yield at 20 kg P ha⁻¹ with inoculation and therefore inoculation of the variety influenced yield. On the other hand, Agondire yield was less affected by inoculation but more by P fertilizer application attaining its maximum yield at 40 kg P ha⁻¹ with or without inoculation. It is concluded that the improved variety requires low rates of P than the local variety when inoculation with rhizobia is applied.

We recommended that farmers adopt the improved variety such as SECOW 2W because it realizes the highest yield at low P rate than the local variety (Agondire) with and without inoculation. In further studies, there is need of harnessing the native rhizobia strains to match the current commercial rhizobia strain used in this study.

Appreciation goes to Regional Agricultural Forum for Capacity Building in Agriculture (RUFORUM) for funding this research. The authors also acknowledge Makerere University, School of Agricultural Sciences, National Crop Resource Research Institute (NaCRRI), National Semi-Arid Resources Research Institute (NaSARRI) and Arua District Famers Association (ARUDIFA) for the infrastructural and technical support rendered throughout this study.

The authors declare no conflicts of interest regarding the publication of this paper.

Nyamaizi, S.M., Tumuhairwe, J.B., Amayo, R., Tumuhe, C.L., Tereka, E., Nabirye, D.L. and Obaa, B.B. (2020) Phosphorus Fertilizer Rating and Rhizobia Inoculation for Improved Productivity of Cowpea in Northern Uganda. American Journal of Plant Sciences, 11, 1505-1519. https://doi.org/10.4236/ajps.2020.119109