Rhizobia inoculant effects on bambara groundnut growth and yield in Namibia
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Date
2023
Authors
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Journal ISSN
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Publisher
University of Namibia
Abstract
Bambara groundnut (Vigna subterranea (L.) Verdc) is the third most significant food
legume in Africa after groundnut (Arachis hypogea) and cowpea (Vigna unguiculata). It
is normally grown in poor soils, is drought tolerant, and has a high potential for nitrogen
fixation. Despite that, Bambara groundnut yields are on a gradual decline due to a
combination of abiotic and biotic stresses such as erratic annual rainfall and climate induced changes in soil microbial community compositions, which negatively impacting
food security. Thus, the application of rhizobial inoculants at planting significantly
improves yields in many leguminous crops. Moreover, symbiotic inoculants are mainly
used in developed countries to improve nitrogen fixation and improve productivity of
grain legumes. Sub-Saharan African countries like Namibia, however, still under-utilise
the above practice. Three (3) studies were conducted, of which two were conducted in
2021, and in 2021/22 season. The first study was on farmer’s appraisal, which was aimed
to assess Bambara groundnut production systems in Kavango East Region. Bambara
groundnut farming systems, farming inputs, and production constraints were explored in
the two constituencies in Kavango East region. Subsequently, an experiment in a
greenhouse of the ZERI at the University of Namibia using soil from the Kavango region
to ascertain how well the various Bradyrhizobium strains promote nodulation and yield
components among three varieties of Bambara groundnut was conducted. The study also
evaluated the strains that are most competitive to reach the roots and promote nodulation
in a given period. For the greenhouse experiment, the treatments included three different
Bambara groundnut varieties that were inoculated with two types of inoculants (MIX
inoculant with 7 Bradyrhizobium strains [1-7; 3B 4-1; 9-5, 36 1-1; 36 3-2; 55 1-1 and 60
2-1] and inoculant with Bradyrhizobium vignae 9-5 only), both of which were obtained
from the University of Bremen in Germany the third treatment was an absolute control.
The treatments were set up in a randomized complete block design with three replications.
In comparison to the single strain 9-5, the mixed strains were found to be more productive
and contagious (p=0.05). Moreover, the two inoculants outperformed the non-inoculated
cultivars in terms of plant height, shoot dry weight. The brown variety, which was not
inoculated, nevertheless, showed some nodules. Hence, in the next season (field
ii
experiment) the mixed inoculant strains of Bradyrhizobium were applied as single
inoculants. The field experiment was conducted at Mashare Agricultural Development
Institute (MADI) in Kavango East Region. The same three Bambara groundnut varieties
which were used in the greenhouse were treated with chemical fertilizer (urea), 7
Bradyrhizobium strains, and negative control with no treatment. After 130 days of planting
the cultivars were harvested and different yield parameters were assessed. The control
Var03 (red) yielded the greatest grain at 1311.7 kg ha-1
, followed by Var01 (brown) treated
with In1-7 at 1237.3kg ha-1
, while Var01 and Var02 (cream) inoculated with In36 3-2
yielded 1171.3kg ha-1
each. Based on the results, farmers are advised to consider
inoculating Bambara groundnut seeds with Bradyrhizobium strains prior to planting, as
this has been shown to have a positive impact on nodulation, grain yield, and biomass
production compared to using urea fertilizer which is expensive and environmentally
unfriendly. However, it is important to note that the control variety Var03 performed the
best, likely due to its compatibility with native soil rhizobia. Therefore, it may be
worthwhile for more studies to investigate and identify the unidentified strains used in the
study as potential bio-inoculants. Additionally, it is recommended that farmers monitor
the performance of different varieties in their specific soil conditions and choose the most
suitable one for their crop management. Finally, it is important to adhere to best practices
for inoculant application, such as using high-quality inoculants and ensuring proper
handling and storage
Description
A thesis submitted in fulfilment of the requirements for the degree of master of science in microbiology in biological sciences
Keywords
Bambara groundnuts, Biofertiliser, Bradyrhizobia, Food security, Legumes (Fabaceae), Productivity