Optimizing legume-rhizobia symbiosis to enhance legume grain yield in smallholder farming systems in Ghana

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May, 2018.
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Soybean and cowpea yields on smallholder farms in northern Ghana are far below the potential yield creating a huge yield gap. This thesis reports on a series of experiments aimed at improving the productivity of soybean and cowpea using effective and persistent strains, as key determinant, within an integrated soil fertility management framework to bridge the yield gap. The field experiments conducted mainly on smallholder farms at Kpachi, Kpalga, Tunayilli, Nyagli, Tanina and Busa in the northern part of Ghana addressed issues of persistence of Bradyrhizobium strains, the response to the application of phosphorus, organic manure, and Bradyrhizobium inoculation singularly or in various combinations using soybean and cowpea as legume hosts in four different studies. The economic benefits of these interventions were also evaluated using value cost ratio (VCR). Four different Bradyrhizobium strains; 532 C (in Legumefix), USDA 110 (in Biofix) and BR 3267 and BR 3262 evaluated for their symbiotic effectiveness on soybean and or cowpea, significantly increased yield over the uninoculated control treatment at Nyankpala but not Nyagli. USDA 110 inoculation of soybean resulted in grain yield 1.5 fold that of 532 C. Strain BR 3267 increased grain yield of cowpea (>2 folds) relative to BR 3262. USDA 110 and BR 3267 were found to be economically profitable with VCRs of 8.7 and 4.6, respectively at Nyankpala. The persistence of B. yuanmingense strain BR 3267 and B. japonicum strain USDA 110 were monitored over a period of 296 days. At 296 days, the numbers of surviving cells of B. yuanmingense strain BR 3267 and B. japonicum strain USDA 110 were log10 1.9 and log10 1.7, respectively. Native Bradyrhizobia population and soil moisture were the predominant factors that influenced the survival of the introduced strains. Addition of P and organic manure (fertisoil and cattle manure) improved cowpea response to Bradyrhizobium inoculation in a cross factorial experiment. Yield increases of 1427 and 1278 kg ha-1 were obtained over the control (without an amendment) when fertisoil and cattle manure with P, respectively were applied in combination with Bradyrhizobium inoculant. The addition of P and Bradyrhizobium inoculant to organic manure was profitable with a VCR of 2. In a single non-replicate trial to test soybean response to P and/ or Bradyrhizobium inoculant (I), a greater yield response of 1371 kg ha-1 was obtained by I+P in the study locations in the Northern region. Both P and I significantly increased grain yield by 17 and 22% respectively over the control. In the Upper West region, yields were relatively low ranging from 128 (control) to 227 kg ha-1 (P+I) in the study locations. Nonetheless, a huge variability in soybean grain yield response to P and / or I was observed in individual farms. Soil nitrogen, phosphorus, cumulative rainfall, soil type, organic carbon, pH and texture explained 42-79 % of the variability in yield in the Northern and Upper West regions. About 75% of the farmers who applied inoculant alone obtained VCR ≥ 1 and 64% of the farmers who applied inoculant in combination with P had VCR ≥ 1. These results imply that Bradyrhizobium inoculation is an effective strategy for increasing grain yield of soybean and cowpea for smallholder farmers. Greater benefits were obtained when inoculants were applied in combination with P and/ or organic manure (fertisoil and cattle manure) and can thus be recommended as soil management option for farmers. These results have important implications for policy makers, government and non government organizations in their quest to bridge the yield gap and improve livelihood for smallholder farmers.
A thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Soil Science.