Legume Research

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Legume Research, volume 44 issue 10 (october 2021) : 1186-1191

Stimulation of Soybean (Glycine max) Growth and Yield using Bradyrhizobium Inoculants in the Semi-arid Environment of Botswana

M. Mosupiemang1, K. Bareeleng1, M.S. Chiduwa2, O.O. Molosiwa3,*
1Ministry of Agricultural Development and Food Security, Department of Agricultural Research, Private Bag 0033, Gaborone, Botswana.
2Agronomy Research Institute, P.O. Box CY 550, Harare, Zimbabwe.
3National Agricultural Research and Development Institute, Private Bag 00177, Gaborone, Botswana.
  • Submitted14-04-2021|

  • Accepted14-07-2021|

  • First Online 10-08-2021|

  • doi 10.18805/LR-624

Cite article:- Mosupiemang M., Bareeleng K., Chiduwa M.S., Molosiwa O.O. (2021). Stimulation of Soybean (Glycine max) Growth and Yield using Bradyrhizobium Inoculants in the Semi-arid Environment of Botswana . Legume Research. 44(10): 1186-1191. doi: 10.18805/LR-624.
Background: Crop yields in the semi-arid regions are low due to climatic and soil related constraints. Soybean as one of the most important legume crops grown worldwide, has a role to contribute nitrogen to improve nutrient poor soils in Africa. A study was conducted to examine the effects of Bradyrhizobium spp inoculations on the growth and yield of soybean varieties in a glasshouse.

Methods: The study was arranged in a randomized complete block factorial design, with factor A being two soybean varieties (Bimha and Status) while factor B was inoculation using four Bradyrhizobium strains and the uninoculated control. 

Result: Bradyrhizobium inoculation significantly (P<0.001) affected days to 50% flowering, days to emergence, nodule number, root dry weight and grain yield and yield traits. Parameters that were affected by both inoculant strain and variety included days to 50% flowering, days to emergence, number of pods per plant, pod weight and number of seeds per pod. The interaction effect of variety and Bradyrhizobium inoculant strain was observed only on number of pods per plants. Our study shows that soybean grows well when inoculated with Bradyrhizobium inoculants, in semi-arid conditions of Botswana.
Mankind is faced with the responsibility to develop an agriculture and food system that will be able to provide sufficient food for the population projected to exceed 9 billion by the mid-century (FAO, 2017). One of the key challenges facing farmers is the capacity of their soils to supply nitrogen for sustainable farming. Therefore, it is essential that nitrogen is replenished through Biological Nitrogen Fixation (Peoples et al., 1995). Some estimates suggest that biologically fixed nitrogen in agricultural fields is about double of nitrogen produced industrially (Chianu et al., 2010). The mutualist relationship between rhizobium and legumes is a cheaper and highly effective practice to ensure supply of nitrogen by legume crop (Zahran, 1999). In addition, the use of mineral fertilizer is scarce, due to high costs and has potential to pollute the pristine environment. Soybean, originally from Asia, now grown worldwide including in Africa is a legume crop with potential to contribute significant amount of nitrogen to agriculture production systems in symbiosis with the appropriate rhizobia microsymbionts. The crop experienced a rapid increase in production due to its  high seed protein (36%) and oil (20%) which are essential in animal and human nutrition (Sinclair, 2004; Sharma et al., 2016). It has a huge commercial potential for use in agro-industries, such as in producing high quality oil and soybean meal (Egbe, 2010). Despite the noticeable importance of soybean, its production has not been fully promoted in Botswana, as a result, its cultivation is extremely low. The crop does relatively well in parts of Southern Africa such as Zambia, Zimbabwe, Malawi, South Africa and Mozambique (Meyer et al., 2018), while issues of high summer temperatures, erratic rainfall and poor soil fertility, may have discouraged investment in the crop in Botswana. Other limitations could be poor knowledge of the crop, low understanding of rhizobial inoculation practices and paucity of its use as a food legume (Pule-Meulenberg et al., 2011). The demand for soybean in Botswana is intensified by soybean use as an ingredient in making Tsabana, which is a weaning food used as a dietary intervention for the under-five age children (Kopong, 2013). With the advent of drought tolerant soybean varieties developed by International Institute of Tropical Agriculture (IITA), there is an opportunity to grow the crop in semi-arid conditions such as in Botswana, which could otherwise not be considered suitable (Kolapo, 2011). The use of rhizobial inoculants would further make it easier for soybean to be planted in new environments where it was difficult to crop. Moreover, in places that do not have a history of soybean, it would be key to use inoculants to promote nodulation. As an initiative of promoting soybean in Botswana, it would be important to evaluate different soybean varieties inoculated with various Bradyrhizobium spp. for their efficacy on increasing the growth and yield of soybean. Therefore, the objective of this study was to explore the effects of Bradyrhizobium inoculation on the growth and yield of different soybean varieties.
Two soybean varieties Bimha and Status which are popular among consumers were obtained from seed dealers in Zimbabwe. A glasshouse experiment was conducted at the Department of Agricultural Research situated in Sebele Content Farm, Gaborone, Botswana (24°35'S; 25°56'E), at 991 meters above sea level. The study was conducted during the period of February 2019 to June 2020 cropping season. The experiment was laid out in a randomized complete block design in 2 × 5 factorial with four replications. The main factor was two varieties Bimha and Status while the sub factor B was inoculation using four Bradyrhizobium strain (a. B. elkanii NAZ599, b. B. diazoefficiens NAZ629. c. B. japonicum NAZ710 and d. B. ottawaense NAZ758 and e. the uninoculated control). The strains of Bradyrhizobium were obtained from Soil Productivity Research Laboratory, Zimbabwe, following isolation from around Zimbabwe (Chiduwa, 2021). Four plants were planted in 5 litre pots each containing 3 kg of soil, with chemical properties shown in Table 1. The soil chemical properties were analyzed at the Soil and Plant Laboratory, Botswana, using the standard procedures. No fertilizers were applied, but, the crops were irrigated three times a week to ensure proper plant growth.

Table 1: Chemical properties of the soil used for the experimentation.

Data collected include days to 50% flowering, days to emergence, plant height (cm), chlorophyll content, nodule number, nodule dry weight (mg), root dry weight (mg) and shoot dry weight (g). Plant height was measured at three different stages (vegetative, flowering and grain filling). Chlorophyll content was measured at the flowering and grain filling stages using chlorophyll meter (SPAD-502 plus, Konica Minolta). Plants were carefully uprooted at flowering, taking care to avoid root and nodule loss, then shoot dry weight and nodulation measurements were obtained. The shoots, roots and nodules were separated and number of nodules from each plant in each pot were recorded based on Howieson and Dilworth (2016), procedures. Subsequently, shoots, nodules and roots were oven dried at 70°C until constant weight to determine dry mass. At harvest four months after sowing, yield and yield component estimates were obtained on number of pods per plant, pods length (cm), pod weight (g), number of seeds per pod from ten randomly selected pods from each pot. Yield per plant was determined by shelling the grain per plant after harvest. Measured variables were analyzed using two-way analysis of variance (ANOVA) using sigma plot analysis software. Treatment means and interaction effects were compared using the Fisher’s Least Significant difference (LSD) procedure at significance level of 5%.
Effect of Bradyrhizobium
Rhizobial inoculation significantly (P<0.001) affected days to 50% flowering, days to emergence, plant height at all the three growth stages and chlorophyll content at grain filling stage (Table 2 and Table 3). Rhizobia inoculant strains NAZ629 and NAZ710 gave plants a growth advantage by early emergence. Plants inoculated with NAZ710 had the longest vegetative period, as they flowered later than the rest. Plants inoculated with NAZ710 and NAZ629 produced taller plants at vegetative (33.4 cm), flowering (53.4 cm), and at grain filling stages (61.5 cm) which was higher than the rest of the inoculation treatments (Table 3). Plants inoculated with rhizobial inoculant strain NAZ599 showed higher chlorophyll content (37.9) followed by NAZ710 (33.8) relative to the control with 20.8 (Table 3), it was also recorded that the control treatment had developed some leaf discolorations by the onset of grain filling (Fig 1) which may be an indication of nutrient deficiencies. As indicated by Tairo and Ndakidemi (2014), that Bradyrhizobium inoculation might have contributed to an increased uptake of macronutrients in the roots and shoots in soybean plants.

Table 2: Effects of inoculation with Bradyrhizobium spp on the number of days to emergence and flowering.


Table 3: Effects of inoculation with Bradyrhizobium spp on the plant height and chlorophyll content of soybean varieties at different growth stages.


Fig 1: Shows soybean plants at the grain filling stage. Fig 1A and 1B represent the uninoculated control and the inoculated plants, respectively.

Rhizobial inoculation significantly (P<0.05) increased nodule number and root dry weight (Table 4). This is in accordance with previous researchers who revealed that inoculation of soybean significantly increased nodule number and root dry weight (Appunu et al., 2014; Samudin and Kuswantoro, 2017).  Rhizobium enhance growth and development through production of indole acetic acid (IAA) which helps in development of plant root system (Kuar et al., 2014). Bradyrhizobium inoculates NAZ599 resulted in highest number of nodules (46.48) and root dry weight (537.96 mg). Our results revealed non-significance of the rhizobial inoculation effect on root dry weight and shoot dry weight (Table 4). This might be due to lower supply of nitrogen to the crop in the pot culture experiments.

Table 4: Effects of inoculation with Bradyrhizobium spp. on nodulation and biomass of soybean varieties at the flowering stage.

Strong positive association between shoot dry weight and nodule dry weight R2 (0.81) show the higher contribution of nodule dry weight to soybean productivity (Fig 2). While a relatively weaker correlation R2 (0.32) between shoot dry weight with nodule number, may imply that an increased number of nodules does not necessarily lead to an increased shoot dry weight.

Fig 2: Regression of shoot dry weight at the flowering stage on nodule dry weight (Fig 2A) and number of nodules (Fig 2B).

Inoculation had significant (P<0.001) effect on number of pods per plant, pod length, pod weight, number of seed per pod and yield per plant (Table 5). Jarecki and Bobreckar-Jamro (2019), in their findings showed that inoculation of seed with symbiotic bacterial result in significant increase in yield and yield components. Rhizobia NAZ710 produced higher number of pods per plant (11.80) while the control had only 3.19 pods per plant. Moreover, genotype × inoculation interaction was significant in number pods per plant (Table 5). This interaction indicates that responses of the two soybean varieties differ in number of pods per plant based on the four Bradyrhizobium spp used. The productive potential of soybean is determined by several yield components. The results illustrated yield per plant had a significant (P<0.05) positive correlation with number of pods per plant (0.77), number of seeds per pods (0.68) and pod weight per plant (0.47) (Table 6). Mofokeng and Mashingaidze (2018) reported that seed yield was highly significant and correlated with seed number per plant, pod weight per plant and pod number per plant.

Table 5: Effects of inoculation with Bradyrhizobium spp. on yield and yield components of soybean varieties.


Table 6: Correlation analysis of yield and yield components parameters.

Soybean variety effect
Among the sixteen traits used to estimate varietal differences, significant difference was identified between six characters, days to 50% flowering, days to emergence, chlorophyll content at flowering (Table 2 and Table 3), number of pods per plants, pod weight and number of seed per pod (Table 5). Bimha took more days to emerge, flowered late and had higher number of pods per plant with higher weight as compared to Status (Table 2 and Table 5). Bradyrhizobium spp used were not specific to a certain soybean variety as there was no interaction between variety and Bradyrhizobium spp. among most of the traits. Similarly, Samudin and Kuswanto (2017), reported a lack of interaction between the inoculation treatment and the genotype among the 12 soybean genotypes they evaluated. Varieties Bimba and Status had similar performance based on plant heights, chlorophyll at grain filling (Table 3), number of nodules, nodule dry weight, root dry weight and shoot dry weight (Table 4) and pod length, and yield per plant (Table 5).
Bradyrhizobium inoculation remarkably increased the growth and yield of soybean as it was revealed by increased plant height, higher chlorophyll content, increased nodulation and grain yield relative to uninoculated plants. Specifically strains B. japonicum NAZ710 and B. diazoefficiens NAZ629 outperformed the other inoculation treatments for growth and yield. Most of the studied parameters lacked interaction with rhizobia used, which shows the soybean varieties used have similar response to the Bradyrhizobium spp applied. Generally, soybean varieties Bimha and Status had similar performance in terms of growth and yield and can both be recommended for use in Botswana. This preliminary study shows that the use of inoculation technology in Botswana cropping system has a huge potential. Going forward, field experimentation would be recommended in multiple locations, to fine tune recommendations around the country.
The study was funded by the Ministry of Agricultural Development and Food Security in Botswana while rhizobia inoculant strains were supplied by Ministry of Lands, Agriculture, Water, Fisheries and Rural Resettlement in Zimbabwe.

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