Increasing yields and nitrogen fixation of soybean, groundnut and mungbean in Vietnam through rhizobial inoculation

• University of Can Tho, Vietnam
• Institute of Agricultural Sciences of South Vietnam, Vietnam
• Oil Plants Institute of Vietnam, Vietnam
• Vietnam National University, Vietnam

Vietnam cultivates about 700,000 hectares of legumes, equally distributed between the north and the south of the country. Production is about 700,000 tonnes, worth $A350 million. Surveys of 155 commercial crops of groundnut, soybean and mungbean in well-established legume-production areas of the Red River Delta in the north of the country and in the regions around HCM City and the Mekong Delta in the south established that Vietnamese farmers do not use rhizobial inoculants to boost crop nitrogen (N). Instead they fertilise their legumes with 30-150 kg N/ha at a cost of A$50-60 million annually.
ACIAR has funded research to test the response of legume crops such as soybean, mungbean and groundnut (peanut) to inoculation with rhizobium bacteria that increase the amount of available nitrogen. This small project built on the earlier research, inoculating to increase the amount of atmospheric nitrogen fixed by crops then measuring the response in terms of increased yield.

The project aimed to test the effect of rhizobial inoculation to increase N2 fixation and thus lift yields of soybean, mungbean and groundnut in Vietnam. Specific objectives of the project were to collect data on Vietnamese farmers' current use of and attitudes to inoculants and fertiliser N for legumes, to quantify levels of N2 fixation of commercial soybean, groundnut and mungbean crops, to determine benefits of inoculation through field experiments at many locations and to train Vietnamese scientists in inoculant technology, including quality assurance. The final project objective was to lay the foundations for a follow-up project to produce large volumes of high-quality rhizobial inoculants to satisfy Vietnam's expanding legume industry.

The scientists examined farmer management practices to determine cultural factors that either depressed or contributed to nitrogen fixation. They tested the quality of inoculants currently produced in Vietnam, identifying the best performing strains for each crop. At the same time they assisted in training Vietnamese scientists to produce high-quality rhizobial inoculants. The training was necessary prior to the development of a project that would lift Vietnam's capacity to produce large volumes of inoculant.

The scientists found that all surveyed crops had nodulated, indicating background soil populations of effective rhizobia, at least in the established legume production areas. Average %Ndfa (plant reliance on N2 fixation) values were 60 per cent for soybean, 46 per cent for groundnut and 58 per cent for mungbean.
Results of the inoculation experiments in the south of the country indicated fertiliser N to be an unnecessary cost that substantially reduced profitability of legume cropping. Fertiliser N inputs could be reduced to basal amounts (i.e. <10 kg N/ha) and replaced by inoculation. The cost of the latter would be in the order of $A2 million annually. In the 23 experiments, profitability was increased by up to A$760/ha, through inoculation and reduction of fertiliser N inputs from high levels (30-150 kg N/ha) down to less than 10 kg N/ha.
Average grain yield responses for inoculated groundnut, relative to the -N and +N uninoculated controls, were 12 and 7 per cent respectively over eight experiments, although responses in the very acidic Mekong Delta averaged 42 per cent (relative to -N control) and 28 per cent (relative to the +N control). Thirteen experiments with soybean showed an average response to inoculation of 19 per cent relative to the -N control. Some individual experiments showed inoculation responses of 40-50 per cent. Yields of the inoculated and +N plots were almost identical.
It became clear that for inoculation to become a reality in Vietnam, high quality rhizobial inoculants would need to be as readily available in the market place as fertiliser N. Farmers would also need to be educated in their use, just as they are educated about using fertilisers and chemicals. Not every legume crop would respond to inoculation and they would need to determine whether or not to inoculate. It would probably require a policy change on N fertiliser and a lot of extension and farmer education, but the economic gain was almost guaranteed.
Considerable effort went into designing a follow-on medium ACIAR project that focused on improved inoculum production in Vietnam and a campaign to promote of its use by grain legume growers - probably soybean growers in some appropriate target areas. Thailand, where soybean is widely inoculated with an effective local product, was to be used as a model. However the proposal did not gain strong support in Vietnam, and there was never a strong Vietnamese champion for the use of good inoculum, something deemed very important if the barriers to production and adoption of inoculum were to be overcome. As a result the proposal was dropped in June 2003.