RE-INOCULAÇÃO DE BACTÉRIAS FIXADORAS DE NITROGÊNIO NA CULTURA DA SOJA E VIABILIDADE ECONÔMICA

Abstract

The practice of inoculation in soybean seeds has been widely used, mainly with the use of bacteria of Bradyrhizobium japonicum species. However, in the search for the increase of the productive indices, techniques such as re-inoculation have been sought where inoculants containing Bradyrhizobium japonicum and Azospirillum brasilense are applied. The objective of this study was to evaluate the agronomic efficiency and economic viability of re-inoculation of nitrogen-fixing bacteria combined with growth promoting bacteria in soybean. Two trials were conducted, the first in the 2017/2018 crop and the second in the 2018/2019 crop at the Instituto Federal Goiano – Campus Rio Verde. The experimental design was in randomized blocks with ten treatments and four replications. The treatments were: T1: no re-inoculation, T2: 100 kg N ha-1, T3: 200 kg N ha-1, T4: 2 doses of B. japonicum, T5: 4 doses of B. japonicum, T6 : 6 doses of B. japonicum, T7: 8 doses of B. japonicum, T8: 10 doses of B. japonicum, T9: 2 doses of B. japonicum + 3 doses of A. brasilense, T10: 10 doses of B. japonicum + 3 doses of A. brasilense. Commercial inoculants were used, being B. japonicum at a concentration of 5x109 CFU mL-1 and A. brasilense at a concentration of 2x108 CFU mL-1. For all treatments, inoculation was performed moments before the sowing operation. At harvest, the yield components were evaluated and the N content of the grains was determined. The results were submitted to variance analysis and the means were grouped by the Scott-Knott test at 5% significance level. Economic viability analysis was also performed using the structure based on the effective operating cost, taking into account the market cost during the harvest. In the first harvest, no significant differences were observed for the variables weight of one thousand grains, number of grains per pod, yield and total nitrogen. Regarding the economic viability in the first crop there were not many positive results, on the contrary, all treatments had lower yields, so the investment made in re-inoculation was lost, as there was no economic return. In the second crop, there was an increase in the weight of one thousand grains, productivity and nitrogen content of the grains in the treatments that received application of 10 doses of B. japonicum, 2 doses of B. japonicum + 3 doses A. brasilense and 10 doses of B. japonicum + 3 doses of A. brasilense. Regarding the economic viability except for treatment 2, all other treatments had higher productivity than the standard treatment, the last two treatments with B. japonicum and A. brasilense obtained better economic results, thus proving what several authors point out about the co-inoculation. The best agronomic indices were in the treatments with higher doses of B. japonicum, combined or not with A. brasilense, and although these higher doses are the ones with higher costs, there is still viability in their application compared to the nitrogen application that by high investment the return is lower.