Nutritional characteristics of marandu grass (Brachiaria brizantha cv. marandu) subjected to inoculation with associative diazotrophic bacteria

Due to the search for viable and cost-effective ways to provide nitrogen for forage grasses, inoculation with diazotrophs presents high potential as an alternative to plant nutrition by reducing the use of nitrogen fertilizers. The objective of this work was to evaluate the nutrition of marandu grass (Brachiaria brizantha cv. marandu) subjected to inoculation with associative diazotrophic bacteria grown in Cerrado soil (Brazilian Savanna). The experiment was conducted in a greenhouse with a completely randomized design using five treatments (three strains of associative nitrogen fixing bacteria [MTH2, MTB1 and Y2], an inoculant formed by combination of strains AbV5 and AbV6 (Azospirillum brasilense) and a control (100 mg dm-3 of N-urea]) and five replications, totaling 25 experimental units. The inoculation was performed by inserting a 5 mL aliquot of bacterial broth containing 109 cells mL-1 in the soil near the root system of each plant. Three cuts were made at 30, 60 and 90 days after sowing. The variables: Falker chlorophyll index, nitrogen concentration in shoots and roots and crude protein in shoots of marandu grass were evaluated. The highest values for Falker chlorophyll index, nitrogen concentration, and crude protein were observed in the three cuts in plants that received N fertilization. The MTH2 strain positively influenced the nitrogen concentration in shoots and roots, and crude protein in shoots in the first and second cuts of marandu grass, whereas in the third cut, these parameters were influenced by nitrogen fertilization. In conclusion, the inoculation with associative diazotrophic bacteria may contribute positively to the nitrogen nutrition of marandu grass.   Key words: Brachiaria brizantha, Cerrado, diazotrophic microorganisms.