Seed and leaf-spray inoculation of PGPR in brachiarias (Urochloa spp.) as an economic and environmental opportunity to improve plant growth, forage yield and nutrient status

Abstract

Brazil has 180 Mha of pastures, 86 Mha occupied with Urochloa spp. (syn. Brachiaria), and 70% in some level of degradation. Inoculation with plant-growth-promoting bacteria (PGPR) may represent an economic and environmental feasible strategy to improve pasture production. Two greenhouse and seven field trials were performed to verify the effects of seed inoculation at sowing or leaf-spray inoculation in established pastures of Urochloa with elite strains of Azospirillum brasilense and Pseudomonas fluorescens. All plants received nutrients including 40\xa0kg\xa0ha−1 of N at sowing, and half of the treatments a supply of 40\xa0kg\xa0ha−1 of N 30\xa0days after emergence. A. brasilense increased shoot biomass by an average of 16.8% with both seed and leaf-spray inoculation, whereas P. fluorescens increased by 15.2 and 14.2%, respectively, always higher with the extra supply of N. Seed and leaf-spray inoculation with A. brasilense increased N content by an average of 11.7 and 20.7%, and K by 9.9 and 11.3%, respectively; for P. fluorescens average increases were of 33.3 and 36.6% for P, and of 10.6 and 13.6% for K, respectively. Benefits were mainly attributed to improvements in root architecture by the synthesis of phytohormones. Biological nitrogen fixation in A. brasilense, P acquisition (solubilization of phosphates and siderophores synthesis) and ACC-deaminase in P. fluorescens also contributed to plant growth and nutrient status. Inoculation with elite PGPR strains translated into more fodder and improved nutritional value of feed for livestock, representing a promising and environmentally-friendly strategy for tropical pastures.