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.

Purpose: 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. Methods: 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 kg ha−1 of N at sowing, and half of the treatments a supply of 40 kg ha−1 of N 30 days after emergence. Results: 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. Conclusion: 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. [ABSTRACT FROM AUTHOR]