Inoculation with Plant Growth-Promoting Bacteria Improves the Sustainability of Tropical Pastures with Megathyrsus maximus

Brazil is the second-largest producer and the first exporter of beef, with herds mainly raised in extensive pastures, where Megathyrsus maximus occupies over 30 Mha. About 70% of the pastures are under degradation, and using plant growth-promoting bacteria (PGPB) may contribute to reversing this scenario. We investigated the effects of PGPB on the growth of six cultivars of M. maximus—Tanzania-1, Massai, BRS Zuri, Mombaça, BRS Tamani, and BRS Quênia—under greenhouse conditions. Plants were inoculated, or not, with the elite strains of Azospirillum brasilense CNPSo 2083 + CNPSo 2084, Bacillus subtilis CNPSo 2657, Pseudomonas fluorescens CNPSo 2719, or Rhizobium tropici CNPSo 103. At 35 days after emergence, plants were evaluated for ten root growth traits, shoot dry weight, and the levels of macro and micronutrients accumulated in shoots. Several root traits were increased due to inoculation in all genotypes, impacting plant growth and nutrient uptake. Despite the differences in effectiveness, all genotypes benefited from PGPB to some degree, but Mombaça and BRS Zuri were more responsive. Scanning electron microscopy indicated that bacterial species differed in their capacity to colonize seeds and rootlets. The results show that inoculation with elite PGPB strains may represent an important strategy for the sustainability of M. maximus pastures.