Multistage Evaluation of Plant Growth-Promoting Microorganisms in Leucaena leucocephala and Megathyrsus maximus cv. Mombasa: Controlled Experiments and Preliminary Field Observations in Northern Peru

Plant growth-promoting microorganisms (PGPM) may contribute to sustainable forage production, but their effects can vary with host species and plant trait. Bacillus subtilis, Pseudomonas putida, Trichoderma harzianum, and Trichoderma viride were evaluated alongside a non-inoculated control in Leucaena leucocephala and Megathyrsus maximus cv. Mombasa in northern Peru. Replicated germination and nursery experiments were complemented by a preliminary field assessment. Inoculation did not significantly affect final germination percentage in either forage species. In the nursery, L. leucocephala exhibited significant inoculant treatment effects on multiple growth traits, including positive responses in plant height and leaf number under B. subtilis, P. putida, and T. harzianum at the final evaluation. Significant treatment effects were also detected for several final aboveground and belowground growth traits, whereas total weight was not significantly affected. By contrast, growth variation in M. maximus cv. Mombasa was mainly associated with evaluation time, while microbial treatment and treatment × time effects were not significant. Multivariate analysis detected significant treatment and species × treatment effects, indicating contrasting joint growth-response profiles between the two forage species without demonstrating specificity of individual host–microorganism combinations. Preliminary field measurements showed numerical variation among treatment-associated rows, but these observations were descriptive only because treatments were neither randomized nor independently replicated. Overall, this comparative evaluation shows that PGPM responses in tropical forage species can be strongly species- and trait-dependent under nursery conditions, while validation of these responses under field conditions requires appropriately randomized and independently replicated experiments.