# Inoculation with Azospirillum brasilense Associated with Nitrogen Rates on the Yield and Nutritional Value of Giant Sorghum Silage

## TL;DR

Inoculating giant sorghum with Azospirillum brasilense did not improve yield or nutritional value and even reduced productivity at higher nitrogen rates in the Brazilian Amazon.

## Contribution

The study reveals that A. brasilense inoculation is ineffective for improving giant sorghum under Amazonian conditions, emphasizing the need for further research on bacterial interactions.

## Key findings

- Inoculation with A. brasilense decreased productivity at higher nitrogen rates.
- A nitrogen rate of 200 kg ha−1 maximized yield without inoculation.
- Crude protein increased with nitrogen but not enough to justify inoculation.

## Abstract

This study evaluated the effect of inoculating with the bacterium Azospirillum brasilense, in conjunction with nitrogen side-dressing. By assessing the agronomic and productive characteristics of the plant and the nutritional value of the silage, we found that inoculation with A. brasilense did not improve the agronomic characteristics of forage sorghum, and resulted in decreased productivity at higher N rates. Although inoculation slightly increased silage crude protein concentration, this effect was insufficient to compensate for yield losses or to reduce the need for mineral N. For forage management, a topdressing application of 200 kg ha−1 of N is recommended, without seed treatment with the bacterium. These findings shed light on the responses of giant sorghum inoculated with the bacterium A. brasilense and highlight the importance of understanding and further investigating the interactions of bacteria in giant sorghum seeds under the soil and climate conditions of the Brazilian Amazon.

The high cost of nitrogen (N) fertilizers has increased interest in sustainable alternatives, such as inoculation with Azospirillum brasilense, to improve forage production. This study evaluated the agronomic performance, yield, nutritional composition, and fermentation profile of giant sorghum silage from crops inoculated with A. brasilense under different N rates. A randomized block design was adopted in a 5 × 2 factorial arrangement, with five N rates (0, 50, 100, 200, and 400 kg ha−1) and the presence or absence of A. brasilense inoculation, with four replicates. Nitrogen rates significantly affected plant height and stem diameter, with maximum values observed at 200 kg ha−1 N. An interaction between inoculation and N rates was detected for first-cycle yield, in which inoculated plants showed reduced productivity at 200 and 400 kg ha−1 N. Total yield was higher in non-inoculated treatments and was maximized at 200 kg ha−1 N, representing a 29.42% increase compared with the unfertilized control. Crude protein concentration increased with increasing N rates, while in vitro dry matter digestibility was influenced only by N fertilization. Inoculation did not improve agronomic performance and negatively affected yield at higher N rates, although both inoculation and N fertilization influenced protein and fiber contents of sorghum silage. Under the edaphoclimatic conditions of the Brazilian Amazon, A. brasilense inoculation is not recommended for giant sorghum cultivation, whereas the application of 200 kg ha−1 N was the most effective strategy to maximize yield.