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

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.