Influences of Growth Stage and Ensiling Time on Fermentation Characteristics, Nitrite, and Bacterial Communities during Ensiling of Alfalfa

This study examined the impacts of growth stage and ensiling duration on the fermentation characteristics, nitrite content, and bacterial communities during the ensiling of alfalfa. Harvested alfalfa was divided into two groups: vegetative growth stage (VG) and late budding stage (LB). The fresh alfalfa underwent wilting until reaching approximately 65% moisture content, followed by natural fermentation. The experiment followed a completely randomized design, with samples collected after the wilting of alfalfa raw materials (MR) and on days 1, 3, 5, 7, 15, 30, and 60 of fermentation. The growth stage significantly influenced the chemical composition of alfalfa, with crude protein content being significantly higher in the vegetative growth stage alfalfa compared to that in the late budding stage (<i>p</i> < 0.05). Soluble carbohydrates, neutral detergent fiber, and acid detergent fiber content were significantly lower in the vegetative growth stage compared to the late budding stage (<i>p</i> < 0.05). Nitrite content, nitrate content, nitrite reductase activity, and nitrate reductase activity were all significantly higher in the vegetative growth stage compared to the late budding stage (<i>p</i> < 0.05). In terms of fermentation parameters, silage from the late budding stage exhibited superior characteristics compared to that from the vegetative growth stage. Compared to the alfalfa silage during the vegetative growth stage, the late budding stage group exhibited a higher lactate content and lower pH level. Notably, butyric acid was only detected in the silage from the vegetative growth stage group. Throughout the ensiling process, nitrite content, nitrate levels, nitrite reductase activity, and nitrate reductase activity decreased in both treatment groups. The dominant lactic acid bacteria differed between the two groups, with <i>Enterococcus</i> being predominant in vegetative growth stage alfalfa silage, and <i>Weissella</i> being predominant in late budding stage silage, transitioning to <i>Lactiplantibacillus</i> in the later stages of fermentation. On the 3rd day of silage fermentation, the vegetative growth stage group exhibited the highest abundance of <i>Enterococcus</i>, which subsequently decreased to its lowest level on the 15th day. Correlation analysis revealed that lactic acid bacteria, including <i>Limosilactobacillus</i>, <i>Levilactobacillus</i>, <i>Loigolactobacillus</i>, <i>Pediococcus</i>, <i>Lactiplantibacillus</i>, and <i>Weissella</i>, played a key role in nitrite and nitrate degradation in alfalfa silage. The presence of nitrite may be linked to <i>Erwinia</i>, unclassified_o__Enterobacterales, <i>Pantoea</i>, <i>Exiguobacterium</i>, <i>Enterobacter</i>, and <i>Allorhizobium–Neorhizobium–Pararhizobium–Rhizobium</i>.