Heterofermentative Lactic Acid Bacteria Enhance the Aerobic Stability of Sweet Sorghum Silage

Sweet sorghum (Sorghum bicolor L.) silage is highly prone to aerobic spoilage due to its high sugar content, leading to significant nutritional losses. This study applied absolute microbial quantification, providing novel insights into how Lactobacillus buchneri and Lactobacillus hilgardii, alone or in combination, influence microbial succession and improve the aerobic stability of sweet sorghum silage. The treatments included: (1) control (CK, sterilised water); (2) Lactobacillus buchneri NX205 (LB); (3) Lactobacillus hilgardii M1814 (LH); and (4) a combination of LB and LH (LBLH). After 60 days of ensiling, lactic acid bacteria (LAB)-inoculated groups exhibited significantly lower pH, butyric acid and ammonia-N (except for the LB group), along with higher acetic acid compared with the CK group (p < 0.05), whereas lactic acid and propionic acid contents did not differ significantly among treatments (p > 0.05). LAB inoculation significantly improved aerobic stability, with the LBLH group exhibiting the longest stability period compared to CK, LB and LH groups (462 h; p < 0.005). During aerobic exposure, the LBLH group delayed nutritional and fermentation losses by maintaining lower pH and ammonia-N levels while sustaining higher lactic and acetic contents compared to CK, LB and LH groups. Microbial analysis showed that LBLH reshaped bacterial and fungal communities, with Gluconobacter oxydans prevailing among bacteria and Zygosaccharomyces bailii and Penicillium paneum dominating fungi. Functional pathway prediction further revealed enrichment in carbohydrate degradation, xenobiotic metabolism and energy utilisation in LAB-inoculated silages. Collectively, these results demonstrate that heterofermentative LAB, particularly the LBLH combination, enhances sweet sorghum silage quality by improving aerobic stability and regulating microbial succession.