Comparison of the improvement efficacies for ensiling quality and biodegradation performance of sweet sorghum silage by different additives

Abstract: Sweet sorghum, a C4 energy crop, has gained much attention recently as a source of biofuel production, due mainly to its high biomass yields while requiring less water and fertilizer. However, the seasonal harvest of sweet sorghum needs to be preserved and stored for a year-round continuous supply. The anti-degradation barrier structure of native lignocellulose can also hinder the biodegradability of sweet sorghum. Particularly, the lignin remains an intact shielding to the degradable carbohydrate polymers against decomposition. Alternatively, the silage pretreatment and bioaugmentation can be combined to serve as an effective way for sweet sorghum, thereby maintaining a sustainable supply, while improving biodegradability. Furthermore, silage additives can also be selected to improve biodegradability and ensiling quality. For instance, the rumen fluid (waste from slaughterhouses) and biogas slurry (waste from biogas plant) display an anaerobic microbial ecosystem with cellulolytic activities. In this study, the rumen fluid and biogas slurry were added into the silages system of sweet sorghum for improved ensiling quality and biodegradation. A comparison was also made on the addition of rumen fluid and biogas slurry, cellulase, and xylanase additives, in terms of the fermentation, structural characteristics, and enzymatic saccharification of sweet sorghum silages. The sweet sorghum was bioaugmented-ensiled at four treatments, including the cellulase treatment (CT) with a dosage of 3 g/kg; xylanase treatment (XT) with a dosage of 3 g/kg; rumen fluid treatment (RT) with a dosage of 70 mL/kg; biogas slurry treatment (BT) with a dosage of 70 mL/kg, and one control group (CK, no-added additive), where all ensiled for 90 days at 18℃. Silage samples were analyzed to determine the content of Dry Matter (DM), the organic Components including Neutral Detergent Fiber (NDF), Acid Detergent Fiber (ADF), Acid Detergent Lignin (ADL), Water-Soluble Carbohydrates (WSC), and the fermentation properties, such as pH value, the content of lactic acid, acetic acid, and ammonia nitrogen (NH3-N). Furthermore, the structural characteristics of sweet sorghum before and after bioaugmented-ensiling were evaluated using an X-ray Diffractometer (XRD), Scanning Electron Microscope (SEM), and Fourier transform infrared spectrometer (FTIR). The results showed that the pH value of four treatments with additives significantly dropped to below 4.3 (P < 0.05), the ratio of Lactic Acid and Total Organic Acids (LA/TOA), and the ratio of Lactic Acid and Acetic Acid (LA/AA) were always higher than 0.6 and 2.0 during ensiling, respectively (Execp for the RT grap at god). Specifically, the content of NH3-N and acetic acid decreased, whereas, the content of lactic acid increased in the silages treated with the addition of cellulase and xylanase, as the pH value decreased. The addition of four additives all reduced the content of DM, WSC, NDF, ADL, and hemicellulose (HC), resulting in the increase of cellulose content and Theoretical Biodegradation Potential (TBP) of silages relative to raw material when ensiling for 90 days (P＜0.05). Therein, the optimal biodegradation effect of lignocellulosic components was achieved in the silages with the addition of biogas slurry. The results of SEM, FTIR, and X-ray showed that the compact lignocellulosic structure in the silages with the addition of RT and BT was disintegrated substantially, compared with raw materials. Specifically, a mass of holes and cracks on the microstructure surface appeared, while, the relative crystallization index in the silages declined sharply. The dynamic enzymatic saccharification for 72 hours showed that four additives efficiently improved enzymatic hydrolysis of bioaugmented-ensiled sweet sorghum, compared with the un-ensiled or control silages (CK). Particularly, the silages treated with BT presented the highest enzymatic hydrolysis yield of 84.69% after ensiled for 90 d. The bioaugmented-ensiled efficacy of biogas slurry was superior to that of rumen fluid, followed by cellulase and xylanase. Four additives improved ensiling quality and the biodegradation potential, further enhancing the enzymatic saccharification yield. The improvement was attributed to the restructuring of organic components, and the positive evolution of the lignocellulosic structure. Consequently, the bioaugmented ensiling with resourceful biogas slurry was recommended as the environmentally friendly and economic way to the simultaneous storage and biological conversion of sweet sorghum. Therefore, this work can open a new pathway for the trans-seasonally storage and bioaugmentation pretreatment for the energy crop of sweet sorghum. The finding can also provide a promising way for large-scale utilization of biogas slurry from biogas plants.