Regulatory Mechanism Of Hybrid Pennisetum Purpureum Silage Quality By Self-Selected Lactic Acid Bacteria LP149 And Cellulase

Hybrid Pennisetum purpureum is one of the most promising high-quality forage in humid and hot areas,with grass production intensive in summer and autumn.Silage can realize the long-term preservation of hybrid Pennisetum purpureum and effectively alleviate the problem of unbalanced seasonal supply of forage in humid and hot areas.However,due to the high moisture and lignocellulose content of hybrid Pennisetum purpureum at harvest,the natural fermentation quality is usually poor.Previous studies have shown the heat-resistant Lactobacillus plantarum 149(LP149)and Acremonium cellulase(AC)have the potential to improve the fermentation quality of hybrid Pennisetum purpureum silage in humid and hot areas.But the effects of these two additives under different fermentation conditions and their regulatory mechanisms are not clear.Therefore,this experiment used hybrid Pennisetum purpureum cv.Guimu No.1 as raw material.A three-factor completely randomized design,set two moisture contents: 80%(M1),70%(M2);two ensiling temperature: 30℃(T1),45℃(T2);four additive treatments: 1)sterile water(CK),2)Acremonium cellulase(AC),3)commercial Lactobacillus plantarum(LP),and 4)self-selected heat-resistant Lactobacillus plantarum 149(LP149).The bags were opened at 1,3,7,14 and 60 d of ensiling,to study the effects of additives on the silage quality of hybrid Pennisetum purpureum under different fermentation conditions,and to explore the regulatory mechanism of LP149 and AC on the silage quality of hybrid Pennisetum purpureum through the analysis of carbohydrate metabolism,changes in microbial communities structure and their correlation.The main findings are as follows:1.Addition of LP149 and AC improved the silage quality of hybrid Pennisetum purpureum.In hybrid Pennisetum purpureum silage with 80% moisture content: the addition of LP149 and AC rapidly decreased p H value to below 4.2 after 3 d of ensiling;at 3-60 d of ensiling,lactic acid and acetic acid contents were significantly higher(P < 0.05)and ammonia nitrogen(AN)contents were significantly lower(P < 0.05)in the LP149 and AC treatment groups than in the control.In hybrid Pennisetum purpureum silage with 70% moisture content:the p H value of LP149 and AC treatment groups at 30℃ rapidly decreased to below 4.2 after3 d of ensiling;but the p H value of all treatment groups at 45℃ decreased to 4.2 after 14 d of ensiling,and the lactic acid content was low(< 40 g/kg DM)and the AN content was high throughout the ensiling process.Compared to the 70% moisture content hybrid Pennisetum purpureum silage,the 80% moisture content silage had higher(P < 0.05)lactic acid content,faster p H decline rate and lower AN levels in all treatment groups.2.Addition of LP149 and AC promoted the degradation of structural carbohydrates in hybrid Pennisetum purpureum silage.At 3-60 d of ensiling,LP149 and AC treatment groups had lower content of neutral detergent fiber(NDF),acid detergent fiber(ADF)and cellulose,and higher content of soluble carbohydrate(WSC),glucose and fructose compared with control and LP treatment groups.In particular,the NDF,ADF,cellulose and hemicellulose contents of the AC treatment group were significantly lower(P < 0.05)than other treatments at 7-60 d of ensiling,while the WSC,glucose,fructose and sucrose contents were significantly higher(P < 0.05)than other treatments.Hence the AC treatment was more powerful in degrading structural carbohydrates compared to LP149.3.Addition of LP149 and AC changed the microbial communities structure and succession in hybrid Pennisetum purpureum silage.The number of lactic acid bacteria in the LP149 and AC treatment groups was significantly higher(P < 0.05)than the control;during the whole ensiling process,the relative abundance of Lactobacillus in the LP149 treatment and Weissella in the AC treatment were higher,which are the main bacteria involved in lactic acid fermentation in ensiling and improve the silage quality of hybrid Pennisetum purpureum by producing large amounts of lactic acid to decrease the p H value.Compared to the 80%moisture content hybrid Pennisetum purpureum silage,the relative abundance of Klebsiella decreased in the 70% moisture content silage,but the abundance of Raoultella,Enterobacter and Acinetobacter increased.In conclusion,the addition of self-selected heat-resistant lactic acid bacteria LP149 improved the fermentation quality of hybrid Pennisetum purpureum silage by increasing the number of lactic acid bacteria,enhancing the relative abundance of Lactobacillus,and promoting lactic acid fermentation to decrease p H.While AC treatment improved the fermentation quality by enhancing the degradation of structural carbohydrates,providing more fermentation substrate for microorganisms,increasing the relative abundance of Weissella,and promoting the production of large amounts of lactic acid to decrease p H in the early stages of ensiling.However,wilting treatment inhibited the effects of LP149 and AC to improve the silage quality of hybrid Pennisetum purpureum in humid and hot areas.