Abstract
This study explored the effects of four sugar source treatments, including no sugar (CON), fructose (FRU), pectin (PEC), and starch (STA), on the microbiota and metabolome of alfalfa (Medicago sativa) silage. The bacterial community was determined via 16S rRNA gene sequencing. The metabolome was analyzed using ultra high-performance liquid chromatography tandem time-of-flight mass spectrometry (UHPLC/TOF-MS). After 60 days of fermentation, the pH values in FRU and PEC were lower than those in STA and CON. FRU had a greater lactic acid concentration compared to STA and CON. Weissella (47.44%) and Lactobacillus (42.13%) were the dominant species in all four groups. The abundance of Pediococcus was lower, and the abundance of Leuconostoc, Pantoea, and Microbacterium was higher, in FRU compared to CON. The abundance of norank_f__Bacteroidales_S24-7_group was higher, and the abundance of Turicibacter was lower, in both FRU and PEC than in CON. Leuconostoc was negatively correlated with the pH value, and Pediococcus was positively correlated with the pH value. No microbiomes were detected as discriminative features between STA and CON. The addition of FRU and PEC presented more peptides, such as Leu–Val–Thr, Leu–Phe, Ile–Pro–Ile, Val–Trp, and Ile–Leu–Leu but a lower abundance of metabolites for triterpene glycosides including sanchinoside B1, medicagenic acid, betavulgaroside IV, and prosapogenin compared to CON. The addition of PEC presented more phenyllactic acid compared to CON. Our study demonstrated that the addition of pectin and fructose improved the quality of alfalfa silage mainly by promoting Leuconostoc, Pantoea, and Microbacterium, and inhibiting Pediococcus in FRU, and promoting norank_f__Bacteroidales_S24-7_group and inhibiting Turicibacter in both FRU and PEC; this was due to altered metabolic profiles resulting from antifungal activity and decreased triterpene glycoside accumulation. This study improves our understanding of ensiling mechanisms related to the contributions of sugar.
Highlights
Alfalfa (Medicago sativa L.) is one of the most important forage crops as animal feed and is commonly used for ensiling worldwide (Dunière et al, 2013)
There were 96, 96, 95, and 98 operational taxonomic unit (OTU) identified in CON, STA, FRU, and PEC, respectively, and 81 OTUs were found in all four groups, which accounted for 77.1% of the total OTUs (Supplementary Figure S2)
The principal coordinate analysis (PCoA) results with Bray– Curtis distances at the OTU level indicated that FRU and PEC were separated from CON, but STA was not (Figure 1A and Supplementary Table S2)
Summary
Alfalfa (Medicago sativa L.) is one of the most important forage crops as animal feed and is commonly used for ensiling worldwide (Dunière et al, 2013). Compared to other forages, it is difficult to make silage with alfalfa because of its high buffering capacity, high crude protein (CP) level, and low water-soluble carbohydrate (WSC) content (McDonald et al, 1991). As one of the main roughages in dairy cow production, alfalfa silage can provide high-quality nutrients for livestock production. The alfalfa showed the beneficial function of improved rumen fermentation in dairy cows partly due to its abundant pectin, an fermented carbohydrate (Wang et al, 2014). Starch is an important fermented carbohydrate for ruminants, which is an important factor for determining the quality of corn silage and its effect on milk synthesis (Ferraretto et al, 2018). Few studies have analyzed the effects of the addition of pectin or starch to see their effects on alfalfa silage fermentation
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