Exploring the diversity mechanism of fatty acids and the loss mechanisms of polyunsaturated fatty acids and fat-soluble vitamins in alfalfa silage using different additives

The purpose of this study was to explore the diversity mechanism of fatty acids (FA) and the loss mechanisms of polyunsaturated FA (PUFA) and fat-soluble vitamins in alfalfa silages treated with molasses (ML), lipase-producing- Lactobacillus plantarum (LP), propionic acid (PA), sunflower seed oil (OL) and without additives as the control (CON) for 70 d. The results exhibited that natural ensiling led to the losses of PUFA, C18:3n3, α-tocopherol, and β-carotene and increased the FA diversity due to the appearances of C11:0 and C18:2 trans -912 relative to the fresh alfalfa material (FAM). Similar to the control silage, great PUFA losses were observed in ML- and OL-treated silages. In contrast, LP ( P = 0.001 and P < 0.001) and PA ( P = 0.053 and P = 0.044) reduced the losses of C18:3n3 and PUFA compared with the control. Adding LP increased the contents of C16:0 ( P < 0.001), C18:1 trans -9 ( P = 0.014), and C18:2n6 ( P = 0.001) than FAM. Furthermore, ML, OL, PA and LP increased ( P < 0.001) the C11:0 content relative to FAM, and LP- ( P < 0.001) and PA-treated ( P < 0.001) silages had lower C11:0 content than the control, ML- and OL-treated silages. Compared with the control, adding ML, OL and PA reduced ( P < 0.001) the β-carotene loss, and PA ( P < 0.001) and LP ( P = 0.001) reduced the α-tocopherol loss. Adding LP slightly increased β-carotene (4.9 %) relative to FAM. Next-generation sequencing results showed that undesired Citrobacter and cocci lactic acid bacteria (LAB) included Weissella, Pediococcus and Lactococcus were predominant in the control, OL- and ML-treated silages. The low abundances of these bacteria were detected in PA- and LP-treated silages but they had high abundances of Lactobacillus (> 58.0 %). Spearman correlation and redundancy analyses revealed that these cocci LAB and Citrobacte r led to the increases of lipoxygenase activity, contents of C11:0, and C18:2 trans -912, and the losses of PUFA, C18:3n3, α-tocopherol, and β-carotene, while Lactobacillus altered these indexes oppositely. Moreover, Pantoea (r = -0.70 and -0.68), Sphingomonas (r = -0.81 and -0.76), Aureimonas (r = -0.81 and -0.78) and Devosia (r = -0.83 and -0.80) correlated negatively with the losses of α-tocopherol and β-carotene. In conclusion, ML, PA, and LP improved fermentation quality. Adding PA and LP altered the bacterial community in alfalfa silage, which in turn reduced the FA diversity and losses of PUFA, α-tocopherol, and β-carotene.