The aim of this study was to evaluate the effect of two organic acids [malic acid (MA) and citric acid (CA)], Lactobacillus plantarum (LP) and their mixtures on the fermentation of alfalfa silage. The wilted and chopped alfalfa was ensiled in vacuum-sealed plastic bags (200-230 g per bag) with following additives: 0.5% distilled water (control), 0.5% DL-MA, 0.5% CA, LP (1 · 106 cfu/g), 0.5% DL-MA plus LP (MA+LP) or 0.5% CA plus LP (CA+LP) (application rates based on fresh weight). Each treatment had four replicates and was stored at room temperature. After 60 d of ensiling, decreases in pH, non-protein N (NPN) and increases in lactic acid were observed in all additive-treated silages (p < 0.01). Compared with treatment LP, higher pH and lower lactic acid concentrations were observed after MA or CA addition (p < 0.05) and further application of MA or CA together with LP did not promote lactic acid fermentation in ensiled alfalfa. Furthermore, additives decreased the proportion of saturated fatty acids and increased the proportion of polyunsaturated fatty acids in ensiled alfalfa, with the highest level in CA-treated silages (p < 0.05). Although MA and CA were not as effective as LP in improving silage fermentation quality, treatments MA+LP and CA+LP were more effective in decreasing silage NPN, and CA was most effective in inhibiting biohydrogenation of fatty acids in ensiled alfalfa.
Keywords: Alfalfa; Lactobacillus plantarum; citric acid; malic acid; proteolysis; silage additives; silage fermentation.