TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Additive: In (I), the liquid by-product obtained from an amino acid fermentation broth is chosen from L-glutamic acid, L-lysine or L-phenylalanine fermentation broth. EXAMPLE - Liquid by-product having pH of 4.7, total nitrogen content of 6.2 wt.%, and ammonia nitrogen content 4.3 wt.% was obtained from glutamic acid fermentation broth, (referred to as Glu mother liquor). Liquid by-product having pH of 4.2, total nitrogen content of 5.9 wt.% and ammonia nitrogen content was 4.7 wt.% was obtained from lysine fermentation broth (referred to as Lys mother liquor). The phenylalanine fermentation broth liquid by-product (referred to as Phe mother liquor) was also obtained. Each of Glu mother liquor, Lys mother liquor and Phe mother liquor was added to Italian ryegrass in an amount of 3% based on the fresh weight of Italian ryegrass. The resultant mixture was filled in a 1 L bottle (filling density of 760 g FW/L) to carry out silage test. As a control group, silage with no mother liquor was prepared. Each of the silages was opened on 40th day after preparation and a sample was taken. The quantity of distilled water equal to 10 times the quantity of the sample was added to the sample, and the mixture was pulverized with a mixer and filtered out with gauze. The filtrate was subjected to centrifugal separation at a rotation speed of 15000 rpm for 5 minutes at 5 degreesC. Two fold amounts of 6% perchloric acid were added to the supernatant liquid. The mixture was stirred and then subjected to centrifugal separation. The supernatant liquid was filtered out with 45 microns filter to measure the pH and to determine volatile fatty acids using a liquid chromatography. The amount of total volatile fatty acid formed and the proportion of lactic acid in total organic acid in the Glu mother liquor-, Lys mother liquor- and Phe mother liquor-treated groups were higher compared with those in the control group, which showed that the mother liquors have effects of promoting the silage lactic acid fermentation and of improving significantly the quality of silage.