Increasing droughts adversely affect grasslands, diminishing the availability and quality of forages for ruminants. We have recently shown that mixed ensiling of drought-impaired grass (DIG) with sugar beet pulp (SBP) improved the conservation and feed value of silage. The application of silage additives may further improve the ruminal degradability, which may thereby shape the fermentation and microbiome in the rumen when those silages are tested as part of dairy diets. Therefore, we performed a long-term in vitro nutritional evaluation of diets containing 50% (DM basis) of mixed silages from DIG and SBP, ensiled either with no additive (T_CON) or with anaerobic fungi culture supernatant (25% in DM
T_AF), mixed ruminal fluid (10% in DM
T_RF) or lactic acid bacteria (1% in FM
T_LAB). The data showed a high degradability of all diets (e.g., >
70% for organic matter), though without differences in nutrient degradabilities among treatments (p >
0.05). Fermentation characteristics, such as ruminal pH, short-chain fatty acid profile, and gas production were only marginally affected by the treatments. Isobutyric acid proportion was higher in T_CON than in T_AF (p = 0.01), whereas isovaleric acid proportion was lower in T_LAB than in T_RF (p = 0.01). The analysis of the bacterial community revealed similar diversity and structure across all treatments in both the liquid and solid fraction. Noteworthy, Lactobacillus was among the predominant genera in the liquid fraction, which may have derived from the mixed silages. In conclusion, mixed silages from DIG and SBP as part of a 50% concentrate diet showed high ruminal degradability, but no beneficial impact by the tested silage additives was observed. Hence, under these conditions, their application appears not justified. Our results warrant further in vivo verification, whereby it would be of interest to determine the impact of the applied silage additives in forage-based diets (e.g., >
50% silage in diet DM) in future research.