Aims: To assess the effect of two additives on alfalfa silage and on in vitro ruminal fermentation when using ruminal inocula from high feed-efficient (HE) and low feed-efficient (LE) lactating cows. Methods and Results: First-and second-cut alfalfa was harvested at 40% bloom stage, treated with control (no additive), Lactobacillus plantarum (LP) or formic acid (Formic), ensiled in 1 +/- 0 l minisilos, and fermented for 60 days. Fermented alfalfa was incubated in vitro for 24 h using ruminal inoculum from HE and LE lactating cows. The pH was lower in alfalfa silage treated with LP and Formic, and produced lower ammonia-N than did the control. In vitro true dry matter digestibility (IVTDMD) was higher with ruminal inoculum from HE than LE cows, but there was no consistent effect of treated alfalfa on microbial biomass yield and in vitro volatile fatty acids. Conclusions: The IVTDMD was numerically greater with ruminal inoculum from higher feed-efficient cows although statistical significance was only demonstrated with the first-cut alfalfa. However, treated alfalfa silage did not show the effect expected on in vitro microbial biomass yield. Significance and Impact of the Study: The feed efficiency of cows used as a source of ruminal inocula may affect IVTDMD and be a source of variation across in vitro runs. Differences in ruminal fermentation between cows of different feed efficiency could help to explain differences in milk yield and other parameters of dairy cattle performance.