Milk production in Uruguay has been traditionally based on direct grazing of grasses and legumes, associated with supplementation of concentrates and/or preserved forages. An alternative is to combine TMR and pastures, also called partial mixed ration (PMR). This system may reach and even enhance both intake and milk production achieved by cows that only eat TMR, depending on the pasture inclusion level. The use of lucerne, a widely recognised forage for its beneficial effects on the ruminal environment, could be the best option to include in PMR in order to maintain nutrient digestibilities and VFA production of TMR diets. Moreover, these effects could be more pronounced with the use of lucerne in earlier stages of development. This study aimed to investigate the effects of replacing 40% of a TMR with mid-vegetative or early bud lucerne on the ruminal environment and nutrient apparent disappearance using the rumen simulation technique (RUSITEC). Treatments consisted of 3 diets formulated for high-production dairy cows: (1) 60:40 TMR: Mid-vegetative lucerne (PMR1); (2) 60:40 TMR: Early bud lucerne (PMR2); (3) 100:0 TMR (control). The diets were incubated in duplicate, performing 3 runs. Each run lasted 12 days (7 days to equilibrate the system and 5 days of sampling). Nutrients disappearance excluding DM disappearance was analyzed using a mixed model with treatment as fixed effects and run as a random effect while parameters of the ruminal environment and DM disappearance were analyzed as repeated measurements using a mixed model with treatment and sample day as fixed effects and run as a random effect. Means were compared using preplanned orthogonal contrasts (PMRs vs TMR and PMR1 vs PMR2). DM, MO, and NDF disappearance were higher in TMR (P < 0.05), with no significant differences between PMRs. Total VFA production was higher (P < 0.05) and pH was lower (P < 0.05) in TMR. The pH was lower in PMR1 compared to PMR2, although PMR2 VFA production was higher (P < 0.05). The propionic acid proportion was higher in TMR (P < 0.05) and no significant differences between PMR1 and PMR2 were found, while butyric acid proportion was higher in PMR1 than in PMR2 (P < 0.05). The acetic acid and propionic acid/acetic acid ratio was lower in TMR (P < 0.05) and were no significant differences between PMRs. Finally, there were no significant differences between treatments, either in gas and NH3–N daily production, CP, or ADF disappearance. In conclusion, a replacement in 40% of TMR with lucerne in early phenological stages fails to maintain fermentation products and nutrient disappearance reached with TMR. Additionally, the substitution of TMR with mid-vegetative lucerne has a lower effect in modifying pH and butyric acid proportion, while early bud lucerne modified to a lesser extent VFA production.