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In Switzerland, fresh herbage is a favoured feed for dairy cows due to its high quality and availability and low production costs. However, transition and early lactation are periods characterized by an increased nutrient demand that may not be covered by herbage alone. To compare the effects of concentrate supplementation in early lactation on nutrient efficiency and ruminal fermentation, 24 multiparous Holstein cows were assigned to two performance groups according to their previous lactation milk yield: high‐ (8,959 ± 984 kg) and low‐ (6,204 ± 1,000 kg) potential cows. Within this group, cows were allocated to two treatment groups receiving either herbage ad libitum (n = 11) or herbage supplemented with concentrate (n = 13). The experiment started for each cow 2 weeks before the predicted calving date (LW‐2) and lasted until lactation week (LW) 8. Milk yield and dry matter intake (DMI) were recorded daily. The reticular pH was measured continuously using a telemetric pH bolus. Milk components and ruminal fermentation traits were analysed in LW‐2, LW2, LW4, LW6 and LW8. Supplemented cows (p < 0.001) and high‐potential cows (p = 0.015) produced more milk than unsupplemented cows and low‐potential cows, respectively. Milk acetone was affected by supplementation (p < 0.001) and milk potential (p = 0.002) and was especially high in unsupplemented, high‐potential cows until LW6. Supplementation caused a decrease in herbage DMI (p < 0.001) but resulted in an increased total DMI (p < 0.001), whereas milk potential had no effect on DMI. Associated with an increasing DMI (p < 0.001), ruminal volatile fatty acid concentration (p = 0.024) increased and reticular pH (p < 0.001) decreased from LW2 until LW6. Apart from that, effects on ruminal fermentation and reticular pH were minor. In conclusion, even though apparent nutrient efficiency was high, high‐potential cows without supplementation seem to struggle more with reduced nutrient availability than other cows; therefore, they appear to be more prone to metabolic stress and consequently to production diseases. [ABSTRACT FROM AUTHOR]