For temperate climate, there is little information on the effects cover crops grown in fall (CCs) on the nitrogen (N) supply for next year's sugar beet (SB). Four field trials were conducted on silty soils to establish the CC N effect (N_eff) compared to bare fallow separately for the periods sowing-summer and summer-autumn harvest. Biomass characteristics of radish (Raphanus sativus L.), spring vetch (Vicia sativa L.), saia oat (Avena strigosa Schreb.), and winter rye (Secale cereale L.) CCs before winter, soil mineral nitrogen in spring (SMN), and SB N accumulation and sugar yield (SY) were measured. In the period sowing-summer, characterized by a high SB N demand, N_eff of overwintering, high biomass yielding rye CC was negative up to –50 kg N ha⁻¹ at three site/years and varied around zero for the other CCs except vetch, for which N_eff was positive. At SB autumn-harvest, N_eff was negative up to –100 kg N ha⁻¹ except for vetch in one trial. SY was lowest after rye CC. Regression analyses indicated a negative impact of CC biomass, C:N ratio and the difference in SMN between fallow (high SMN) and CCs (low SMN) on N_eff. To conclude, if CCs yield a high amount of biomass surviving until spring and thus remove SMN from the soil which otherwise remains available for SB, early season mineralization of CC biomass N can be too low to ensure a N supply sufficient for maximum SB yield. Choosing leguminous CCs or early termination of CCs might alleviate this constraint.