A Norwegian dairy herd was examined during and after conversion to ecological management and low concentrate feeding (maximum 3 FEm per day and cow) in 1989–1994. Records from 34 Norwegian Red cows representing 81 lactations were subjected to ANOVA. Mean annual milk yields of 4667 kg, 5478 kg and 6155 kg per cow during first, second and later lactations, respectively, were obtained after conversion. A reduction in number of cows and hence, total milk yield resulted from decreased feed import and reduced forage yields. Decreasing milk yield and increasing protein percentage was found during conversion when effects of calving season, lactation month and number, concentrate ration and cow were corrected for. However, years with large deviations from this trend were observed, demonstrating sensitivity to variations in the farm’s feed production. Periods with pasturing showed the highest milk yield and protein percentage for most seasons of calving, and hence spring calvers obtained the highest milk yield for the whole lactation. Lower somatic cell count in milk was also observed for cows calving in spring and summer compared to calving in autumn and winter. With respect to daily milk yield and protein percentage, cows with higher sire index for protein yield and protein percentage were found superior in this production system. Cows sired by high genetic merit bulls for protein yield also showed higher somatic cell counts than cows sired by low genetic merit bulls, but cows sired by bulls with high overall breeding value showed lower somatic cell counts.