The development of production systems, which allow increased nutrient intakes to be achieved, is a key issue in the management of high genetic merit dairy cows. Consequently, forty high genetic merit autumn calving dairy cows (PTA95 fat + protein = 38.2 kg) were managed on either a ‘high forage (HF)’ or ‘high concentrate (HC)’ based system of milk production for the first 305 days of lactation, with the study encompassing both the indoor winter and outdoor summer grazing periods. System HF involved a high feed value silage, a lax grazing regime, and a low concentrate input (842 kg DM), while system HC involved a medium feed value silage, a tighter grazing regime and a higher concentrate input (2456 kg DM). Total milk outputs with each of systems HF and HC were 7854 and 8640 kg respectively (P<0.01), illustrating that high genetic merit cows can perform satisfactorily on very different inputs over a single lactation. However animals on system HF experienced a more extreme and prolonged period of negative energy balance post partum than those on system HC, and completed the winter with a significantly lower condition score. Detailed fertility records were maintained for all animals on the study. Days to first observed heat were 51.2 and 59.3 with systems HF and HC respectively, while the respective conception rates to first service were 26 and 21%. The number of services/conception were 2.22 and 2.50, while the calving interval was 390 and 404 days for systems HF and HC respectively. Despite the greater degree of negative energy balance associated with system HF, none of the fertility measures was significantly affected by system of milk production (P>0.05), although fertility with both systems was poor. There were no obvious reasons for the poor fertility noted in this trial.