Twenty-eight high genetic merit and 32 medium genetic merit Holstein/Friesian dairy cows with Predicted Transmitting Abilities for milk fat plus protein yield, calculated using 1995 as the base year (PTA95 fat plus protein) of 43·3 and 1·0 kg respectively, were used in a continuous design, 2 (cow genotypes)×4 (concentrate proportion in diet) factorial experiment consisting of eight treatments. Concentrate proportions in the diet were 0·37, 0·48, 0·59 and 0·70 of total dry matter (DM), while the forage component of the diet was grass silage. Diets were offered ad libitum in the form of a complete diet. Animals remained on these concentrate regimes for a mean of 84·7 days before completing a standard 98-day grazing period. At pasture, cows received either 5·0 or 6·0 kg concentrate daily according to turnout date. There were no significant genotype×nutrition interactions for any of the variables examined during either the indoor feeding or post-turnout grazing periods (P<0·05). Outputs of milk, fat, protein and fat + protein were greater for high merit cows than for medium merit animals (P<0·001). Milk output and milk protein output responses to increasing concentrate proportion were linear for both cow genotypes (P<0·001), while high merit animals showed a linear response in terms of milk fat plus protein output (P<0·01) with these responses being statistically parallel for both merits. High merit cows had a significantly higher DM intake than medium merit animals (P<0·01). With high merit animals, concentrate proportion had little effect on body tissue reserves, while medium merit animals showed a tendency for increased condition score and backfat thickness with increasing concentrate inclusions (P<0·05). In terms of the output of milk and milk constituents during the grazing period, there were no significant residual effects from winter concentrate feed rate (P>0·05), while high merit cows continued to have higher outputs of milk and milk constituents (P<0·001). Body tissue reserves of both genotypes changed little during the grazing period. It is likely that the higher milk yields of the high merit cows can be attributed both to their higher DM intakes and their ability to partition a greater proportion of the nutrients consumed into milk rather than body tissue reserves. Although statistically both high and medium merit cows responded in a similar manner to an increasing proportion of concentrate in the diet, high merit cows did exhibit a 49% greater fat plus protein yield response during the indoor period, compared to animals of medium genetic merit, perhaps hinting at the existence of a genotype×nutrition interaction.