A comparative study of grazing behaviour, herbage intake and milk production was conducted using three strains of Holstein-Friesian (HF) heifer : 33 high production North American (HP), 33 high durability North American (HD) and 33 New Zealand (NZ) animals. Heifers were assigned, within strain, to one of three grass-based feeding systems : (1) the Moorepark (control) system (MP), (2) a high concentrate system (HC), (3) a high stocking rate system (HS). Strain of HF had no significant effect on grazing time or number of grazing bouts. The NZ strain had longer grazing bouts (P < 0.01) and spent a lower proportion of time ruminating (P < 0.05) than both the HP and HD strains. There was a significant strain ✕ feeding system interaction for biting rate. The biting rate of the NZ strain was reduced in the HC system. Biting rates in the HS feeding system were significantly higher (P < 0.001) than in the MP system. Heifers on HC had shorter grazing time (P < 0.01) with grazing bouts of shorter duration (P < 0.01). Increasing stocking rate (HS) decreased the proportion of time ruminating (P < 0.001) and tended to shorten grazing bouts (P = 0.06). The HP strain had higher (P < 0.05) herbage and total dry matter (DM) intakes than the NZ strain, while the HD strain was intermediate. Concentrate supplementation reduced (P < 0.001) herbage DM intake but increased (P < 0.001) total DM intake. The reduction of herbage DM intake per kg of concentrate DM intake (substitution rate) was greater for the NZ than the HP strain. The HP produced significantly higher milk, fat, protein and lactose yields than the NZ, while the HD strain was intermediate. The milk fat content of the NZ was higher than both the HP and HD strains, while the protein content was higher than the HP strain. Concentrate supplementation (HC v . MP) significantly increased yields of milk and milk components. Milk production responses to the HC system were much greater with the HP than the NZ strain. Increasing stocking rate (MP v . HS) significantly decreased milk protein yield. The results indicate that the choice of strain of HF may depend on the feeding system.