1. One-hundred-and-ten British Friesian, British Friesian x Ayrshire or successive backcrosses to British Friesian heifer calves were allocated to three groups. Groups A and B were rapidly reared and fed a barley-beef diet which resulted in mean body-weight gains exceeding 1 kg/day (13 to 39 weeks) and group C was normally reared on summer grazing and hay plus concentrates in winter at a mean bodyweight gain never exceeding 0-74 kg/day. Animals in group A were first mated at an average age of 42-9 weeks (body weight, 302 kg). Groups B and C were mated later at average ages of 78-4 and 78-1 weeks (average body weights, 443 and 353 kg respectively). After the first calving all animals were fed and managed as a single group. 2. There were no significant differences between the proportion of heifers conceiving at first service in groups A (55-5%), B (66-7%) and C (72-4%). 3. There were no differences in the incidence of dystocia at first calving in heifers served by an Aberdeen Angus bull but 12 out of 19 heifers in group A served by a British Friesian bull had dystocia. 4. Average 305-day fat-corrected milk yields in the first four lactations in group A (18 animals) were 1959, 2918, 3545 and 3210 kg and in the first three lactations in group B were2450,3216and3310kgand in group C 3863,4694 and 4813 kg. Thus milk yield was significantly lower in all lactations for rapidly-reared animals irrespective of the age at breeding and was further significantly lowered in the first lactation of animals mated early. 5. There was a high incidence of laminitis and bloat in heifers reared on the barley-beef diet, but during lactation, there was a lower incidence of mastitis in the lower-yielding, rapidly-reared groups.
