Optimum growth rate of Belgian Blue double-muscled replacement heifers

________________________________________________________________________________ Abstract Belgian Blue double-muscled (BBDM) heifers (n = 341) were used to investigate the effect of body weight (BW) at first calving at an age of two years (BWC). Females with a birth weight (BWB) <40 kg realized a lower body weight gain (BWG) up to an age of four months and they also had a significant lower BWC. Dams with a higher BWC gave birth to calves with a higher birth weight. Milk yield during the first four months after calving linearly increased with a significantly higher BWC (R = 0.34). Females with a daily BWG from zero to four months of age of <0.6 kg have a lower BWC than females with a higher growth rate. A higher rate of gain from zero to four months did not affect subsequent heifer BWG. There was a tendency for a BW-loss during the last trimester of the gestation when BW at conception was lower, especially when this period coincided with the grazing season. This may be due to a reduced intake capacity of BBDM animals and a heavy BWB of the offspring. With mean adult BW of BBDM cows of ca. 710 kg, heifers should gain approximately 0.8 kg/d from birth to conception, to achieve 55 60% of their adult BW at the conception at 15 months of age. A lower BWC was only partly compensated from the first up to the third calving, but there was no clear effect on the interval from the first to the third calving. From the present study we can conclude that it is desirable to strive for a pre-partum BW of about 600 kg in BBDM heifers when their first calving occurs at an age of 24 months. Extra attention should be paid to the nutrition of BBDM heifers to realize an accelerated growth rate before they become pregnant. ________________________________________________________________________________