First calving performance of beef cattle. 1. Effects of moderate and slow growth between weaning and joining at 15 months of age

In each of 4 years, Shorthorn and Dairy x Shorthorn heifers were fed for moderate (55 to 101 kg) or slow (1 7 to 50 kg) growth from weaning at 7 months until joining at 15 months of age. Thereafter they grazed together on improved pasture in a 430 mm rainfall Mediterranean environment. Their performance was compared until their second calving and their calves' performance until weaning. The heifers that had grown moderately prior to their first joining had 25 to 73 kg higher average liveweight and 3 to 7 mm higher ultrasonic backfat reading during first joining, a 9% higher calving rate, 14 kg higher average liveweight but a 10% lower dystokia rate at calving, and 12 kg higher average liveweight and 1 mm higher ultrasonic backfat reading at their second joining, than those that had grown slowly. Survival of dams and calves, date of first calving, calf birth weight, second calving rate, intercalving interval and date of second calving were not significantly affected by the rate of growth prior to first joining. The effect upon dams' weight at weaning (moderate 11 kg higher than slow) was significant only at P< 0.1. Most aspects of performance differed widely amongst years, but significant year x growth interactions were only found for liveweight and condition of heifers at first joining, and for weaning weight of calf. The calves of moderately-grown heifers were of significantly lower mean weaning weight than those of the slowly-grown heifers, but only in those years when the mean mid-joining weight of the moderately grown heifers exceeded 315 kg. It is suggested that high fertility and satisfactory first calf growth are not incompatible provided that growth between the heifers' weaning and first joining is limited to that which results in a mean mid-joining liveweight not more than 20 kg higher than the target weight for 90% calving.