The effect of weaning age and post-weaning feeding regime on growth and fertility of pasture-based Holstein-Friesian and Jersey dairy heifers

Achieving weight-for-age targets while rearing heifers is important for future productivity, particularly in pasture-based systems of production where heifer growth trajectory is not linear. The present study investigated the effect of weaning age (eight or 12 weeks) and post-weaning feeding regime (high or low) on the growth and fertility performance of pasture-based Holstein-Friesian (n = 130) and Jersey (n = 57) dairy heifers over two years. Live-weight and linear body measurements of the heifers were monitored from birth until first breeding. Fertility performance measures, i.e., the achievement of puberty prior to first breeding, days from mating start date to conception, submission rate, pregnancy rate to the first service, 42-day pregnancy rate, pregnancy result, and age at first calving, were also observed. At three months of age, the calves weaned at 12 weeks of age were superior compared to those weaned at eight weeks of age in terms of live-weight and linear body measurements; however, this advantage had completely disappeared by nine months of age. There was an interaction between pre and post-weaning treatment for body length at three, six, and nine months, respectively. Live-weight, average daily gain, and linear body measurements were significantly different for Holstein-Friesian compared with Jersey heifers throughout the experiment. The heifers weaned at 12 weeks of age were more likely to exhibit pre-breeding estrus activity than their herd mates weaned at eight weeks. There were significant interactions between weaning age and post-weaning feeding regime for days from mating start date to conception, pregnancy to first service, and 42-day pregnancy rate, respectively, such that heifers on a consistent plane of nutrition throughout the rearing period, i.e., heifers weaned at eight weeks and offered a low post-weaning feeding regime, and the heifers weaned at 12 weeks and offered a high post-weaning feeding regime had improved fertility performance relative to their herd mates weaned at eight weeks and offered a high post-weaning feeding regime, and heifers weaned at 12 weeks and subsequently offered a low post-weaning feeding regime. These findings provide an indication of heifer growth on fertility performance.