The effect of growth rate on production and reproduction in replacement dairy heifers in seasonally calving, pasture-based systems.

There is continued pressure on agricultural systems to become more environmentally and economically efficient. Meanwhile, modern pasture-based dairy systems have resulted in the selection of a dairy cow type, with management requirements, production goals and possibly even growth targets distinct from those of a cow in a confinement system. A 365-day calving interval is key to maintaining the seasonal nature of calving and hence maximising milk production from grazed grass. Maximising milk production from grazed grass begins with the achievement of an appropriate age at first calving, in which pre-breeding growth is an integral factor. However, to date much of the research into replacement dairy heifer growth has been carried out in confinement systems. The objectives of this thesis were to investigate the effect of pre-breeding growth rate on production and reproduction in dairy heifers in seasonally calving, pasture-based systems. In the first study, I analysed the effect of growth rate between birth and mating start date (MSD) on days between MSD and date of conception (days open). Using survival analysis, I found that increasing pre-breeding average daily weight gain (ADG) was associated with a reduction in days open. The predicted median days open for a heifer with a pre-breeding ADG of 0.40, 0.70, or 0.90kg/day were 27, 16 and 11 days, respectively. In the second study, I investigated the effect of growth rate on first lactation milk yield. Linear regression analysis demonstrated a quadratic relationship between ADG from birth to breeding and first lactation milk yield. Increasing ADG had a positive effect up to a maximum beneficial effect of 0.88kg/day, and a minor negative effect thereafter. Combining the findings of these two studies, while recognising their limitations, it was concluded that an overall birth to breeding ADG of 0.88kg/day will minimise days open while maximising first lactation milk yield.