Advances in bovine theriogenology in New Zealand. 1. Pregnancy, parturition and the postpartum period.
Unique systems of breeding and calving management have been developed to allow New Zealand cattle herd owners to maintain seasonally concentrated calving patterns. Improved pastures are used as the sole ration for most beef and dairy cows with limited conservation and no grain feeding. Patterns of calving are described in relation to herd Planned Start of Calving (PSC) dates, calculated as 282 days after the first date of insemination in the preceding seasonal artificial breeding (AB) programme. Corticosteroids have been systematically used in over 80% of New Zealand dairy herds to induce premature parturition (calving induction) during the third trimester of pregnancy for about 25 years, to condense calving patterns into periods of 6-12 weeks. Widespread use of calving induction has minimised the adverse effects of declining reproductive performance in dairy herds on the sustainability of seasonally concentrated calving. The use of calving induction is now discouraged on animal welfare grounds and due to possible consequences for the marketing of New Zealand dairy products. Recent declines in the reproductive performance of dairy herds are linked to increased prevalence of extended periods of postpartum anovulatory anoestrus (AA), which has become the major form of infertility in dairy herds in New Zealand and can be related to reliance on grazed pasture as the sole diet for cows of increasing genetic merit. The prevalence of AA is influenced by by parity, breed, recent genetic origin within a breed, body condition score, herd and year. management strategies such as once-daily milking, preferential grazing or feeding of silage, have not proven as successful as hormonal treatments. Although seasonal concentrated calving has remained a standard feature of the industry's low-cost, pasture-based system of management, continuing research will be required to economically maintain this pattern.
Macmillan
K L
KL
University of Melbourne, Department of Veterinary Science, 250 Princes Highway, Werribee 3030, Victoria, Australia. k.macmillan@unimelb.edu.au