Calving inductions on New Zealand's pasture-based dairy systems
New Zealand is known for its pasture-based dairy system where cattle are provided pasture year-around and given supplemental grain. A rigid timeline must be kept in order to get cows pregnant, dry them off, and have them calve during the appropriate seasonal window, all so the process can be repeated the following year with maximum success. A 365-day calving interval is maintained in order to maximize feed cost efficiency. The most challenging part of a seasonal system required all cows to be bred withing a very short time period, commonly 14-18 weeks. As cows become pregnant later in this window, allowing them to calve naturally prevents them from conceiving the following year during the same time period. A common management practice used to return these late conception cows to the normal timeline is early induction of calving. This practice involves injecting a long-acting ester of dexamethasone followed, most commonly, by a short acting dexamethasone injection. These injections mimic fetal cortisol and directly act on the placenta to increase conversion of progesterone to estrogen and initiate the changes that lead up to birth. Calves born from induced cows are usually stillborn or unable to breath unaided. If born alive and non-viable, the farmer must humanely euthanize these calves using a shot or blow to the head. The dams of these calves are also affected by the induction sequence and are less likely to get milk fever, more likely to have retained fetal membranes, and have a significantly decreased pregnancy rate than cows that complete gestation. In addition, they have a slightly lower milk yield over an entire lactation than non-induced cows. This paper wil discuss the meaning and importance of the balance date, planned start of mating (PSM), and planned start of calving (PSC), and will detail the early induction sequence, its physiological basis of action, and the outcome for induced calves. It will focus on management practices in the Waikato region of the North Island of New Zealand.
Senior seminar paperSeminar SF610.1 2007 M39
Cattle -- Reproduction; Cattle -- Parturition