Short communication: Replacement heifer mortality from weaning until second mating in seasonal-calving, pasture-based dairy herds in New Zealand

The primary aim of this prospective study was to determine the postnatal mortality risk of replacement dairy heifers from weaning until the start of their second mating period (∼27 mo of age) in seasonal-calving, pasture-based dairy herds. Data were analyzed from 24 farms from the Waikato (n = 15) and Canterbury (n = 9) regions of New Zealand. All animals included in the study had an identified weaning date. From this point onward, data on animals that were euthanized, died unassisted, were culled, or were sold were recorded by the farmer on the home farm or by the grazier, according to animal location, and validated using calving, mating, culling, and sold records in their herd improvement database (MINDApro LIC, Newstead, Hamilton, New Zealand). The mortality risk from weaning to the start of the second mating period was calculated by dividing the number of deaths by the total number of enrolled animals at weaning. Mortality rate was calculated by dividing the number of deaths over the study period by the total days at risk, and reported as the mortality rate per 100 cow years; this measure was also calculated as the mortality rate per farm. A total of 3,770 animals from 24 farms had data from weaning until the farm planned start of mating when animals were ∼27 mo old. The animal-level mortality incidence risk from weaning (∼13 wk of age) to the start of their second mating (∼27 mo old) was 2.7% (95% confidence interval: 2.2 to 3.3%; 102 deaths/3,770 animals). The median farm-level mortality incidence risk was 3.0%, with a range across farms from 0 to 7.9%. There was a total of 102 deaths over 2,429,362 cow days at risk, with the mean time at risk for the animals of 646 d. The animal-level mortality was 1.53 deaths (95% confidence interval 1.26 to 1.86) per 100 cow years from weaning to second mating start date. The range in farm-level mortality rate was 0 to 4.52 deaths per 100 cow years. The hazard of death did not change throughout the study period. The results from this study are difficult to compare with international studies due to differences in study timing and duration, reporting method (mortality risk vs. mortality rate), and concerns with data validation. However, to the best of our knowledge, the results from this study indicate that postweaning, postnatal mortality under the New Zealand seasonal-calving, pasture-based system is lower than most other reported studies.