On-Farm Cow Mortality in Swedish Dairy Herds

A high rate of on-farm cow mortality (i.e. unassisted death and euthanasia) is both a financial concern and an important animal welfare issue. The overall aim of this thesis was to evaluate the development of mortality in Swedish dairy herds and to identify characteristics associated with on-farm mortality at cow and herd levels. In paper I, two analyses were performed using data from the cattle database with the objective of identifying risk factors at the herd level: one multiple-year study of 6898 herds between 2002 and 2010; and one single-year study of 4252 herds in the year 2010. Paper II is based on information from a designed questionnaire sent in 2012 to herds with either high or low mortality rates to evaluate differences in herd characteristics. In paper III, data were retrieved from the cattle database to assess hazard rates for mortality at the cow level and included cows with a calving between 1 July 2008 and 30 June 2009 (209,236 lactations). In paper IV, a field study on destruction plants was performed to assess the relative proportion of unassisted death and euthanasia. Dairy cow cadavers were examined and a hole in the forehead (caused by a bullet or a captive bolt) was used as an indication of euthanasia. Telephone interviews were carried out with the farmer to verify the type of death and to obtain a short anamnesis. The results show that the cow mortality rate has gradually increased between 2002 and 2010, from 5.1 to 6.6 deaths/100 cow-years. At the herd level, a larger herd size, longer calving intervals and the Swedish Holstein breed were associated with greater mortality. Lower mortality was observed in herds with a higher average milk yield, during autumn-winter, and in organically managed herds. In the questionnaire, the same effects of breed and herd size were identified, but also having cows on exercise pasture (instead of production pasture) during summer was associated with high mortality herds. At the cow level, the highest mortality hazards were found for traumatic events and diseases. The mortality hazard was higher in early lactation and increased with parity. Of the 433 cows in the destruction plant study, 30% had died unassisted. A high herd average stillbirth rate increased the risk of unassisted death. In conclusion, the Swedish mortality rates were found to be high from an international perspective, and several risk factors at both the cow and the herd level were identified.