Key factors affecting mortality of adult ewes in extensive Australian conditions: Applications for welfare assessment

In on-farm animal welfare assessment schemes mortality is a commonly used measure to assess welfare states, as well as to ascertain the effectiveness of different management strategies. While decreases in welfare and mortality are linked it is likely that monitoring and measuring the factors that influence mortality will be key in using this relationship to improve animal welfare. Therefore, this paper aimed to identify various factors that influence mortality in adult ewes. Once identified these factors may then be suitable for inclusion in an on-farm welfare assessment, or for a broader framework designed to manage risks to mortality. The data used in this analysis was collected on an Australian sheep flock from 2007 to 2010, and focused on identifying variables that increase the risk of mortality of ewes from weaning to a mature age (∼ 660 days). Given that neither mortality, nor the factors affecting it, were routinely recorded in this data set an individual ewe was considered dead when she was identified as missing at consecutive regular husbandry events. Analysis of the data indicated that sire (P < 0.005), location of birth (P < 0.001), year of birth (P < 0.005) and weaning weight (P < 0.001) are all factors that had an impact on mortality to 660 days. Additionally, mortality was increased by 1.8% in the 7 days following a fly strike event while light ewes (< 40 kg) or ewes with poor body condition score (< 2) were also at an increased risk of mortality in the 90 days following measurement. Given these results, risk factors for mortality that may be valuable for inclusion in an on-farm welfare assessment include weaning weight, body condition score and liveweight change over time and incidence of fly strike. Factors that have been identified as useful for a risk management framework include genetic and environmentally-influenced factors, such as individual sire and year of birth, respectively.