Unveiling the factors associated with ewe death, casting in an extensively farmed sheep flock

Introduction
Ewe deaths affect the productivity and profitability in sheep farming systems and have issues in terms of potential animal welfare implications and consumer perceptions. Internationally, there is scant data on the timing and causes of ewe deaths in extensive grazing systems. Reported ewe mortality rates are in the range of 2.8–27%, and few studies have described relationship between productive parameters and ewe mortality in outdoor grazing systems. There is no published literature on the incidence and risk factors associated with casting (ewe in late gestation accidentally immobilised, often in dorsal recumbency). This study reports some factors associated with production parameters (live weight, body condition score, and litter size) on both ewe mortality, which becomes cast through an almost two-year period, along with causes of death during peripartum periods.

Material and methods
This study utilised a cohort of 1789 ewes on a New Zealand farm, the ewes were 16 to 19 months of age. Ewes were weighed and body condition score assessed at eleven key management times. Additionally, ewes were monitored for a period of time over lambing (20–24 days). During these observations, dead or compromised ewes were identified. The compromised ewes that required intervention were restrained and assessed based on the severity of presenting conditions and either treated if possible or euthanized on welfare grounds. Scoring systems were developed to record interventions for compromised ewes.

Results and discussion
Ewe deaths occurred throughout two years of the study but were most frequent during the peripartum (pre-lambing to mid-lactation) period. Over the study period, in the absence of intervention, it was assumed that 222 of the 1789 ewes would have died (12.4%). However, due to intervention during the lambing monitoring period, 180 ewes died (10.1%).   Casting was the most commonly identified cause of death, responsible for approximately a quarter to a third of potential annual mortality. This study identified other causes including vaginal prolapse and dystocia. Few risk factors for death or casting were identified. The factor associated with increased odds of becoming cast was litter size, such that triplet-bearing ewes were more likely to become cast than singlebearing ewes (OR 13.53, 95% CI 1.50–122.36; p = 0.03).

Conclusion and implications
The peripartum period is a high-risk time period for ewe deaths (and, by extension, will also contribute to lamb perinatal mortality). In extensively grazed flocks where casting events occur, it is recommended that all ewes are monitored daily during the peripartum period. It would be remarkable for farmers to routinely check ewes during lambing to proactively reduce mortality rates.