Investigating the occurrence and causes of ewe mortality during the peri-parturient period for commercial non-Merino flocks: A thesis submitted for the degree of Doctor of Veterinary Studies

Sheep meat is an important commodity to Australia, being the world’s largest exporter of sheep meat, and the world’s second largest producer of lamb and mutton. Australian sheep farming is extensive in nature whereby sheep are grazed in often large paddocks and rarely, if ever housed for any length of time. Under these conditions, the estimated annual ewe mortality in Australia is between 2 and 10%. Overseas, the highest risk period for ewe mortality has been determined as the peri-parturient period but limited data is available in Australia to support this. Furthermore, the factors that cause ewe mortality are not routinely determined in sheep production systems due to labor and time constraints. Understanding the prevalence of peri-parturient ewe mortality and reducing it should be a high priority for the Australian sheep industry, both in terms of animal welfare and improving profitability. This project estimates the prevalence of ewe mortality during the peri-parturient period and identifies the causes and risk factors associated with ewe mortality during this period in non-Merino ewes in Southern Australia.
To achieve the aims of this project, an observational, cross-sectional study was conducted involving 50 non-Merino commercial farms across southern Australia over a 2-year period (two lambing seasons). The study population of this research project was the breeding flock of non-Merino ewes on each farm. Ewes were monitored by farmers over the peri-parturient period, from the time ewes were first placed in their lambing paddocks, up until lamb marking. During this period, all ewe deaths were recorded by farmers in a project designed farm diary; including the suspected cause of death, if it was obvious to the farmer. A range of other descriptors were recorded in the farm diary for each lambing paddock, including ewe age, litter size, ewe breed, sire of lamb breed, feed on offer (FOO) at the start and end of lambing and the number of ewes in the paddock at the start of recording and at lamb marking. Concurrently, the project veterinary team conducted post mortem examinations on a sample of ewes that died during the study period. Veterinarians attended each farm at three time points and conducted post mortem examinations on all ewes that died in the preceding 48 hours. Using data collected during the study period, the prevalence of non-Merino ewe mortality was estimated and the association of mortality with risk factors investigated. In addition, post mortem findings provided insights into the major causes of peri-parturient ewe mortality.
Firstly, this thesis provides benchmarking data for peri-parturient ewe mortality in Southern Australia and identifies dystocia, septicaemia, trauma, and dorsal vaginal wall rupture (DVWR) as significant contributors to ewe mortality during this period.
Secondly, risk factors associated with various causes of peri-parturient ewe death have been determined using Bayesian Network predictive models, confirming previous findings while also revealing new risk factors. Foetal malpresentation was found to be the primary cause of dystocia- related ewe mortality.
Based on the results from this work, three key recommendations have been made:
i. Separate management of older ewes and those with multiple foetuses is recommended to reduce overall mortality rates.
ii. Maintaining ewes at a normal body condition score between 2.5 and 3.5 is protective against multiple diseases and significantly reduces mortality risk.
Additionally, femoral cortical bone thickness on PM examination may serve as a useful ancillary test to diagnose hypocalcaemia, and further research on this is warranted.
The most significant finding from this work is that dystocia is a major cause of peri-parturient ewe mortality accounting for 36.0% of the veterinary diagnosed causes of death overall (Year 1, 41.2% and Year 2, 30.9%). As such, better preventative, diagnostic and treatment methods are needed to address this "silent" killer. Research into selective breeding for freedom from dystocia in both ewes and rams, including the ASBV LAMBPLAN Lambing ease measurement, would be valuable. Additionally, investigating the potential role of excessive progesterone in overweight ewes in causing hormonal imbalances during parturition and difficult births could add to the knowledge base surrounding dystocia.