Lambing outcomes with provision of parturient and neonatal assistance: 8 Evidence from a longitudinal study in Australian Merino ewes

Context Improving lamb and ewe survival during the lambing period is a priority for the Australian sheep industry. Aim To determine main causes, predictors, and intervention effects associated with neonatal lamb mortality in an extensive production system with supervised and assisted lambing. Methods A longitudinal study was conducted using the New England flock of the Merino Lifetime Productivity (MLP) project (n = 1350 ewes), whereby records of birth, neonatal mortality, and lamb necropsy examination were collected over a 5-year period with 5475 lambing events and 7798 lambs born. Where needed, ewes and lambs were provided assistance during parturition and/or in the neonatal period. Lamb mortalities were categorised as ‘actual’ and ‘theoretical’ deaths to determine ‘predicted’ mortalities. Predicted mortality represents an informed estimate of lamb mortality in the absence of assistance and intervention. Key results Actual lamb survival to marking was 88%, and predicted survival in the absence of interventions was 80.5%. Assistance at parturition was provided at 215 lambing events and was more likely required for triplet births (12.5%) than single or twins (4.2% and 3.2% respectively). Year, dam age, lamb sex, chill index at birth, and birthweight were all predictors of lamb survival (P < 0.05). Collectively, the predictor variables and random effects (parentage) explained approximately 50% of the variation in lamb mortality. Notwithstanding the provision of assistance, the major causes of lamb mortality were dystocia/birth difficulty (38%) and the starvation–mismothering–exposure (SME) complex (30%), with severe weather events and disease outbreaks affecting lamb survival in certain years. Conclusion Despite potential biases associated with the lambing system and associated protocols, the findings were consistent with other observations in extensive Australian production systems. Outcomes from lambing systems that incorporate intervention during lambing can, therefore, be generalised to commercial systems conditional of record keeping that captures interventions that constitute theoretical death and the inclusion of necropsy protocols to confirm proportional death causes. Implications This study reinforces the impacts of dystocia and SME on lambing outcomes, the need for targeted management for high-risk groups such as ewes carrying multiples, and the value of veterinary diagnostics in identifying infectious disease outbreaks.