A genetic analysis of the causes of lamb mortality determined by an on-farm postmortem procedure

Lamb mortality is a major challenge in sheep production with significant implications for animal welfare and farm profitability. This study investigated the causes of lamb mortality within the first three days of life in a lowland outdoor lambing flock in the Scottish Borders, UK, over three lambing seasons (2021-2023). Dead lambs were collected from the pastures during checking rounds three times a day, and simple on-farm postmortems were conducted once a day. A total of 468 lamb postmortem examinations were conducted to classify the most likely cause of death as dystocia, starvation/mismothering/exposure (SME), stillbirth, other, or unknown causes. Overall mortality to three days of age was 12 %, with SME (28 %) and stillbirth (27 %) the most common causes, followed by dystocia (13 %). Single and triplet born lambs (p < 0.01) were most likely to die from dystocia compared to other litter sizes and the risk of death from SME increased with litter size (p < 0.01). Direct lamb heritability estimates for death by dystocia and stillbirth were moderate (0.31 and 0.27, respectively), indicating potential for improvement via genetic selection. However, heritability for SME was not significant, highlighting the increased influence of environmental factors for this cause of lamb death. The findings demonstrate that the use of simple postmortems could be used to increase the accuracy of selection for lamb survival in breeding programmes, through the integration of breeding values for specific causes of death. This would be most applicable in well-recorded nucleus flocks that are well connected to the rest of the breeding programme.