Timing, risk factors, and causes of fetal and pre-weaning lamb mortality in lowland production systems involving a range of ewe genotypes

Reducing lamb mortality has production, economic and animal welfare benefits. The
timing and causes of death and associated risk factors were investigated in a study
conducted over two consecutive years (involving 1 103 and 1 038 ewes in 2017 and
2018, respectively) in three prolific (average litter size 1.91) indoor-lambing, lowland
flocks (in Ireland) that consisted of a range of genotypes, managed in grass-based
systems of production. Data were collected from all foetuses and lambs that died
(between ~120 days gestation and weaning at 14 weeks of age); 221 cases in 2017
and 241 cases in 2018. All cases were submitted to a Regional Veterinary Laboratory
for necropsy examination using standardized protocols that were developed in
advance of case submissions. The majority (60%) of lamb mortality occurred prior to
or within 24 h of birth: 46% at or prior to birth and 14% within the first 24 h. Infection
(32%) and dystocia (20%) accounted for over half of the mortality. Chlamydia abortus
was detected more often in lambs from 2-year-old ewes lambing for the first time than
in lambs from older ewes. Dystocia accounted for a statistically significant higher
proportion of deaths among purebred lambs born to Texel ewes (49.4%, 95%CI
(confidence interval) 36.0 – 62.9) compared to purebred lambs born to Belclare ewes
(12.8, 95%CI 2.2 – 23.5). More lambs failed to yield a diagnosis of the cause of death
when born to Belclare ewes (29.2%, 95%CI 17.8 – 40.6) than to Suffolk-X ewes (7.4%,
95%CI 0.1 – 14.8). About one third of lamb mortality cases were adjudged to be
preventable through more consideration of management factors during pregnancy,
parturition and early post-partum. The use of good hygiene practices at lambing time
and optimising lamb birth weight should reduce the level of pre-weaning lamb mortality
in indoor lambing flocks.