Abstract
BackgroundPerinatal mortality is a major cause of loss in the sheep industry. Our aim was to explore time trends in crude population stillbirth and neonatal mortality rates in Norway. We used data on 6,435,715 lambs from flocks enrolled in the Norwegian Sheep Recording System (NSRS) from 2000 through 2010 for descriptive analysis of trends. Longitudinal patterns of mortality rates were compared for lambs within different levels of variables suspected to be associated with perinatal loss.ResultsThere was an approximately linear increase in the annual proportion of stillborn lambs during the study period, from 3.3 % in 2000 to 4.7 % in 2010. In the same time period, average litter size of ewes in NSRS flocks increased from 2.00 to 2.19. However, a steady rise in stillbirth rate was observed within each litter size group, suggesting a gradually increasing impact on stillbirth risk of other, yet unidentified, factors. Average flock size increased during the study period. The highest stillbirth rates were found in the largest and smallest flocks. Early neonatal mortality rates (0–5 days of life) varied from year to year (minimum 2.2 %, maximum 3.2 %) and were invariably higher among triplets and quadruplets than among singletons and twins. Annual fluctuations were parallel within the various litter sizes. A significant overall decreasing trend was present within all litter sizes with the exception of singletons. Weather data for the prime lambing months (April and May) 2000–2010 indicated a relationship between low temperatures and high neonatal mortality rates. At the flock level, there was a significant positive correlation between stillbirths and early neonatal mortality rates (r = 0.13), between stillbirth rates in two consecutive years (r = 0.43) and between early neonatal mortality rates in two consecutive years (r = 0.40).ConclusionsThe substantial increase in ovine stillbirth rate in recent years in Norway was to some extent related to a corresponding increase in the proportion of lambs in triplet or larger litters; however, other factors apparently have contributed. Early neonatal mortality rate exhibited year-to-year variations, partly following temperature fluctuations, which is somewhat unexpected, considering that lambing mainly occurs indoors in Norway.
Highlights
Perinatal mortality is a major cause of loss in the sheep industry
Based on data from the Norwegian Sheep Recording System (NSRS), we examined crude perinatal mortality rates in Norway in the period from 2000 through 2010
We examined whether there was any relationship between stillbirth and neonatal mortality rates and we compared losses in two consecutive years
Summary
Perinatal mortality is a major cause of loss in the sheep industry. Our aim was to explore time trends in crude population stillbirth and neonatal mortality rates in Norway. Perinatal lamb mortality rates until 7 days of age recorded in studies from various countries range from 9 % to around 20 % [1,2,3,4]. These losses have a great impact on profitability in sheep production [5, 6] and are of concern from an animal welfare perspective [7, 8]. When combining mortality data with records describing various characteristics of flocks and individual animals, exploration of time trends and comparison of distributions of subgroups may reveal patterns leading to hypotheses about causal relationships. When mortality records are included, factors associated with lamb loss can be investigated, and studies identifying risk factors and providing heritability estimates for perinatal death based on such registries have been published [13, 14]
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