Early life reproductive investment affects longevity in ewes

To increase lifetime production of ewes, it has been suggested to mate ewe lambs. However, research on the effect of age at first lambing on longevity has shown conflicting results. This study investigated the impact of early life reproduction (i.e., the effect of age at first lambing and litter size) and environmental conditions on longevity in a prolific sheep breed traditionally bred as lambs. Lifetime performance data were obtained from 550 884 Norwegian White Sheep born between 2000 and 2013 spread across Norway. All ewes lambed their first litter either as 1-year-old or 2-year-olds. Longevity was investigated using a linear mixed model with age at first lambing, litter size, county, the covariates early- and late parturition, and two categorical variables describing cross-fostering and success in weaning as fixed effects. Herd x year was fitted as a random effect. The lifespan of the ewes was affected both by age of the ewe at first reproduction and the reproductive investment (i.e., litter size). The predicted lifespan for ewes mated as lambs was 1 548 days (4.2 years) whereas the lifespan for those mated as 2-year-olds was 1 700 days (4.7 years). There was a curvilinear relationship between lifespan and litter size. Lifespan increased from 1 272 days (3.5 years) to 1 618 days (4.4 years) when litter size increased from 0 to 1 lamb. Further increased reproductive investment resulted in a decreased lifespan with a litter size of 4 having the shortest lifespan of 1 468 days (4.0 years). The relation between litter size and lifespan was similar but at different levels for both age groups of ewes, except for ewes rearing less lambs than they gave birth to. In that group, ewes mated as lambs had a peak predicted lifespan at 1 504 days (4.1 years) with a litter size of one lamb, whereas those first mated as 2-year-olds had their peak predicted lifespan of 1 650 days (4.5 years) with a litter size of two lambs. The relationship between longevity and reproductive investment was in some cases affected by environmental conditions. However, the effect was not consistent with latitude. In conclusion, ewes mated as lambs, in general, live shorter lives compared to those first mated as 2-year-olds but environmental importance for longevity and reproduction needs further investigation.