Seasonal variations in lamb's birth weight and mortality

The aim of the study was to determine weather lambs born in different seasons manifest difference in birth weight and mortality rate. All year-round mating of Merinolandschaf ewes was used based on natural occurrence of oestrus in different months of the year. The lambing seasons were grouped after birth month of lambs as follows: winter (December-February), spring (March-May), summer (June-August) and autumn (September-November). Data concerning identification number, birth date, type of birth and sex were collected on 401 lambs. Lamb mortality was recorded as number of lambs born dead and lambs born alive but died within first 48 hours after birth (perinatal mortality). All lambs were weighed, within 12 h after birth, using suspended scales weighing from 0 to 20 kg in minimal 200 g increments. Lambs born in summer had the highest mean birth weight (4.8 kg) which was significantly (P 0.05) differences were also observed for birth weight of male twins born in different season. Between all female groups (singles and twins), birth weight of lambs born in winter, spring, summer and autumn showed relatively small and not significant differences. Season had significant (P<0.01) influence on perinatal lamb mortality. Low relative values (1.0 % ; 2.0 %) were observed for lambs born in autumn and spring, while during winter and summer, 11.0% and 20.0% of lambs were born dead or died within 48 hours after birth. Within the same season, lambs that stayed alive were always heavier than lambs which were born dead or died after birth. Although the largest difference was observed for birth weights in spring (4.5 kg - alive vs. 3.5 kg - dead), statistical significant (P<0.05) differences were only found within winter (4.5 kg - alive vs. 4.1 kg - dead) and within summer (5.0 kg - alive vs. 4.2 kg - dead).