Abstract
Background:
The seasonality in lamb supply challenges the development of sheep production. Increasing the duration of the breeding season, aimed at distributing births throughout the year, enables a constant supply of lambs. However, the birth season can influence their productive performance.
Aim:
The objective was to evaluate the effect of birth season on birth live weight (BW), daily live weight gain from birth to weaning (DWGBW), weaning live weight (WW), and daily live weight gain from weaning to slaughter age (DWGW-150), as well as the influence of ewe age and body condition score (BCS), sex of the lamb, and type of pregnancy (single or twin) on these productive indices.
Methods:
The study analyzed production data comprising ten breeding seasons (from 2015 to 2019), 643 matings, and 531 lambs, using binary logistic regression, ANOVA, Tukey’s and t tests.
Results:
Lambs born in summer exhibited lower productive performance, as evidenced by lower DWGBW (0.22 ± 0.08 kg), WW (18.88 ± 7.82 kg), and DWGW-150 (0.13 ± 0.07 kg). Ewes with prepartum BCS between 3 and 4 gave birth to heavier lambs. After birth, lambs from ewes with BCS between 3 and 4 had a higher DWGBW, while lambs from ewes with postpartum BCS of 1.5 and 2 had a lower WW. Ewes aged 2 to 8 years gave birth to heavier lambs, and ewes aged ≥8 years weaned lambs at a lower weight. After weaning, the weight gain was similar among all age groups. Male lambs had higher DWGBW and WW than female lambs. Lambs from twin pregnancies had lower BW (3.95 ± 1.27 kg), lower DWGBW (0.21 ± 0.08 kg), and lower WW (17.59 ± 8.18 kg). The average lamb mortality rate between lambing and weaning was 12.5%, ranging from 9.8 to 13.9%, with no significant variations between birth seasons. Lambs born in spring and summer needed more anthelmintic treatments than those born in autumn and winter.
Conclusion:
The production of lambs outside the traditional season is possible; however, the productive performance of lambs born in the summer is lower than that in the other seasons of the year.

Key words: Birth season, Finishing lambs, Live weight gain