Reproductive traits are of paramountimportance in an efficient lamb production system. In Croatia, values of reproductive traits of Romanov sheep are below the average expected for this breed, indicating the need for improvement of these traits to exploit the breed’s full reproductive potential. This study was conducted on 260 Romanov ewes located at a commercial Romanov sheep farm in Croatia. All ewes were kept under similar conditions, and reproductive management was based on accelerated lambing with continuous mating. Linear models with fixed effects were used to estimate the influence of year of birth/lambing, season of birth/lambing, ram, parity and litter type on litter size (LS), age at first lambing (AFL) and lambing interval (LI). Average LS was 2.11 ± 0.71, while the year of lambing and parity were the most important factors affecting this trait. The smallest litters were recorded after 1st parity (1.77 ± 0.06), and the largest after 5th parity (2.21 ± 0.08). AFL averaged 388.5 ± 72.4 days. All investigated non-genetic factors had a significant (P<0.05) influence on AFL, with year of birth as most important. The average LI was 241.2 ± 70.8 days. A significant (P<0.05) influence of all factors on LI was observed. Ewes lambing in spring (200.2 ± 7.9 days) or summer (190.5 ±6.6 days) had a significantly (P<0.05) shorter LI than ewes lambing in autumn (227.7 ± 8.3 days) or winter (237.2 ± 6.7 days). The longest LI was observed after the first parity (284.0 ±5.5 days). Average values for reproductive traits in the examined population of Romanov sheep were higher than those reported in the official Croatian database for breeding sheep. Non-genetic factors have a significant influence on the variation of reproductive traits of Romanov sheep, and therefore should be considered when assessing ewes’ reproductive performance.

The aim of this study was to analyse the non-genetic factors influencing the birth weight of Romanov lambs, and to determine which of them may be a useful source of information in the selection of female lambs for replacement. The study included 1712 lambs born in the period from 2016 to 2019. The linear model used in birth weight estimation included: birth type, sex, parity, year of birth, season of birth, and breeding group as fixed factors. Year of birth x season of birth interaction was also included in the model. All factors, except the season of birth, significantly affected birth weight (P<0.05). Birth weight decreased significantly (P<0.05) with increasing birth type and singles, twins, triplets and quadruplets weighing 4.11 ± 0.07, 3.56 ± 0.05, 3.07 ± 0.05 and 2.66 ± 0.08 kg, respectively. Male lambs (3.44 ± 0.05 kg) were significantly (P<0.05) heavier than female lambs (3.25 ± 0.05 kg). The lightest lambs (2.99 ± 0.05 kg) were born in the first litter, after which the birth weight gradually increased up to the sixth litter (3.54 ± 0.10 kg). Litter weight showed a nonlinear increase with an increase in birth type (P<0.05). The birth weight of female lambs from litters of twins and triplets with different ratios of male and female lambs did not differ significantly (P>0.05). Variations in birth weight are influenced by non-genetic factors, with birth type, sex, and parity being the most important. These factors should be considered in breeding and selection programs for lamb meat production, where fertility and growth traits have the key role.