11A cross-sectional study was performed to evaluate factors which influence birth weights of 12 beef suckler calves in Norway. Data were from a national beef cattle registry, and lifetime 13 production data of cows slaughtered between January 2010 and January 2013 were included 14 in the study population. The study population consisted of 20,541 cows and 53,819 calves. 15The analysis was performed on the subset of singleton calvings from which birth weights 16 were recorded. The study sample consisted of 9,903 cows with birth weights available for 17 29,294 calves. The mean birth weight was 43.47 kg (95% CI 43.40; 43.53). Two multilevel 18 linear regression models were built; the first was for all calves and included parity of dam as 19 one of the explanatory variables (with herd and cow as random effects), the second model 20 was for calves born to primiparous dams only where age of first calving was included as an 21 explanatory variable (with a random herd effect). The multilevel regression models estimated 22 that female calves were 2.3 kg lighter than males (95% CI 2.2-2.4, P< 0.001), that calves of 23 Norwegian Red, Charolais, Aberdeen Angus and "Other" born in the western part of Norway 24were lighter than from all other regions, and that calving in the autumn yielded lighter 25 offspring than calving other parts of the year. Furthermore, calves born from primiparous 26 cows were heavier than calves from older cows. Herd explained a large proportion of the 27 variation in birth weights (40% and 37%, in the full and heifer models, respectively), and 28 both the herd and cow random effects were highly significant. In conclusion, birth weights of 29 beef calves in the Norwegian Beef Cattle Recording System were influenced by sex of the 30 calf, breed of the dam, parity, age at first calving, calving season, cow, herd and region. 31 32 -3 -