A cross-sectional study of factors associated with birth weights of Norwegian beef calves.

A cross-sectional study was performed to evaluate factors which influence birth weights of beef suckler calves in Norway. Data were from a national beef cattle registry, and lifetime production data of cows slaughtered between January 2010 and January 2013 were included in the study population. The study population consisted of 20,541 cows and 53,819 calves. The analysis was performed on the subset of singleton calvings from which birth weights were recorded. The study sample consisted of 9903 cows with birth weights available for 29,294 calves. The mean birth weight was 43.47kg (95% CI 43.40; 43.53). Two multilevel linear regression models were built; the first was for all calves and included parity of dam as one of the explanatory variables (with herd and cow as random effects), the second model was for calves born to primiparous dams only where age of first calving was included as an explanatory variable (with a random herd effect). The multilevel regression models estimated that female calves were 2.3kg lighter than males (95% CI 2.2-2.4, P<0.001), that calves of Norwegian Red, Charolais, Aberdeen Angus and "Other" born in the western part of Norway were lighter than from all other regions, and that calving in the autumn yielded lighter offspring than calving other parts of the year. Furthermore, calves born from primiparous cows were heavier than calves from older cows. Herd explained a large proportion of the variation in birth weights (40% and 37%, in the full and heifer models, respectively), and both the herd and cow random effects were highly significant. In conclusion, birth weights of beef calves in the Norwegian Beef Cattle Recording System were influenced by sex of the calf, breed of the dam, parity, age at first calving, calving season, cow, herd and region.