مشخصات پژوهش

This experiment was conducted to determine the association between several risk factors which may affect culling of the Iranian Holstein cows. The data consisted of 329,772 lactation records collected by the Animal Breeding Center of Iran (2003 to 2012).The logistic regression was applied to investigate the associations between the response variable, the risk of culling, and predictor variables including the calving season, parity, birth type, calving ease, standardized 305-d milk yield, age at the first calving, region and the number of calving cows in the herd. The risk of culling increased (P< 0.001) with parity and twinning. The odds ratio (OR) of a cow leaving the herd were 1.32, 1.55, 1.62, 1.67 and 1.88 for parities 2 to 6, respectively, and 1.09 for twinning. The OR increased with increasing in calving difficulty score except for the farmer- assisted cows. The OR were 1.12, 1.36 and 1.23 for herds with 26 to 53, 54 to 125 and ≥126 calving cows, and the risk of culling was increased with increases in the number of calving cows in the herd. Calving in hot season increased the risk of culling and cows calved in cool seasons had lower risk of culling. The OR were 0.87, 0.79 and .077 for summer, fall and winter respectively. Cows in the warm and dry and warm and semi-arid regions had a higher (P< 0.001) risk of culling compared with the cows in temperate or cold and wet regions. Risk of culling (P< 0.001) decreased with increases in the standardized 305d milk yield. Cows that calved first at about 24 months of age had a lower risk of culling; higher ages at first calving increased the risk of culling. The findings showed that the studied risk factors significantly affected the culling rate in Iranian Holstein cows, and can be used as management tools by dairy farmers.