Risk factors for retained placenta (RP), as a common and economically important disorder, have been widely investigated. However, comprehensive studies are rare. Therefore, this study sought to identify herd-cow factors and their interactions on the risk of RP in Iranian dairy herds. The data were collected from nine Holstein dairy herds in Isfahan province, Iran. The final dataset included 154,048 records for 59,610 cows which calved between March 2011 and December 2018. A logistic regression model was used to separately analyze the risk factors and their interactions for RP in primiparous and multiparous cows. The average rate of RP was 12.3% (9.0-15.4%) at the herd level. The highest rate of RP was recorded in spring compared with other seasons. Regardless of calving season, occurrence of dystocia, stillbirth and twinning increased the odds ratio (OR) of RP. In primiparous cows, occurrence of dystocia and stillbirth increased OR of RP 4.30 and 3.33 times, respectively. In multiparous cows, dystocia, twinning and stillbirth increased OR of RP 4.36, 3.94 and 1.29 times, respectively. Cows with an age at first calving of >28 months had the highest rate of RP compared with other cows. Multiparous and primiparous cows with a short (less than 271 d) and long (more than 281 d) pregnancy, respectively, had the highest risk of RP compared to cows in other groups. Furthermore, multiparous cows with an extended dry period length (more than 75 d) and produced milk yield more than 13,000 kg/lactation were at higher risk of RP with an average of 13.5%. With an interaction analysis, although the potential effect of each factors depends on the effect of other factors, but in general dystocia, stillbirth, an extended age at first calving (>28 months), and calving during the spring increased risk of RP for both groups. In conclusion, identification of risk factors for RP with an interaction analysis can help farm managers to employ the best strategies to reduce the occurrence of this reproductive disorder.

中文翻译：

---

农场和奶牛因素及其对荷斯坦奶牛胎盘残留发生率的影响

胎盘滞留 (RP) 是一种常见且具有重要经济意义的疾病，其风险因素已得到广泛研究。然而，全面的研究很少见。因此，本研究试图确定畜群因素及其对伊朗奶牛群 RP 风险的相互作用。数据来自伊朗伊斯法罕省的九个荷斯坦奶牛群。最终数据集包括 2011 年 3 月至 2018 年 12 月产犊的 59,610 头奶牛的 154,048 条记录。使用逻辑回归模型分别分析初产和经产奶牛 RP 的风险因素及其相互作用。在畜群层面，RP 的平均率为 12.3% (9.0-15.4%)。与其他季节相比，春季记录的 RP 率最高。不论产犊季节，难产的发生，死胎和孪生增加了 RP 的优势比 (OR)。在初产奶牛中，难产和死产的发生率分别使 RP 的 OR 增加了 4.30 倍和 3.33 倍。在经产奶牛中，难产、双胞胎和死胎分别使 RP 的 OR 增加了 4.36、3.94 和 1.29 倍。与其他奶牛相比，首次产犊年龄 >28 个月的奶牛的 RP 率最高。与其他组的奶牛相比，分别妊娠短（少于 271 天）和长（超过 281 天）的经产和初产奶牛患 RP 的风险最高。此外，干奶期延长（超过 75 天）且产奶量超过 13,000 公斤/泌乳的经产奶牛患 RP 的风险较高，平均为 13.5%。通过交互分析，虽然每个因素的潜在影响取决于其他因素的影响，但一般而言，难产、死产、首次产犊年龄延长（>28 个月）和春季产犊都会增加两组的 RP 风险。总之，通过交互分析确定 RP 的风险因素可以帮助农场管理者采用最佳策略来减少这种生殖障碍的发生。