In the present study, records on 115,291 heifers distributed in 113 herds were used to investigate the association between age at the first calving (AFC) and lactation performance, lactation curve, the length of the first calving interval (CI), calf birth weight (CBW), and the incidence of dystocia in Holstein heifers in Iran. Based on the AFC, the heifers were classified into eight classes: AFC of 541 to 690 d, 691 to 720 d, 721 to 750 d, 751 to 780 d, 781 to 810 d, 811 to 840 d, 841 to 900 d, and 901 to 1200 d (AFC1 to AFC8, respectively). Multiple regression mixed models were used to investigate the association between AFC and lactation curve parameters, partial and 305-d lactation performance, 100- and 305-d SCS, and the length of the first calving (CI) interval. The mean (SD) and median AFC across all heifers was 760.2 (74.01) and 750 d, respectively. Of 115,291 heifers included, 28,192 and 7,602 heifers were, respectively, ≤ 720 and > 900 d when calving for the first time. More than 44% of the heifers were at 691 to 750 d (23 to 25 months) of age when calving for the first time. An increased AFC was associated with increased partial and 305-d lactation performance, 100- and 305-d SCS, initial milk yield, milk production at the peak of lactation, upward and downward slopes of the lactation curve. The 305-d fat percentage was associated with AFC; however, there was no association between AFC and 305-d protein percentage. An increased AFC was also associated with decreased milk production persistency, delayed peak time, longer CI, and higher calf birth weight. Compared to heifers calving for the first time between 691 to 780 d (23 to 26 months) of age, both increasing and decreasing AFC were associated with increased risk of dystocia. Controlling AFC is an important management factor in achieving a lower risk of dystocia, higher lactation performance, lower SCS, and shorter length of the calving interval.

在本研究中，使用记录在113个牛群中的115,291头小母牛的记录来研究第一次产犊的年龄（AFC）与泌乳性能，泌乳曲线，第一次产犊间隔的长度（CI），小牛出生体重（CBW），以及伊朗霍尔斯坦小母牛难产的发生率。根据AFC，将小母牛分为八类：AFC 541至690 d，691至720 d，721至750 d，751至780 d，781至810 d，811至840 d，841至900 d，和901至1200 d（分别为AFC1至AFC8）。多元回归混合模型用于研究AFC和泌乳曲线参数，部分和305天泌乳性能，100和305天SCS以及第一次产犊（CI）间隔的长度之间的关联。所有小母牛的平均（SD）和中值AFC分别为760.2（74.01）和750 d。首次产犊时，在包括的115,291头小母牛中，分别有≤720 d和> 900 d的28,192头小母牛。第一次产犊时，超过44％的小母牛年龄在691至750 d（23至25个月）。AFC的增加与部分和305天泌乳性能，100天和305天SCS的增加，初始产奶量，泌乳高峰期的产奶量，泌乳曲线的上下坡度有关。305天的脂肪百分比与AFC相关；但是，AFC和305-d蛋白百分比之间没有关联。AFC增加还与牛奶产量持续性下降，高峰时间延迟，CI延长和小牛出生体重增加有关。与首次在691至780 d（23至26个月）之间产犊的小母牛相比，AFC的增加和减少都与难产的风险增加有关。控制AFC是降低难产风险，提高泌乳性能，降低SCS和缩短产犊间隔时间的重要管理因素。