Dairy herd efficiency depends on an adequate replacement program, and one of the main management factors affecting the replacement cost is age at first calving (AFC). The primary objective of this study was to evaluate the impact of AFC on performance and economic results in commercial dairy herds in Argentina. A retrospective study was conducted to analyze data from a total of 26,614 first lactation cows from 290 argentinian dairy farms calved for the first time during the calendar year 2016. Milk yield for first lactation cows having at least 5 milk test days was used to analyze production (n=15,181). Records were stratified by AFC into 5 groups: 1) 18–21 mo (n=202), 2) 22-25 mo (n=5,569), 3) 26-29 mo (n=5,579), 4) 30-33 mo (n=2,389), 5) 34-37 mo (n=1,442). The overall median of AFC was 27 mo. Although milk yield at the beginning of the lactation was lower for lower AFC categories, the differences between categories 2, 3, and 4 disappeared by the fourth test day at approximately 113 days in milk. No effect of AFC category was detected for milk fat and protein content and for SCC. Considering AFC categories, cows calving for the first time at 22–25 mo of age needed the shortest period after calving to conceive. The lower income associated with the mild milk losses may be compensated by the cost of more than 80 extra d in the rearing period and the increment in days open estimated for the upper categories.

奶牛群的效率取决于适当的替代计划，影响替代成本的主要管理因素之一是初产犊年龄（AFC）。这项研究的主要目的是评估AFC对阿根廷商业奶牛场绩效和经济结果的影响。进行了一项回顾性研究，以分析来自2016日历年首次产犊的290个阿根廷奶牛场的总计26,614头第一头泌乳牛的数据。使用具有至少5个牛奶测试日的第一头泌乳牛的产奶量来分析产量（n＝ 15181）。AFC将记录分为5组：1）18-21 mo（n = 202），2）22-25 mo（n = 5,569），3）26-29 mo（n= 5,579），4）30-33 mo（n = 2,389），5）34-37 mo（n = 1,442）。AFC的整体中位数为27 mo。虽然对于较低的AFC类别，泌乳开始时的牛奶产量较低，但在牛奶中大约113天的第四个测试日，类别2、3和4之间的差异消失了。未检测到AFC类别对乳脂和蛋白质含量以及SCC的影响。考虑到AFC类别，在22-25个月大时第一次产犊的母牛在产犊后需要最短的时间。与温和的牛奶损失相关的较低收入，可以通过在饲养期间多花费80 d的费用以及较高类别的未加工天数增加来补偿。