Hare E, Norman H D, Wright J R
Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350, USA.
J Dairy Sci. 2006 Jan;89(1):365-70. doi: 10.3168/jds.S0022-0302(06)72102-6.
Trends since 1980 for calving age and calving interval, 2 factors that influence herd life, were examined by parity for 5 breeds of US dairy cattle. Calving data were from cows with records that passed edits for USDA genetic evaluations and were in herds that remained on Dairy Herd Improvement test. First-calf heifers calved at progressively younger ages over time, but the age decline was less for later parities because of longer calving intervals. Breed differences for calving age were evident for all parities; current mean age at first calving ranged from 24 mo for Jerseys to 28 mo for Ayrshires. Mean calving age across all parities declined over time for all breeds, primarily because of increased turnover rate, and ranged from 48 mo for Holsteins to 54 mo for Ayrshires. Across parity, annual increase in calving interval was reasonably consistent (0.90 to 1.07 d/yr) for all breeds except Jersey (0.49 d/yr). Within parity, regressions of calving interval on year were generally similar to overall breed trend. Breed means for first calving interval across time ranged from 390 d for Jerseys to 407 d for Brown Swiss.
对美国5个奶牛品种按胎次研究了1980年以来影响牛群寿命的两个因素——产犊年龄和产犊间隔的变化趋势。产犊数据来自于其记录通过美国农业部基因评估编辑标准且所在牛群仍在奶牛群改良测试中的母牛。随着时间推移，头胎母牛的产犊年龄逐渐变小，但由于产犊间隔变长，后面胎次的年龄下降幅度较小。所有胎次的产犊年龄均存在品种差异；目前头胎产犊的平均年龄，泽西牛为24个月，艾尔夏牛为28个月。所有品种所有胎次的平均产犊年龄均随时间下降，主要是因为更替率增加，范围从荷斯坦牛的48个月到艾尔夏牛的54个月。在各胎次中，除泽西牛（每年0.49天）外，所有品种的产犊间隔年增加量基本一致（每年0.90至1.07天）。在同一胎次内，产犊间隔对年份的回归通常与品种总体趋势相似。不同时间的头胎产犊间隔品种均值范围从泽西牛的390天到瑞士褐牛的407天。
