A total of 2302 high-yielding cows from the active population used in Pomerania and Kujawy regions were analyzed for milk yield, fertility and culling levels. Lifetime performance of the cows (lifetime milk yield in kg, life span, length of productive life, milking days, milk yield per day of life (utilization), mean calving interval (CI), number of calvings and number of calves born alive) was estimated according to the length of the first calving interval (≤340, 341-370, 371-400, 401-430, 431-460, 461-490 and >490 days). It was found that excessive shortening of the first calving interval in high-yielding cows was an error resulting in significant losses in the form of reduced first lactations and lifetime milk yields. It is debatable whether the first calving period should be lengthened. This had a favorable effect on the yield of first calvers (r = 0.6xx), but had no statistically significant effect on lifetime milk yield (r = 0.01) or length of productive life (r = 0.01). The length of the first CI was negatively and significantly correlated with the number of calvings and the number of live born calves. The group of cows with the longest first CI included cows with the lowest fertility, indicating that their mean CI was very long. As the first CI increased even up to 490 days, some parameters of cows’ lifetime performance (such as lifetime milk yield, life span and length of productive life) tended to increase slightly, but lengthening the period of productive life was not directly proportional to the increase in the lifetime number of milking days, because this was accompanied by the lengthening of the dry periods.
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