Özet:

The study involved 132 Polish Black–and–White Holstein–Friesian cows, kept in a public sector farm located in the Kujawsko–Pomorskie province. The yield of the analysed cows averaged 12,800 kg milk. The aim of the study was to determine the effect of lactation length in high–yielding cows on milk yield, milk composition and indicators of fertility. The results show that in high–yielding herds, cows often have lactations beyond the traditional 305 days. In the analysed herd, 89.4% lactations were extended.With extended lactations, milk production increased from 9,278 kg for a 305–day lactation to 16,497 kg for lactations >420 days. With extended lactations, milk yields during the extended lactation period and % yield in relation to 305–day lactation gradually increased, with statistically significant differences between the groups. The cows that completed their standard 305–day or shorter lactations were characterized by optimum indicators of fertility, which decreased with extended lactations. The longest lactations were found in third and fourth lactation cows, and the shortest in older cows beyond the fourth lactation. First calvers were characterized by poorer indicators of fertility compared to older cows.

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