The aim of research was to state the daily milk yield of Polish Holsztyn-Friesian for cows before and after estrus for more precise determination of the effective insemination time. The daily milk yield of 125 cows in estrus cycle was analyzed in herd with efficiency of about 7000 kg of milk in 305-day lactation. The milkiness level and occurrence dates of 474 estrus cases were taken from documentation of that herd from January 2004 to December 2005. The milk yield was analyzed 3 days before estrus, on estrus day and 3 days after estrus. On estrus day it was observed that the milkiness decreased significantly of about 2.9 kg (11.9%) with relation to 3-day periods before and after estrus. The breed and genotype of cows, the next lactation and its stage and the daily yield levels had statistically significant influence on the milk yield decrease on estrus day. Differences in milkiness decrease on estrus day between old cows (IV lactation and higher) and younger (I, II, III lactation) and between cows with the lowest milk yield level (< 20 kg) and the highest (> 30 kg) turned out to be statistically significant. The cows milkiness reduction on estrus day (about 11.9%) can be the valuable information mak-ing it possible to determine estrus occurrence date.

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