The aim of this study was to determine the effect of the origin of service sires on calving ease in dairy cattle herds. The experiment involved a total of 1257 Polish Holstein-Friesian Black-and-White cows. Variability of traits associated with calving ease was estimated on the basis of direct observation. Calving heifers and cows after the second and third pregnancy lasting at least 260 days were taken into consideration. Twin pregnancies were not taken into account, because by nature they are associated with a high proportion of difficult calvings. The experimental factors were as follows: calving ease (1 - birth independently without human assistance; 2 - birth requiring a little help from one person; 3 - severe birth, requiring the use of much strength and the help of several people; 4 - very heavy labor needing the assistance of a veterinarian, including a caesarean section), the age of the cows (1 - first calving, 2 - second calving, 3 - third calving), the country of origin of 100 sires (France, USA, Poland, Germany) and calf birth weight. Overall, 15.67% of the calvings were considered hard pulls and required the assistance of a veterinarian (10.26% and 5.41% respectively). The number of difficult births decreased in successive calvings. Calving difficulty increased with calf birth weight. Calving ease was significantly affected by the sire. The highest percentage of hard pull and veterinary-assisted calvings (24.79%) was noted in the group of calves sired by a German bull, while the greatest calving ease was reported for American service sires. The high variation in the results of a phenotypic evaluation of bulls with respect to calving ease indicates that this trait should be considered in dairy herd improvement programs, and suggests that the country of origin of sires is less important than their actual test scores.
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