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Material for the study was collected from the SYMLEK database and provided information on 148385 parturitions of Polish Black-and-White Holstein-Friesian cows representing the active population (part of the performance evaluation) in the Pomorze and Kujawy regions. The animals first calved in 2006 and were used or culled by the end of 2012. GLM and chi-square procedures of the SAS package were used for the statistical calculations. The number and sex of calves born had an effect (P≤0.01) on the parturition process and on the level of perinatal mortality. As the number of foetuses increased, the proportion of difficult parturitions increased from 4.3 to 28.6%. A decrease in calving ease was also related to the fact that the calf was of male sex. The risk of perinatal mortality increased after multiple pregnancy and when a bull calf was born (among single calves perinatal mortality was over 3 times as frequent in bull calves compared to heifers; among samesex twins perinatal mortality was around twice as frequent when two bull calves compared to two heifers were born; for births of opposite-sex twins, perinatal mortality was more frequent among bull calves). It was also found that perinatal mortality of calves increased with increasing level of calving difficulty. For very difficult parturitions, the risk of perinatal mortality among calves, regardless of sex, increased 10-fold in relation to unassisted parturition.

Effect of Single and Multiple Pregnancies and Calf Sex on Parturition Process and Perinatal Mortality