Abstract
The objectives of this study were (1) to evaluate the level of inbreeding by estimating inbreeding coefficients and (2) to estimate the effect of inbreeding on lactational milk traits in Polish Simmental cows. Two reference populations were defined for pedigree analysis: animals born between 1984 and 2009 (dataset A, n=20,827) and animals born between 2010 and 2020 (dataset B, n=19,554). Mean inbreeding coefficients for the analysed cows were generally low in both reference populations, however, approximately 72% of the inbred cows were born in the second studied decade. Notably, an increased number of ancestors did not result in an increase in genetic diversity. 7 founders exhibited the highest gene contribution. The principal ancestors with the highest gene contribution were 11 sires in dataset A and 15 animals in dataset B. The coefficients of regression on inbreeding were low and negative for milk yield and fat or protein content for all three lactations, whereas positive effects were observed for fat or protein percentage. Overall, the genetic variability of Polish Simmental cows is at a satisfactory level, nevertheless in young cows an increase in the number of inbred individuals is observed. Analysis of ancestral gene contributions indicates significant changes in the composition of ancestors in this younger population.