Estimated breeding values for Lifetime Performance Index (LPI), production traits, mammary system (MS), somatic cell score (SCS) and herd life (HL) from Canadian Ayrshire bulls and cows born from 1950 and 1960 respectively, were used to estimate and study realized genetic selection differentials (GSDs). The objectives of the study were to define and determine the progress that has been achieved (realized GSDs) along the sire of bull (SB), dam of bull (DB), sire of cow (SC) and dam of cow (DC) pathways of genetic improvement and the variabilities among herds in realized GSD. The mean annual realized GSDs of LPI between 1980 and 2015 were 101.7, 322.9, 83.0 and −7.2 points for SB, DB, SC and DC respectively. There were, however, little to no selection for LPI in the DC path. The mean realized GSDs per year of conception of offspring (YOC) for the DB were higher than the other pathways and increasing for LPI, 305-d milk yield (MY), 305-d fat yield (FY) and 305-d protein yield (PY). For mammary system, there were increasing realized GSD in the DB and SC paths, but an irregular pattern of selection in the SB path. There were no clear trends in realized GSDs of the SB, DB and SC paths for HL between 1980 and 2015 while favorable and decreasing trends were observed for SCS in the 3 most influential paths of selection. Year of conception significantly influenced realized GSD of AI bulls, but not replacement cows in the Ayrshire population. Housing system (HS), agricultural region and the interaction between them and with YOC were largely not important in realized GSDs of AI sires and replacement cows. Variations in realized GSD due to herd and herd × YOC were significant and contributed between 0.6 and 5.5% and 4.5 to 30.2% respectively of the total variation in realized GSD for sires of bulls. There were also significant variations due to herd and herd × YOC for the DC ranging between 6.3 and 13.8% and between 0 and 1.1% respectively. This study demonstrates considerable variations among Ayrshire cattle herds in their selection of economically important traits.