Changes over time in the genetic parameters of milk production and functional traits in a Holstein population

Changes over time in the genetic parameters for milk yield (MY), fat yield (FY), protein yield (PY), fat content (FC), protein content (PC), longevity (LONG), somatic cell score (SCS), calving interval (CI) and age at first calving (AFC) were estimated with information from records of 335,155 pedigreed Chilean Holstein cows with first calvings from 1998 to 2017, using multi-trait linear mixed models within four 5-year periods. Additive genetic variances for MY, FY and PY decreased by 45, 33 and 34% between the first and last periods, increased by 21% for FC and remained constant for PC. Environmental variances for MY, FY, PY, FC and PC increased by 14, 40, 34, 74 and 20%. Heritabilities for MY, FY, PY and FC declined by 43, 44, 44 and 16%. In functional traits, genetic variances and heritabilities remained unchanged, while a decrease over time was observed in environmental variances for SCS, CI and AFC. Genetic and environmental correlations between MY, FY and PY decreased over time, while environmental correlations of FY and LONG with SCS and LONG with CI increased. These findings underscore the importance of changes in genetic and environmental parameters over time for important traits in selected populations of dairy cattle.