Genetic and environmental parameters were estimated using a random regression model and applying the average-information REML approach. 
REML
Age-dependent genetic and environmental variance of semen quality in Nordic Holstein bulls.The aim of this study was to estimate genetic and environmental parameters, across bull's age, for semen quality traits including pre- and postcryopreservation semen concentration, sperm motility, and sperm viability as well as ejaculate volume and number of doses per ejaculate.  A data set on 96,595 ejaculates from 2,831 Nordic Holstein bulls collected between 2006 and 2019 was used.  Genetic and environmental parameters were estimated using a random regression model and applying the average-information REML approach.  Spline functions were chosen to fit the additive genetic and permanent environmental effects across bull's age, and the optimal number of knots was chosen using cross validation.  Residual variance heterogeneity was assumed in different bull age classes.  The estimated repeatabilities of semen quality traits ranged from 0.16 to 0.85 across different ages of bulls.  The estimated heritabilities of semen quality traits ranged from 0.02 to 0.56 across different ages of bulls.  The results indicate possibilities for genetic improvement of semen quality traits through selective breeding.
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