Data on insemination records of Holstein Friesian (HF) purebred (n=45,497) and crossbred (n=58,497) collected from the BAIF Research Foundation were utilized. The conception rate was modeled as a binary trait, using linear repeatability models. Random regression models (RRM) were used to obtain the trajectory of variance components across age of the bulls. Legendre Polynomials up to order of fit of 4 were used for the random effects of additive genetic and permanent environmental effects.

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... 0 Gibbs samples were generated with a burn-in of 20,000 and thinning interval of 50 using the THRGIBBS1F90 program. Heritability estimates were very low (0.1) in both breeds but peaked at the extreme ages. Heritability and repeatability estimates ranged between 0.038 (8 years) to 0.627 (10 years) and 0.060 to 0.809 respectively in purebreds. Narrower ranges of 0.010 (4 years) to 0.087 (11 years) and 0.532 to 0.832 for heritability and repeatability respectively, were obtained in crossbreds. Effect of permanent environment was low in purebreds compared to the crossbreds. The study using RRM was able to provide genetic parameter estimates of fertility for all ages throughout the productive lifespan of bulls.