Inbreeding depression leads to the reduction of the mean phenotypic value. There has been a steady increase in inbreeding (F) in the UK since the introduction of reproductive techniques (AI, MOET). There has been an increase in the percent Holstein (%H) in the UK population due to the influx of North American Holstein genes. Crossing these Holsteins to British Friesians can result in the favourable effect of heterosis (het), whereby crossbred progeny out-perform the mid-parent mean for that trait. Of the heterosis in the F1 population, a proportion is lost due to recombination (rec) between parental line genes and is a measure of the epistatic interaction of genes. The purpose of this study was to examine the impact of nonadditive genetic effects (F, het, rec, %H) on the estimation of dairy cow fertility breeding values in the UK.