Poor reproductive performance is a major problem on dairy farms throughout the United Kingdom, resulting in reduced financial returns for the farmer. As the genetic merit of the cow increases, an increasing proportion of nutrients are partitioned towards milk production and this can result in the cow entering into a more severe negative energy balance (NEB). Previous studies have shown that the extent of NEB may influence the expression of oestrous behaviour (Orihuela, 2000). The aim of the current study was to examine the effect of offering a high or low proportion of concentrate (70% and 40% respectively, with the balance being grass silage) in the total diet, on the expression of oestrous behaviour of high genetic merit Holstein Friesian lactating dairy cows, using optical electronic oestrous detection equipment.