Adulthood of a dairy cow is reached when she is about 60 months old, but cows first calve at a much younger age. Therefore, most heifers are still growing during their first lactation and part of the energy and protein intake will be assigned to this purpose. As the cow grows closer to mature body weight and is able to use more energy and protein for the milk production, milk production will increase from 1st to 2nd to 3rd lactation, but the amount of increase differs among cows. A gradual production increase is believed to be found in trouble-free cows (i.e. cows with good fertility and high disease resistance), and is a measure of the maturity rate (MR), where maturity is defined as reaching mature body weight. Genetic variance for this production increase exists (Krogmeier et al., 2003). The aim of this study was to define MR in a way that enables selection, and to estimate genetic correlations with functional traits.