<b>The aim of the study was to use evolutionary operations (EVOP) to evaluate changes in the behaviour of dairy cows depending on the strategy according to which they were grouped after calving. The experiment was carried out on a Danish farm keeping about 200 Holstein cows. The farm had two separate areas for lactating cows: a small area with 54 waterbeds and a larger area with 133 beds with mattresses. During the experiment both areas were filled with cows. Cows after calving were introduced to either the small area (group) or the large area (group). After 14 days, the cows from the small group were transferred to the large group. The impact of different cow grouping strategies on their lying time, considered a welfare indicator, was examined. During the 210-day experiment, 195,703 observations were collected. The behavioural data were analysed using a linear mixed-effect regression model fitted by the maximum likelihood method. Days in milk, parity number, and lying time of herd mates were found to significantly affect the lying time of individual cows, while the strategy of cow grouping after calving had no effect on lying time.</b>