The objective of this paper is to describe the random variation in voluntary dry matter intake (VDMI) and to discuss the application of the results for monitoring purposes. Furthermore, the objective is to review and quantify the influence of day length or photoperiod on VDMI. VDMI was recorded in fifteen bulls, steers and heifers, respectively, fed a complete diet ad libitum. The diet was based on chopped straw formulated to secure physical regulation of feed intake. Total random variation of recorded VDMI corresponded to a coefficient of variation (CV) of 24.6%. Two thirds of this variation was due to random day to day variation in VDMI. Random variation between weeks, random variation between animals and residual random variation corresponded to a CV of 3.6, 8.0 and 10.0%, respectively, and they explained 2.5, 12.2 and 18.8% of the total random variation. In the present experiment VDMI was increased by 0.32% per hour increase in day length. This is in agreement with the increase found in reviewed literature when photoperiod was manipulated artificially. Practical application of the results for monitoring purposes are exemplified and discussed.