Circadian clocks and their integration with metabolic and reproductive systems: our current understanding and its application to the management of dairy cows.

The circadian system is an inbuilt timekeeping mechanism that tracks the 24-h day through the generation of circadian rhythms. Circadian rhythms enable animals to forecast and anticipate regular changes in their environment, and orchestrate biochemical, physiological and behavioral events so that the right process occurs at the right time. The 24 h rhythms generated by circadian clocks are integrated into homeostatic feedback loops and repair pathways. Metabolic and reproductive systems are highly integrated with the circadian timing system and demonstrate reciprocal regulation. Circadian clocks set the timing of circadian rhythms by gathering temporal information from external and internal signals to include light and nutrients. Exogenous and endogenous factors that function as inputs to the circadian clocks can disrupt their timing when applied at unusual and inappropriate times, and are referred to as chronodisruptors. Changes in the natural light-dark cycle perturb the circadian system. Other chronodisrupters include inappropriately timed food intake and physical activity and biological stress. Knowledge of the biology underlying circadian clock timing is critical to understanding how to maximize health and production efficiency of cattle. Here we review circadian clocks and their function in the regulation of metabolic and reproductive systems as well as the consequence of circadian disruption on mammary development and lactation with a particular focus on recent research findings from studies of dairy cows.