The influence of different feeding times and frequencies on the circadian rhythm of serum cortisol concentration in horses (Equus caballus)

With the exception of dogs, all mammals studied so far (including the horse) have shown a circadian rhythm in blood glucocorticoid concentration. The rhythm is tightly regulated and may be influenced by light, feeding, and sleep. Light is the strongest candidate for being the entrainer of the rhythm, although, meals and paradoxical sleep (PS) have been associated with increases in glucocorticoid concentration. Of most interest in this study is the meal associated rises in glucocorticoid concentration. This is because glucocorticoids are associated with glucose metabolism, and it is this association that may offer a functional explanation for the circadian rhythm in glucocorticoids. The study was carried out on horses because of the large differences in feeding patterns between feral horses (continuously grazing throughout the day) and stabled domestic horses (one or two large high energy meals per day). The purpose of the study was to investigate whether feeding large meals of high DE (Digestible Energy) concentration once a day could be detrimental for horses. Nine horses were fed over three periods either at 0900 hours, 2100 hours or hourly, for eight days in a cross over design so all horses were on each treatment once. Behaviour sampling occurred on day six and hourly blood samples were collected on day eight. Behaviour sampling consisted of monitoring sleep-wake patterns. Blood samples were analysed for serum cortisol concentration, and serum glucose concentration. Horses showed a circadian rhythm in cortisol (p0.05). Sleep cycles were also not affected by feeding times or frequencies (p>0.05). Glucose concentration profiles differed markedly between treatments (p