24-hour endocrine profiles of Quarter Horses under resting conditions

The aim of this researchwas to evaluate the changes in b-endorphin, cortisol, total and free iodothyronines, and estradiol-17b levels of 10 unfit Quarter horses undergoing training under resting conditions during a 24-hour period. All animals that were sedentary since 3 months were divided intotwogroups comprisingfive animals each: group I, pre-trained for western riding (age: 8.8 6.7 years; two stallions, one gelding, and two mares) and group II, not pre-trained for western riding (age: 9.4 6.8 years; one stallion, three geldings, and one mare). Blood samples were collected from horses under resting conditions every 4 hours, from 5.30 AM to 5.30 AM of the next day. A two-way analysis of variance with repeated measures showed that the interaction between pre-training status and sampling timewas not significant for hormone changes, but tended to be significant for fT3 changes (P <.06). The effects of pre-training status were considered significant for fT3 (P < .02). Time had significant effects on b-endorphin (P <.01), cortisol (P <.02), fT3 (P <.0001), and estradiol- 17b (P < .0001) changes. Therefore, it seems to be extremely useful to take into account hormone circadian changes to ensure correct performance assessment and to monitor training in Quarter horses.