24-Hour Endocrine Profiles of Quarter Horses under Resting Conditions

The aim of this research was to evaluate the changes in β-endorphin, cortisol, total and free iodothyronines, and estradiol-17β 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 into two groups comprising five 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 time was 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 β-endorphin (P < .01), cortisol (P < .02), fT3 (P < .0001), and estradiol-17β (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.