Some owners appear to experience problems with their mares due to excessive oestrous signs and/or changes in their mare’s behavioural characteristics across the oestrous cycle. Few such mares are examined at equine clinics and, thus, the cause of such behaviour is often unknown. Adrenal sex steroids have been suggested to cause oestrous behaviour in ovariectomised (OVX) mares and, if secreted in an aberrant manner, aggressive behaviour in intact mares. The aims of this study were to gain further knowledge regarding oestrus in the mare by (1) studying intact mares with a history of deviant behaviour (‘problem’ mares), intact control mares and OVX mares using objective methods and (2) by investigating mare adrenal gland function, specifically sex steroid hormone production after synthetic adrenocorticotropic hormone (ACTH) treatment (tetracosactide) in intact mares (‘problem’ mares and controls) and OVX mares, and adrenal steroid hormone receptors using immunohistochemistry in intact mares, OVX mares and oestradiol-treated OVX mares. In addition, since treatment with progestins to suppress oestrous cyclicity in competition mares is not endorsed by most European Community countries, a preliminary study was performed to determine if dioestrous ovulations and a subsequent prolonged luteal phase can be induced using human chorionic gonadotropin (hCG). Neither oestrous-cycle stage nor whether a mare was a ‘problem’ or control mare affected the mare’s response to a novel object test or isolation test. In the novel object test, mares (both control and ‘problem’) tended to touch the novel object earlier when tested in dioestrus first, indicating mares in the luteal phase may be more responsive to a novel environment. An effect of learning or habituation to the novel object test was found. OVX mares showed oestrous behaviour during both conventional teasing and during a ‘paddock teasing’ method, but without a normal cyclic pattern. Adrenal production of progesterone, androstenedione and testosterone was demonstrated and the hormones were shown to be of both ovarian and adrenal origin in intact mares at oestrus. Ovariectomy affected the diurnal cortisol rhythm, with a more pronounced rhythm after ovariectomy. ‘Problem’ mares showed a lower cortisol response to ACTH treatment compared with controls. Staining for oestrogen receptor alpha (ERα) and progesterone receptor (PR) (weak) was found in cell nuclei of the adrenal cortex (all mares) and in the cytoplasm of medullary cells (for ERα, in OVX mares and one intact mare only). Ovulation was induced in three of four hCG-treated mares that also developed prolonged luteal phases, as assessed by progesterone analysis. However, although no control mare ovulated after saline treatment, two control mares showed prolonged luteal activity during the study period. Further studies are needed to determine the method’s efficacy and applicability in practice. The present study confirms that mares after ovariectomy show oestrous signs, but without normal cyclicity. The adrenal gland produces a significant amount of androgens, which may be involved in such paradoxical sexual behaviour. The presence of ERα and PR in the adrenal cortex indicates a direct effect of ovarian steroids on adrenal gland function. ‘Problem’ mares showed few deviations from control mares, but the low number of animals and their heterogenicity may have confounded the results
