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Conference paper2005Peer reviewed

Stress and reproduction in the mare including studies on the effect of ACTH on steroid hormone levels and oestrous behaviour

Hedberg, Ylva; Dalin, Anne-Marie; Kindahl, Hans


The hypothalamo-pituitary-adrenal (HPA) axis interacts with the hypothalamo-pituitary-gonadal (HPG) axis at several sites and there are both species and stress related differences in how a particular stressor affects the gonadal axis. In the pregnant mare, negative effects of stress on reproduction are often measured in terms of effect on plasma progesterone levels or abortion rate. In cycling mares, plasma levels of sex hormones, oestrous cycle duration and ovulation rates are examples of end-points measured in response to stress. Disease and low nutrition levels have been shown to cause lowered progesterone concentrations and, in the case of inadequate nutrients, abortion in pregnant mares. In non-pregnant mares, poor body condition has been shown to delay the onset of the ovulatory season and lower conception rates. Transportation, on the other hand, does not seem to adversely affect reproductive success in either pregnant or cycling mares. Feral horses have shown significantly higher foetal loss if they were removed for adoption or gathered and then re-released into the wild. The mare’s adrenal gland is capable of producing progesterone and androgens. Such extra-ovarian hormone production may be a reason why some ovariectomised mares or mares with inactive ovaries show symptoms of oestrus. In a recent study, we found that 100 % (5/5) of ovariectomised mares showed from two to 78 days of behavioural oestrus during a three month period. In another study, we investigated whether mares believed by their owners to suffer from oestrous related behavioural problems had an altered hormonal response to adrenocorticotrophic hormone (ACTH), but found they did not differ from control mares. All mares in the study showed significant increases in progesterone, androstenedione and testosterone as a result of the ACTH treatment. In other species, oestradiol has been shown to have a stimulatory effect on adrenal hormone production. However, ovariectomy in the mare was found to have no clear effect on the cortisol response to ACTH. However, either time from ovariectomy or season seemed to effect basal cortisol levels, with ovariectomised mares having lower basal cortisol concentrations in November as compared to June. Subsequent oestradiol treatment in November had no effect on basal cortisol levels. Ovariectomised mares showed a lower response in androstenedione production after ACTH stimulation, indicating androstenedione of gonadal origin in intact mares

Published in


56th Annual Meeting of the European Association for Animal Production