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

The hypothalamo-pituitary-adrenal (HPA) axis interacts with the hypothalamo-pituitarygonadal (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.