Research in farm animals has long been a cornerstone of our understanding of reproduction and holds similar opportunities for understanding mechanisms by which stress affects fertility. Sheep and cow experimental models are appropriate to humans, especially as detailed longitudinal studies are possible because of size and access to ovaries.
Failure to realise genetic potential is a better definition of stress than an increase in corticoids; mainly because corticoid secretion is suppressed during chronic stress. Indeed, the response to a repeated acute stressor is lower in normal cows than in those enduring chronic lameness. Furthermore, cows that develop behavioural coping strategies have reduced responses to an acute stressor. Intensive studies on sheep have revealed the existence of tightly regulated feedback mechanisms that keep the HPA axis in control.
Profiles of sex hormones in cows milk indicate that reproductive endocrine activity does not always predict sexual behaviour we propose that even in clinically normal animals, there are defects in endocrine control of pheromone production/detection. Intricate endocrine/behavioural interactions of hypothalamic changes in sheep are now underway.
There is overwhelming evidence that common (clinical) conditions in farmed animals reduce fertility. Most dairy cows are bred by artificial insemination, timed by behavioural signs of oestrus. However, lame cows display less intense signs of oestrus - interestingly peripheral oestradiol patterns/values are normal, but progesterone concentrations are lower. Furthermore in sheep, acute stress in the late follicular phase decouples the simultaneous onset of oestrus behaviour, the preovulatory LH surge and possibly ovulation.
The major lesson to be learnt by researchers is to choose appropriate models for their studies; ask the right questions; and maintain a holistic approach. Farmers/vets/consumers need to appreciate the consequences of increased production and lower costs; and medics require a deeper understanding about the impact of stress on fertility.