Oxidative stress and its effects on reproductive performance in thermally-stressed ewes

Abstract

This paper reviewed the impact of thermally-induced oxidative stress on reproduction in ewes. Thermally-induced oxidative stress is associated with excess generation of reactive oxygen species (ROS) during aerobic respiration and metabolic activities within the body. Excess ROS have been implicated in pathophysiologic mechanisms impairing reproductive processes, including changes in cyclic luteal and endometrial processes, poor follicular development, ovulation, fertilisation, embryogenesis, embryonic implantation, and placental differentiation and foetal growth. High levels of ROS during pregnancy result in pregnancy disorders such as spontaneous abortions, embryopathies, pre-eclampsia, foetal growth restriction, pre-term abortion and low-birth weight. Thermally-induced stress occurs as a result of extreme cold and hot temperatures, and its impact on animals is determined by nature of management practices being used in the farm. Heat stress suppresses reproductive hormonal expression and activities, resulting in poor reproductive performances, including the exhibition of poor oestrus behaviour and mating ability; while cold stress suppresses reproductive neuroendocrine axis, which adversely affects conception rates and mating behaviours. Other oxidative stress factors exerting negative effects on reproduction are poor nutrition, solar radiation, environmental pollutants, high ambient temperature and high relative humidity, expressed as temperature-humidity-index. The stressors contribute to adverse pregnancy outcomes, increase in the susceptibility of offspring to disease by compromising the offspring immune status and overall reproductive performance in ewes. The link between oxidative stress and reproductive processes such as spermatogenesis, oogenesis, fertilisation, embryonic development, implantation and foetal growth are critical therapeutic and prophylactic interventions in reproductive medicine. The role of oxidative stress in reproductive processes and the state of knowledge on the association between oxidative stress and pregnancy outcomes in ewes are elucidated. It is concluded that oxidative stress is an important silent condition undermining reproductive performance in sheep resulting in serious economic loss. Reduction of its impacts will enhance productivity and performance in ewes.