Role of oxidant–antioxidant balance in reproduction of domestic animals Saranika Talukder A , Kendra L. Kerrisk A , Gianfranco Gabai B and Pietro Celi C,D,E A Faculty of Veterinary Science, The University of Sydney, Camden, NSW 2570, Australia. B Dipartimento di Biomedicina Comparata e Alimentazione, Università di Padova, Italy. C DSM Nutritional Products, Columbia, MD 21163, USA. D Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, Vic. 3010, Australia. E Corresponding author. Email: pietro.celi@dsm.com Abstract. Reproductive process leads to dynamic changes in metabolism and energy consumption, which may be responsible for the excessive production of free radicals (oxidants) that are generated during the physiological process of oxygen consumption. As the ovary is a metabolically active organ, it produces oxidants. Growing follicles, granulose cells of Graffian follicles and ovulated follicles all produce both enzymatic and non-enzymatic antioxidants to preserve themselves from the oxidative damage of oxidants. Oxidants and antioxidants are involved in several reproductive functions such as the regulation of follicular fluid environment, folliculogenesis, steroidogenesis, corpus luteum function, and luteolysis. In this article, the currently available literature is reviewed in relation to the roles of oxidants and oxidative stress in both normal and abnormal reproductive physiological processes. Additional keywords: antioxidants, female reproduction, ovulation, oxidative stress. Received 16 September 2015, accepted 26 February 2017, published online 5 April 2017 Introduction The characterisation of the oxidant–antioxidant balance is attracting a high level of interest in ruminant physiology (Celi 2011b) as it may have a pivotal role in the regulation of several physiological functions, including reproduction (Agarwal et al. 2005; Celi 2011b). The literature suggests that oxidative stress (OS) has a dual role in the regulation of the female reproductive system as it is involved not only in several physiological functions but also in the pathogenesis of female infertility (Agarwal et al. 2005). The association between OS, female reproduction and adverse reproductive outcomes encompasses several key issues in animal and human medicine (Al-Gubory et al. 2010). In dairy cows, disorders of the oxidant–antioxidant balance have been observed during several reproductive pathologies, such as retained placenta (Kankofer et al. 2010), mastitis, embryonic mortality (Celi et al. 2011, 2012), follicular cysts and repeat breeder syndrome (Rizzo et al. 2007, 2009; Talukder et al. 2014a). Moreover, one of the main constraint on conception rate is the elevated rate of early embryonic death that occurs before Day 21 post-insemination, causing 30–50% losses (Cook 2009). It is well established that reproduction increases energy expenditure by augmenting the metabolic rate (Nilsson 2002; Alonso-Alvarez et al. 2004). Elevated metabolism results in increased production of reactive oxygen species (ROS) and, therefore, unless the antioxidant defences are also enhanced, it might expose the organism to the negative effects of OS (Alonso-Alvarez et al. 2004). The ovary is a metabolically active organ and, hence, it produces ROS (Fujii et al. 2005; Pandey et al. 2010). It is important to consider that excessive consumption of reducing equivalents, provided by reduced nicotinamide adenine dinucleotide phosphate (NADPH), by both antioxidant defence and metabolic reactions, can result in NADPH depletion (Fujii et al. 2005). The OS-induced depletion of reducing equivalents can reduce the amount of NADPH required by other important physiological processes and the consequent upregulation of the monophosphate shunt would redirect glucose from other pathways such as milk production, resulting in decreased productivity ( Lean et al. 2014). Therefore, when ROS accumulates because of high metabolism, this may lead to a disruption of the oxidant–antioxidant balance, resulting in OS (Celi and Gabai 2015). An increase in both fetal and maternal metabolism can be observed during the final stages of pregnancy as a consequence of rapid fetal growth and, at the start of the lactation, as a result of colostrum and milk production. The resulting enhanced mitochondrial activity in both fetal and maternal tissues results in an over-production of ROS (Aurousseau et al. 2006) as observed in dairy cows during the last trimester of pregnancy (Castillo et al. 2005) and the beginning of the lactation (Pedernera et al. 2010). The aim of the present review is to discuss the roles that oxidants and antioxidants have in the regulation of female reproductive physiological and pathophysiological functions. CSIRO PUBLISHING Animal Production Science Review http://dx.doi.org/10.1071/AN15619 Journal compilation Ó CSIRO 2017 www.publish.csiro.au/journals/an