Liquid Storage and Cryopreservation of Ram Semen: Storage-Associated Damage and the Role of Non-Enzymatic Antioxidants

The successful use of Assisted Reproductive Technologies (ARTs) such as artificial insemination (AI) in small ruminants and other mammals depends on many factors, the most important of which is the quality of the semen used. The liquid storage and cryopreservation of spermatozoa at lower temperatures are associated with artificial insemination and rapid genetic improvement programs in the sheep production industry. Several studies have reported increased lipid peroxidation (LPO) and highly reactive oxygen species (ROS) production during liquid storage or cryopreservation of ram semen, leading to oxidative stress (OS), decreased antioxidant defense, and changes in sperm quality parameters like biokinetic and biochemical characteristics, viability, functional membrane and DNA integrity, along with mitochondrial activity. Therefore, supplementing ram semen extenders with exogenous antioxidants before preservation could mitigate this harmful effect. Various in vitro studies have reported improvements in ram sperm quality parameters like motility indexes, vitality, functional membrane/DNA integrity, antioxidant enzyme activity, total antioxidant content, mitochondrial activity, in vivo/in vitro fertility with significant decline in sperm abnormality, free radical production, LPO, ROS, apoptosis rate, and cytochrome C release from the mitochondrial matrix after the addition of various natural and synthetic antioxidant (vitamins, glutathione, taurine, pyruvate, melatonin, cysteine, selenium, zinc, plant extracts, sugars, amino acids, polyphenols) substances during preservation. Therefore, this review summarizes recent findings on oxidative stress-induced damage to sperm quality parameters in various ram breeds during chilling storage and cryopreservation. Moreover, supplementing basic semen extenders with different non-enzymatic antioxidant substances as a method to maintain sperm quality—along with their efficacy in reducing or preventing sperm damage during preservation—was discussed in detail.