ADDITIVES USED IN EXTENDERS TO IMPROVE THE FREEZABILITY OF RAM SEMEN IN RECENT YEARS: a mini review
One of the most important reasons why artificial insemination does not spread as much in cattle as sheep is that ram sperm is highly fragile against cryodamage and consequently the optimum fertility results can not be obtained from cervical inseminations compared to laparoscopic insemination. To establish an ideal ram sperm freezing model, many studies have been carried out for more than 60 years and various methods have been tested by making great efforts. One of the topics where these tests have been done intensively in recent years are additives used in freezing extenders in order to increase the tolerance of sperm against oxidative stress in frozen-thawed ram semen. In this review, covering the studies carried out between the years 2000-2016, we have mostly compiled the additives used in ram semen freezing media which have given better results compared to others supplements. It is understood that, usually, seminal plasma and proteins, thiol compounds, enzymatic antioxidants, sugars, fatty acids and vitamins have been studied as additive agents in recent years. As a result, this review suggests us that further studies are needed to explore the novel techniques and new additives, and their combinations should be tested in different doses in order to install an ideal cryopreservation template for ram semen. Furthermore, even though addition of various compounds to freezing media have improved freezability of ram semen in experimental conditions, anyway these results must be confirmed in field studies. Therefore, the value of sperm which potential fertility is predicted from laboratory survey must be compared with conception or lambing rates.