Tharasanit T. et al./TJVM 37(4): 39-48. 39 The Effect of Cold Storage on the Quality of Stallion Semen and Pregnancy Rate after Artificial Insemination

While the use of frozen-thawed semen has been hampered by poor cryopreservability of equine sperm, the technique of choice in equine practice remains artificial insemination (AI) using cold storage semen. This study examined the effect of cold storage on semen quality and pregnancy rate after AI. In experiment I, ejaculates were collected from 3 stallions of proven fertility and then diluted to a final concentration of 25-50 x 106 sperm/ml. After transportation, the semen was then divided into two aliquots. The first part of semen was centrifuged to partially remove the seminal plasma, while the remaining was left intact with seminal plasma. Subsequently, the semen was slowly cooled and maintained at 4 oC for 24, 48, 72, 96 and 120 h where quality of semen was examined. At 6 h and 72 h post cold storage, spermatozoa were also morphologically analyzed. Experiment II aimed to examine fertilizability of equine sperm in vivo. Collected semen was stored at 4 oC for 6, 24 and 48 h and then used for AI. Fresh semen extended with semen extender without cooling (0 h) served as a control. Overall, viability and progressive motility of equine sperm were dramatically decreased during cold storage, while partial removal of seminal plasma improved sperm viability. Of stallions used, two were classified as “cold tolerant ” stallions, judged by progressive motility and viability that were greater than 50 % after cold storage for 48 h. When cold semen (6, 24 and 48 h storage times) was inseminated to estrus mares (n=21), fourteen (66.7%