18 EFFECTS OF SKIM MILK ON THE QUALITY AND FERTILITY OF BOAR SEMEN FOLLOWING LIQUID PRESERVATION AT 5°C AND 15°C

Abstract

Liquid preservation of semen can be an alternative to frozen–thawed semen for artificial insemination. The success of a selection of boar semen extenders has been studied over storage periods of 5 to 7 days. The objective of this study was to evaluate the effects of skim milk on the viability and in vitro fertility of boar spermatozoa preserved in Modena-based extenders at 5°C and 15°C for 2 weeks. A total of 7 ejaculates were collected from one boar. The sperm-rich fraction of each ejaculate was centrifuged and diluted in Modena extenders supplemented with 0 (control), 7.5, and 15 mg mL^–1 of dry skim milk. The final sperm concentration was adjusted to 1 × 108 cells mL^–1, and then the semen was stored at 5°C and 15°C for 2 weeks. In the first experiment, the motility, viability (live/dead fluorescence viability assay), plasma membrane integrity (hypoosmotic swelling test; HOST), and acrosome integrity (FITC-labelled peanut agglutinin staining) of semen stored for 2 weeks were assessed. In the second experiment, the fertilization of stored semen after 20 h of co-incubation with in vitro matured oocytes and their development were examined. Data were analysed using ANOVA. When the semen was stored at 5°C for 2 weeks, the mean total sperm motility of semen stored with 7.5 and 15 mg mL^–1 of dry skim milk was significantly higher than that of semen in the control group (41.4% and 41.5% v. 17.4%; P < 0.05). However, the beneficial effects of skim milk on the sperm motility were not observed in the semen stored at 15°C. Moreover, there were no significant differences in the other parameters of semen quality among the groups in each storage temperature. Significantly higher penetration rates of semen stored with 7.5 and 15 mg mL^–1 of dry skim milk were observed in the storage at 5°C (41.1% and 34.8% v. 19.8%; P < 0.05) but not at 15°C (38.9% and 26.0% v. 30.0%; P > 0.05) when compared with the control group. When the semen was stored at 5°C, the development rate to the blastocyst stage of oocytes fertilized with semen stored with 7.5 mg mL^–1 of dry skim milk was significantly higher than that with control and 15 mg mL^–1 of dry skim milk (15.4% v. 1.1% and 7.8%; P < 0.01). However, there were no significant differences in the development rates of oocytes fertilized with semen stored at 15°C among the groups (9.6–11.9%). In conclusion, our results indicate that the effect of skim milk on the viability and in vitro fertility of liquid-stored boar spermatozoa is dependent on the storage temperature. The addition of 7.5 mg mL^–1 of dry skim milk may be effective for the improvement of viability and fertility of semen stored at 5°C but not at 15°C.