Effect of sperm concentration on characteristics of frozen-thawed semen in donkeys

In this study, the effect of donkey sperm concentration in the straw during cryopreservation on the quality of thawed semen was evaluated. Samples from seven adult Martina Franca jackasses were collected three times using a Missouri artificial vagina. After estimation of volume and concentration, raw semen was evaluated for motility using a computer-assisted sperm analyzer (CASA); viability and acrosome integrity were also determined. Fresh semen was then centrifuged and re-suspended at five different concentrations (100, 250, 500, 750, and 1000 × 106 sperm/ml) with a commercial extender, packaged in 0.5 ml straws, and frozen. After thawing, motility parameters, viability, and acrosome integrity were analyzed. The analysis of the data showed similar parameters of fresh semen compared with those of centrifuged and cooled samples. The sperm concentration in the straw affected the semen parameters analyzed after thawing, as suggested by evidence that when the concentration increased, the quality of the post-thawed semen decreased. Furthermore, the differences in total and progressive motility among samples at different concentrations are due to the immobilization of spermatozoa, as suggested by the finding that the percentage of static spermatozoa increased when the concentration increased. The reason for the impairment of semen quality when the sperm concentration increased was discussed. A great variability in cryo-resistance was found between jackasses but not within the same male, suggesting the presence of donkey males with semen that has acceptable and unacceptable freezing qualities.