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 cryopreservationon the quality of thawed semen was evaluated. Samples from seven adult Martina Francajackasses were collected three times using a Missouri artificial vagina. After estimation ofvolume and concentration, raw semen was evaluated for motility using a computer-assistedsperm analyzer (CASA); viability and acrosome integrity were also determined. Fresh semenwas 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, andfrozen. After thawing, motility parameters, viability, and acrosome integrity were analyzed.The analysis of the data showed similar parameters of fresh semen compared with those ofcentrifuged and cooled samples. The sperm concentration in the straw affected the semenparameters analyzed after thawing, as suggested by evidence that when the concentrationincreased, the quality of the post-thawed semen decreased. Furthermore, the differencesin total and progressive motility among samples at different concentrations are due to theimmobilization of spermatozoa, as suggested by the finding that the percentage of staticspermatozoa increased when the concentration increased. The reason for the impairmentof semen quality when the sperm concentration increased was discussed. A great variabilityin cryo-resistance was found between jackasses but not within the same male, suggestingthe presence of donkey males with semen that has acceptable and unacceptable freezingqualities.[...]