Comparison of different diluents based on liposomes and egg yolk for ram semen cooling and cryopreservation

Sperm cryopreservation is an important tool for breed improvement, nonetheless, spermatozoids of rams are extremely sensitive to cryopreservation. The present research was to compare a liposome-based (OptiXcell: OX) diluent, a commercial TRIS-egg yolk (Optidyl: OP) and a citrate egg yolk-based (CY) diluent on ovine semen quality through the cryopreservation process. Semen was collected from four sexually mature Dorper rams during the natural breeding season. After collection, semen was evaluated and diluted in OX, OP or CY diluent and was cooled from 37°C to 4°C for 2 h (refrigerated semen, RS), after that semen-filled straws were placed in liquid nitrogen (LN) vapour for 10 min, then immersed into LN at -196°C (cryopreserved semen, CS) and stored until evaluation. For fresh semen (FS), similar values (P>0.05) were obtained from the 3 diluents [motility (4.2 ± 0.3), viability (75.4 ± 3.2), hypo-osmotic swelling test (HOST) (59.2 ± 2.1), and normality (84.7 ± 3.5)]. The motility values were higher for RS with OX and CY (4.0 ± 0.2 and 3.6 ± 0.3, respectively) compared to OP (3.0 ± 0.21; P<0.05). The viability was reduced after refrigeration and freezing (P<0.05). Refrigerated semen viability was similar for OX (65%), CY (63%) and OP diluents (60%; P>0.05), but for frozen semen, viability was lower in the CY diluent (P<0.05). Membrane integrity (HOST) in OX (53.6 ± 1.7) was similar to that in OP (50.7 ± 1.5; P>0.05) but higher than in CY (48.7 ± 1.5; P<0.05). No difference was found between the OX diluents and those made with egg yolk in terms of sperm parameters; however, the OX diluent was more efficient in protecting the integrity of membrane in freezing/thawing semen.