Fertility of ram spermatozoa pellet-frozen in zwitterion-buffered diluents

The fertility (positive ultrasound scanning 60-75 days after insemination) of ram spermatozoa frozen in Tris- or zwitterion/glucose/egg yolk/glycerol-based diluents was examined during the natural breeding season (autumn). In Experiment I Tris-citrate, high glucose and Hepes-NaOH diluents were used and ewes (n = 266) were inseminated (induced oestrus) with 20 (uterus) or 180 (cervix) million motile spermatozoa. There was no difference in fertility (range: 10.9-71.1%) between diluents, despite the superior post-thawing motility and progressive velocity of spermatozoa in Hepes-NaOH compared with the other diluents (P < 0.001). In Experiment 2, there was no difference in fertility (range: 4.1-50.0%) between Tris-citrate and Hepes-NaOH diluents after insemination (induced oestrus) of ewes (n = 290) with IO(uterus) or 100 (cervix) million motile spermatozoa, respectively. In Experiment 3 Tris-citrate, Hepes-NaOH and Pipes-NaOH diluents were used and ewes (n = 492) were inseminated (natural oestrus) with 8 or 24 (uterus), or 80 or 240 (cervix) million motile spermatozoa. There were no differences in fertility (range: 35.4-52.9%) between insemination doses or diluents despite the superior motility and acrosome integrity in the zwitterion compared with the Tris-citrate diluents (P < 0.001). In all experiments, the fertility of frozen spermatozoa was higher for intrauterine than cervical insemination (51.9 vs 22.1%, n = 1 048; P < 0.01). It was concluded that post-thawing in vitro quality was superior and the fertility comparable after freeze-thawing spermatozoa in zwitterion compared with Tris-citrate diluents.