Variation between ejaculates in the fertility of frozen ram semen used for cervical insemination of merino ewes

Two field experiments were conducted to investigate the amount of variation between rams, and between ejaculates within rams, in the fertility of frozen-thawed semen used for cervical insemination. Individual ejaculates from seven Merino rams were frozen and used to cervically inseminate Merino ewes at a synchronized oestrus at two sites. In Experiment 1 (N = 491), pregnancy rates (determined 70–80 days after insemination) for individual rams ranged from 1.8 to 11.9%. Individual ejaculates produced pregnancy rates between 0 and 21.4%. Overall conception rate was 6.5%. No significant differences were detected. Pregnancy rates for the same rams in Experiment 2 (N = 449) varied from 10.3 to 32.6% (P < 0.05). Ejaculate pregnancy rates ranged between 0 and 60% (P < 0.01). Fertility of individual ejaculates within rams differed for three of the seven rams used (P < 0.01). It is concluded that considerable variation in fertility of frozen ram semen exists between ejaculates within rams as well as between rams. Possible sources (biological and arising from freezing artefacts) of such variation and their practical implications are discussed.