The impact of storage temperature and sperm number on the fertility of liquid-stored bull semen

In Ireland, liquid bull semen is stored at unregulated ambient temperatures, typically at 5×106 spermatozoa per dose, and inseminated within 2.5 days of collection. In Experiment 1, the effect of storage temperature (5, 15, 22, 32°C and fluctuations (Flux) between these temperatures) on progressive motility, viability, acrosomal status, DNA fragmentation and osmotic resistance was assessed. In Experiment 2, the field fertility of liquid semen at 5, 4 and 3×106 spermatozoa per dose, up to Day 2 after collection, was assessed in comparison to frozen-thawed semen at 20×106 spermatozoa per dose (n≤35328 inseminations). In Experiment 1, storage at 15°C resulted in the highest progressive motility (P<0.01). The osmotic resistance of spermatozoa declined with duration of storage; however, after Day 3 this decline was reduced in the 5°C and Flux 15°C treatments (P<0.01). In Experiment 2, the non-return rate of liquid semen stored at 4 and 3×106 spermatozoa per dose on Day 2 of storage was reduced in comparison to frozen-thawed semen (P<0.01). In conclusion, liquid semen is versatile between storage temperatures of 5 and 22°C, but demonstrates reduced fertility on Day 2 of storage at lower sperm numbers in comparison to frozen-thawed semen.