Pregnancy rates in heifers and cows with cryopreserved sexed sperm: Effects of sperm numbers per inseminate, sorting pressure and sperm storage before sorting

Field trials were conducted to increase fertility with AI of flow-sorted, sexed bovine sperm. In the first trial, a novel competitive fertilization approach was used to compare pressures (30 psi vs 50 psi) for sorting sperm. Both X- and Y-sperm were sorted to approximately 95% purity at 30 and at 50 psi; X-50 + Y-30 (and the converse) were mixed in equal numbers for AI of heifers. Fetal sex divulged which treatment produced the pregnancy; 82% of pregnancies resulted from the 30 psi treatment (P < 0.05). Based on a similar approach, a new-pulsed laser did not damage sperm any more than the previous standard continuous wave laser. In a large field trial, sorting sperm at 40 psi increased pregnancy rates in heifers relative to 50 psi (42.3% vs 34.1%, n = 367/group, P < 0.05). Storing sperm for 20 h before sorting at 40 psi decreased pregnancy rates from 42.3% (n = 367) to 36.8% (n = 368; P < 0.05). Breeding heifers with sexed sperm 55-56 h after CIDR removal and PGF2α resulted in 34% (n = 32) pregnant, compared to 49% (n = 35) with fixed-time insemination 67-68 h after CIDR removal (P > 0.1). Lactating dairy cows pre-screened for normal reproductive tracts when OvSynch injections (GnRH, prostaglandin, GnRH) were initiated, had similar (P > 0.1) pregnancy rates to timed AI, with 10 × 106 sexed sperm (43.9%, n = 57), 2 × 106 sexed sperm (40.5%, n = 57) and 10 × 106 unsexed control sperm (55.6%, n = 58). A final field trial with unselected, lactating dairy cows resulted in similar pregnancy rates for 2 × 106 sexed sperm in 0.25 mL straws (25.0%, n = 708) and 0.5 mL straws (24.4%, n = 776), but lower (P < 0.05) than unsexed control sperm (37.7%, n = 713). Younger cows and those >84 days in milk had the highest pregnancy rates for both sexed and unsexed sperm. These studies improved sperm sexing procedures, and provided insight into appropriate commercial use of sexed sperm.