Improved Pregnancy Outcomes in Beef Heifers Through Delayed Insemination of Sexed Semen.

This study evaluated the effect of the timing of Y-sorted sexed semen (SS) insemination after the onset of estrus on reproductive outcomes in beef heifers and examined the influence of AI sires and their sperm DNA fragmentation (%SDF) over time. Angus heifers (n = 718) from two locations were synchronised using a CIDR + Select-Synch protocol and blocked by age, body condition score, and reproductive tract score. Heifers expressing estrus were randomly assigned to AI at 12, 20, or 28 h post-estrus onset using SexedULTRA 4 M semen from one of three bulls. Post-thaw %SDF at 0, 12, and 24 h was assessed by acridine orange staining. Pregnancy per AI (P/AI) differed by timing: 12 h (45.4%), 20 h (48.1%), and 28 h (56.5%) (p < 0.05), with the 28 h group achieving significantly higher P/AI than the 12 h group. Stillbirth incidence and gender ratio (bull: heifer) did not differ significantly among groups. No overall difference in P/AI among sires was observed; however, a significant sire × time of AI interaction existed (p = 0.05), with Bull 2 showing the greatest improvement in P/AI at 28 h (up to 15.2 percentage points higher than earlier times). For Bulls 1 and 2, %SDF increased significantly from 0 to 24 h post-thaw, while Bull 3 showed no change. These findings indicate that delaying insemination to 28 h post-estrus enhances P/AI when using Y-sorted SS, potentially due to improved synchrony with ovulation and reduced exposure to sperm with increasing DNA damage.