Evaluation of SexedULTRA 4M™ sex-sorted semen in timed artificial insemination programs for mature beef cows

Abstract

An experiment was designed to compare fertility of SexedULTRA 4M™ sex-sorted semen and conventional, non-sex-sorted semen following either fixed-time artificial insemination (FTAI) or split-time artificial insemination (STAI) of mature suckled beef cows. Units of sex-sorted and conventional semen were produced using contemporaneous ejaculates from three commercially available sires. Units of conventional semen were generated with 25.0 × 106 live cells per 0.25 ml straw prior to freezing, and units of sex-sorted semen were generated using the SexedULTRATM Genesis III sorting technology with 4.0 × 106 live cells per 0.25 ml straw prior to freezing. Sex-sorted units were sorted to contain X chromosome-bearing sperm cells at an accuracy level of >90%. Cows (n = 1620) across four herds were treated with the 7-d CO-Synch + CIDR protocol [administration of gonadotropin-releasing hormone (GnRH) and insertion of a progesterone insert (CIDR) on Day -10, followed by administration of prostaglandin F2α (PG) and removal of CIDR inserts on Day -3]. Cows were preassigned based on age, body condition score, and days postpartum to one of the following four treatments: FTAI with SexedULTRA 4M™ sex-sorted semen, FTAI with conventional semen, STAI with SexedULTRA 4M™ sex-sorted semen, or STAI with conventional semen. On Day -3, estrus detection aids (Estrotect®) were applied. For cows in FTAI treatments, AI was performed on Day 0 at 66 h after PG administration and CIDR removal, and 100 μg GnRH was administered concurrent with AI. For cows in STAI treatments, AI was performed on either Day 0 or 1, at 66 or 90 h after PG administration and CIDR removal, based on timing of estrus expression. On Day 1 at 90 h after PG administration and CIDR removal, 100 μg GnRH was administered concurrent with AI to any STAI-treated cows that had failed to express estrus. Pregnancy rates to AI were affected (P = 0.04) by the interaction of bull and semen type. Greater pregnancy rates were obtained with conventional semen versus SexedULTRA 4M™ sex-sorted semen when using semen from Bull A (64% [176/277] versus 36% [100/278]; P < 0.0001) and Bull B (72% [200/277] versus 57% [156/276]; P < 0.01), whereas pregnancy rates to AI did not differ between conventional and SexedULTRA 4M™ sex-sorted semen when using semen from Bull C (58% [149/258] versus 52% [131/254]). Pregnancy rates did not differ significantly between cows inseminated using a STAI versus FTAI approach, regardless of whether insemination was performed with conventional semen (65% [265/409] versus 65% [260/403] or SexedULTRA 4M™ sex-sorted semen (50% [200/403] versus 48% [187/405]). However, due to the additional 24 h for potential estrus expression when performing STAI, total estrous response prior to AI was greater (P < 0.001) among cows receiving STAI (84%; 686/812) compared to FTAI (72%; 585/808), and greater pregnancy rates (P < 0.0001) were obtained among cows that expressed estrus prior to AI. In summary, the relative fertility of SexedULTRA 4M™ sex-sorted semen and conventional semen varied across bulls. Although overall pregnancy rates to timed AI did not differ between STAI and FTAI approaches, use of a STAI approach allowed for greater total estrous response prior to AI. Therefore, to achieve acceptable conception rates per unit and service the maximum number of cows with sex-sorted semen, one viable approach may be to use STAI to maximize total estrous response and restrict use of SexedULTRA 4M™ sex-sorted to only those cows expressing estrus.