A series of experiments was conducted to evaluate alterations to long-term progestin-based estrus synchronization protocols that may result in enhanced pregnancy rates to timed artificial insemination (AI). In the first series of experiments, a new approach called the 9-d CIDR-PG protocol was developed and tested in mature beef cows (Chapters 2-4) and also evaluated in beef heifers (Chapter 5). In the second series of experiments (Chapters 6 and 7), a modified timed AI approach called split-time AI was evaluated in conjunction with use of sex-sorted semen. In Chapter 2, a pilot experiment was designed to test the hypothesis that administration of prostaglandin F2[alpha] prior to progestin treatment would allow for a reduced duration of progestin treatment in a long-term progestin-based estrus synchronization protocol. A modified pre-synchronization treatment was compared to a standard long-term controlled internal drug release (CIDR) treatment, and treatments were compared on the basis of ovarian follicular dynamics, estrous response rate, synchrony of estrus expression, and pregnancy rates resulting from timed AI in postpartum beef cows. Estrus was synchronized for 85 cows, with cows assigned to one of two treatments based on age, days postpartum (DPP), and body condition score (BCS).Cows assigned to the 14-d CIDR-PG protocol received a CIDR insert (1.38 g progesterone) on Day 0, CIDR removal on Day 14, and administration of PG (25 mg im)on Day 30. Cows assigned to the 9-d CIDR-PG protocol received PG concurrent with CIDR insertion on Day 5, PG concurrent with CIDR removal on Day 14, and administration of PG on Day 30. In both treatments, split-time AI was performed based on estrous response. At 72 h after PG (Day 33), cows having expressed estrus received timed AI; cows that failed to express estrus by 72 h received timed AI 24 h later (96 h after PG on Day 34), with gonadotropin-releasing hormone (GnRH; 100 [mu]g im)administered to non-estrous cows. Estrus detection transmitters were used from CIDR removal until AI to determine onset time of estrus expression both after CIDR removal and after PG. Ovarian ultrasonography was performed at CIDR removal on Day 14, at PG on Day 30, and at AI on Day 33 or Day 34. At CIDR removal on Day 14, diameter of the largest follicle present on the ovary (LFD) was similar between treatments. The proportion of cows expressing estrus following CIDR removal tended to be higher (P =0.09) among cows assigned to the 9-d CIDR-PG treatment (93%; 40/43) than among cows assigned to the 14-d CIDR-PG treatment (81%; 34/42). Following PG, a significantly higher proportion (P = 0.02) of cows expressed estrus following synchronization with the 9-d CIDR-PG treatment (91%; 39/43) than the 14-d CIDR-PG treatment (69%; 29/42). Consequently, pregnancy rate to TAI tended to be increased (P =0.09) among the 9-d CIDR-PG treatment (76.7%; 33/43) compared to the 14-d CIDR-PG (59.5%; 25/42). In summary, a long-term CIDR-based estrous synchronization protocol for postpartum beef cows was enhanced through administration of PG at CIDR insertion and at CIDR removal. In Chapter 3, an experiment was designed to evaluate endocrine parameters, ovarian dynamics, and pregnancy rates to fixed-time artificial insemination (FTAI) following the 9-d CIDR-PG protocol in comparison to the 14-d CIDR-PG protocol. While both are long-term protocols using CIDR treatment for presynchronization, the 9-dCIDR-PG protocol differs from the 14-d CIDR-PG protocol in that prostaglandin F[subscript 2[alpha]] (PG) is administered at CIDR insertion and removal to facilitate a decreased length of progestin treatment and potentially enhance response to the presynchronization treatment. Estrus was synchronized for 393 mature beef cows across five locations. Treatments were represented in each location, and cows within each location were randomly assigned to one of the two protocols based on age, DPP, and BCS. Cows assigned to the 14-d CIDRPG treatment received a CIDR insert on Day 0 with removal of CIDR on Day 14, and 25mg PG 16 d after CIDR removal on Day 30. Cows assigned the 9-d CIDR-PG treatment received 25 mg PG and a CIDR insert on Day 5; 25 mg PG and removal of CIDR on Day14; and 25 mg PG 16 d after CIDR removal on Day 30. In both treatments, cows received FTAI on Day 33, 72 h after PG. All cows were administered 100 [mu]g GnRH concurrent with insemination. For a subset of animals in each treatment, ovarian ultrasound was performed and blood samples were collected for determination of serum estradiol concentrations at CIDR removal, PG administration, and FTAI. Protocols were compared on the basis of estrous response and pregnancy rate resulting from FTAI. Serum estradiol concentrations, follicle size, and estrous response did not differ based on treatment. However, cows assigned to the 9-d CIDR-PG protocol tended to achieve greater FTAI pregnancy rates than cows assigned to the 14-d CIDR-PG protocol (62% versus 52%; P =0.07). Across treatments, greater pregnancy rates tended (P = 0.10) to be achieved by cows that expressed estrus prior to FTAI (69% for 9-d CIDR-PG, 58% for 14-d CIDRPG)than by cows that failed to express estrus (55% for 9-d CIDR-PG, 47% for 14-dCIDR-PG). In summary, the 9-d CIDR-PG protocol is an effective protocol for synchronization of estrus among mature beef cows, and pregnancy rates to FTAI tended to be improved through use of the 9-d CIDR-PG compared to the 14-d CIDR-PG protocol. In Chapter 4, response of mature beef cows following the 9-d CIDR-PG protocol was characterized in reference to a short-term protocol commonly used in commercial production. Mature beef cows (n = 480) across four locations were assigned based on age and days postpartum to either the 9-d CIDR-PG protocol [25 mg PG and insertion of a CIDR on Day -28; 25 mg PG and CIDR removal on Day -19; and 25 mg PG on Day -3] or the 7-d CO-Synch + CIDR protocol [100 [mu]g GnRH and CIDR insertion on Day -10; 25mg PG and CIDR removal on Day -3]. On Day -3, estrus detection aids (Estrotect[superscript registered trademark]) were applied, and a subset of cows (n = 203) were fitted with radio telemetric, pressure sensitive devices (HeatWatch[copyright symbol]) to characterize timing of estrus onset. Split-time artificial insemination (STAI) was performed on Day 0 or 1 based on timing of estrus expression, with cows failing to express estrus by Day 1 administered 100 [mu]g GnRH at AI. In Locations 1, 2, and 3, blood samples were collected for radioimmunoassay on Day -38 and Day -28 to determine pretreatment estrous cyclicity status based on serum progesterone concentrations. Additional blood samples were collected at PG administration and at STAI for determination of serum estradiol concentrations. Transrectal ovarian ultrasound was performed at PG administration and AI for a subset of TAI on Day 33, 66 h after PG. Estrus detection aids were applied at CIDR removal on Day 14 and at PG on Day 30 to evaluate estrous response rate. Mean RTS differed (P less than 0.0001) based on biological type due to higher rates of estrous cyclicity (RTS 4 and 5)among Bos taurus heifers (72%; 416/574) than among Bos indicus-influenced heifers(27%; 150/565). The proportion of heifers expressing estrus following CIDR removal was greater (P = 0.01) among heifers assigned to the 14-d CIDR-PG treatment (88%; 492/559) compared to the 9-d CIDR-PG treatment (83%; 480/580). Estrous response following CIDR removal was also higher (P less than 0.0001) among Bos taurus (95%; 547/574) compared to Bos indicus-influenced (75%; 425/565) heifers. Rate of estrous response prior to FTAI did not differ significantly based on treatment but was higher (P less than 0.0001) among Bos taurus heifers (60%; 344/574) than among Bos indicus-influenced heifers (45%; 253/565). However, the effect of biological type on estrous response was not significant when RTS was included in the model, as RTS significantly (P less than 0.0001) affected the rate of estrous response both at CIDR removal and prior to FTAI. Across treatments and biological types, heifers that expressed estrus prior to AI achieved higher (P less than 0.0001) AI pregnancy rates than heifers failing to express estrus. Pregnancy rates to FTAI did not differ significantly based on treatment in either biological type. Higher rates of estrous cyclicity among Bos taurus heifers resulted in higher FTAI pregnancy rates among Bos taurus (51%; 290/574) compared to Bos indicus-influenced heifers (39%; 218/565). However, pregnancy rates of respective RTS did not differ based on biological type. In summary, long-term CIDR-based protocols provide a simple, effective method of estrus synchronization in Bos indicus-influenced and Bos taurus beef heifers. Moreover, these results highlight the importance of management practices that result in high rates of estrous cyclicity prior to protocol initiation, particularly among later maturing breeds and biological types. In Chapter 6, an experiment was designed to evaluate the relative fertility of SexedULTRA[superscript TM] sex-sorted semen compared to conventional, non-sex-sorted semen when used among beef heifers in conjunction with split-time AI following the 14-dCIDR-PG protocol. Units of conventional semen were generated with 25.0 x 10[superscript 6] live cells per 0.5 ml straw prior to freezing, and units of sex-sorted semen were generated using the SexedULTRA[superscript TM] Genesis III sorting technology with 4.0 x 10[superscript 6] live cells per0.25 ml straw prior to freezing. Sex-sorted units were sorted to contain X chromosome bearing sperm cells at an accuracy level of >90%. Estrus was synchronized in 851 heifers at four locations using the 14-d CIDR-PG protocol: CIDR insert on Day 0, CIDR removal on Day 14, and administration of PG (25 mg im) on Day 30. Estrus detection aids were applied at PG on Day 30 to evaluate estrous response rate, and split-time AI was performed based on estrous response. At 66 h after PG (Day 33), heifers having expressed estrus received timed AI. Heifers failing to express estrus by 66 h received timed AI 24 h later (90 h after PG on Day 34). Heifers failing to express estrus by 90 h were administered GnRH (100 [mu]g im) concurrent with AI. Heifers were preassigned to treatment (insemination with either conventional or Sexed ULTRATM sex-sorted semen), and treatments were balanced within each location based on source, reproductive tract score, and weight. Heifers were exposed for natural service beginning 14 d after AI for the remainder of a 60 d breeding season. Pregnancy rates to AI across locations tended to be higher (P = 0.09) for heifers inseminated with conventional semen (60%; 257/429) compared to sex-sorted semen (52%; 218/422). Higher pregnancy rates to AI (P less than 0.0001) were obtained among heifers that expressed estrus prior to AI than among heifers that failed to express estrus prior to AI at 90 h. Total pregnancy rates at the end of the 60d breeding season did not differ between heifers that received sex-sorted semen at AI (89%; 376/422) and heifers that received conventional semen at AI (89%; 382/429). In summary, the pregnancy rates observed suggest that SexedULTRA[superscript TM] sex-sorted semen and conventional, non-sex-sorted semen was compared following either FTAI or STAI of mature suckled beef cows. Units of sex-sorted and conventional semen were produced using contemporaneous ejaculates from three commercially available AI sires. Units of conventional semen were generated with 25.0 x 106 live cells per 0.25 ml straw prior to freezing, and units of sex-sorted semen were generated using the Sexed ULTRA[superscript TM] Genesis III sorting technology with 4.0 x 10[superscript 6] 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 = 922) across seven locations were treated with the7-d CO-Synch + CIDR protocol [administration of GnRH and insertion of a CIDR on Day -10, followed by administration of 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 SexedULTRATM sex-sorted semen, FTAI with conventional semen, STAI with Sexed ULTRA[superscript TM] sex-sorted semen, or STAI with conventional semen. On Day -3, estrus detection aids (Estrotect[superscript registered trademark]) were applied. For cows in FTAI treatments, AI was performed on Day 0 at 66 h after PG administration and CIDR removal, and 100 [mu]g 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 90h 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. Total estrous response prior to AI was greater (P less than 0.001) among cows receiving STAI (90%) compared to those receiving FTAI (80%) due to the additional 24 h for potential estrus expression when performing STAI. Greater pregnancy rates (P less than 0.0001) were obtained among cows inseminated with conventional semen (68%; 315/464) compared to cows inseminated with SexedULTRA[superscript TM] sex-sorted semen (51%; 235/458). However, pregnancy rates did not differ between cows inseminated using a FTAI versus STAI approach, regardless of whether insemination was performed with conventional semen (69% versus 67%) or Sexed ULTRA[superscript TM] sex-sorted semen (50% versus 53%). Pregnancy rates to AI were affected (P = 0.001) by the interaction of bull and semen type. Greater pregnancy rates were obtained with conventional semen versus SexedULTRA[superscript TM] sex-sorted semen when using semen from Bull A (69% [111/161] versus 38% [59/157]; P less than 0.0001) and Bull B(75% [121/161] versus 58% [92/157]; P less than 0.01), whereas pregnancy rates to AI did not differ between conventional and SexedULTRA[superscript TM] sex-sorted when using semen from Bull C (58% [82/142] versus 58% [84/144]). In summary, greater pregnancy were obtained among cows receiving timed AI with conventional semen compared to SexedULTRA[superscript TM] sex-sorted semen, although the relative fertility of Sexed ULTRA[superscript TM] sex-sorted semen and conventional semen varied across bulls. Although likely resulting in lower average pregnancy rates than those obtained with conventional semen, use of SexedULTRA[superscript TM] sex-sorted semen for timed AI of mature beef cows may result in pregnancy rates of a satisfactory level for some production settings.
