Comparison of progestin-based estrus synchronization protocols before fixed-time artificial insemination on pregnancy rate in beef heifers1

Evaluation of later timepoints for fixed-time artificial insemination of beef heifers and cows when using sex-sorted semen

Two experiments were designed to evaluate later timepoints for fixed-time artificial insemination (FTAI) of beef heifers and cows, with the hypothesis that use of a later timepoint would allow a greater proportion of animals to express estrus prior to FTAI and result in greater conception rates among estrous females inseminated with sex-sorted semen. In Experiment 1, estrus was synchronized for 1640 heifers using the 14 d CIDR-PG protocol: insertion of an intravaginal progesterone-releasing insert (CIDR; 1.38 g progesterone) on Day -33 and removal on Day -19, and administration of prostaglandin F2α (PG; 500 μg cloprostenol sodium) on Day -3. Heifers were inseminated at one of three FTAI timepoints: 66 h, 70 h, or 74 h after PG administration. In Experiment 2, estrus was synchronized for 414 beef cows using the 7 & 7 Synch protocol: administration of PG coincident with CIDR insertion on Day -17, gonadotropin-releasing hormone (GnRH; 100 μg gonadorelin) on Day -10, and PG coincident with CIDR removal on Day -3. Cows were inseminated at one of two FTAI timepoints: 66 h or 72 h after PG administration. In both experiments, only animals that expressed estrus prior to FTAI were inseminated with sex-sorted semen. In Experiment 1, the proportion of heifers that expressed estrus prior to FTAI (66 h: 62 %; 70 h: 67 %; 74 h: 71 %) was greater when FTAI was performed at 74 h versus 66 h (P = 0.0097); however, conception rate of heifers that expressed estrus and were serviced with sex-sorted semen did not differ among treatments (P = 0.67; 66 h: 56 %; 70 h: 53 %; 74 h: 53 %). In Experiment 2, the proportion of cows expressing estrus prior to FTAI did not differ between treatments (P = 0.30; 66 h: 71 %; 72 h: 76 %). Additionally, conception rate of estrous cows inseminated with sex-sorted semen did not differ between treatments (P = 0.24; 66 h: 45 %; 72 h: 40 %). These results indicate that performing FTAI later following the 14 d CIDR-PG protocol increases the proportion of heifers that express estrus and are serviced with sex-sorted semen but does not improve conception rates. Later timing of FTAI following the 7 & 7 Synch protocol was not observed to increase the proportion of cows expressing estrus prior to FTAI or improve conception rates among estrous cows inseminated with sex-sorted semen. Together, these results provide further insight into optimal timing of FTAI when using sex-sorted semen.

Pre-synchronization of ovulation timing and delayed fixed-time artificial insemination increases pregnancy rates when sex-sorted semen is used for insemination of heifers

This study was conducted to determine effects of pre-synchronization of ovulation timing among heifers and delayed fixed-time artificial insemination (TAI) with sex-sorted semen on proportion of heifers pregnant after TAI (PR/AI). Heifers were assigned to one of eight treatments: 1 and 2), 7-d CO-Synch + CIDR treatment regimen with administration of gonadotropin-releasing hormone and a CIDR insert on Day 0, prostaglandin F_2α (PGF) at CIDR removal on Day 7, and TAI occurring 54 h later with conventionally processed (CTRL54-CNV) or sex-sorted semen (CTRL54-SEX); 3 and 4), same as CTRL54 but TAI delayed to 72 h with conventionally processed (CTRL72-CNV) or sex-sorted semen (CTRL72-SEX); 5 and 6), same as CTRL54 but additional administration of PGF on Day -7 and TAI with conventionally processed (PRE54-CNV) or sex-sorted semen (PRE54-SEX); 7 and 8), same as PRE54 treatments but TAI delayed to 72 h with conventionally processed (PRE72-CNV) or sex-sorted semen (PRE72-SEX). Proportion of heifers pregnant after TAI was greater (P ≤  0.02) with conventionally processed semen compared with sex-sorted semen, yet PR/AI did not differ (P =  0.14) between heifers in PRE72-CNV and PRE72-SEX groups. There were greater PR/AI in the PRE72-SEX (P =  0.03) than CTRL54-SEX group (46.1 % and 36.9 %) and there was no difference (P =  0.31) in PR/AI between CTRL54-CNV and PRE72-SEX groups (50.4 % and 46.1 %). In conclusion, pre-synchronization of ovulation timing among heifers combined with delayed TAI resulted in increased PR/AI with sex-sorted semen compared with the 7-d CO-Synch+CIDR treatment regimen.

Presynchronization with prostaglandin F2α and prolonged exposure to exogenous progesterone impacts estrus expression and fertility in beef heifers

To determine the effects of two presynchronization strategies in conjunction with delayed fixed-time artificial insemination (TAI) on pregnancy rates to TAI (PR/AI), 1700 Angus beef heifers at three locations in South Dakota were enrolled in a completely randomized design with a 2 by 2 factorial arrangement of treatments. Within location, all heifers were randomly assigned to one of four treatments: 1) PG54 (n = 434), heifers were administered prostaglandin F_2α (PGF; 25 mg im) 7 d prior [Day -14] to the initiation of the 7-d CO-Synch + controlled internal drug releasing (CIDR) protocol wherein they received gonadotropin-releasing hormone (GnRH; 100 μg im) and a CIDR insert on Day -7, PGF at CIDR removal on Day 0, and a second injection of GnRH concurrently with TAI 54 ± 2 h later; 2) PG72 (n = 426), heifers were exposed to the same treatment as PG54, however, TAI was performed 72 ± 2 h after CIDR removal; 3) PG-CIDR54 (n = 422), same as PG54 but heifers received a CIDR insert on Day -14 rather than Day -7, in addition to PGF administration; 4) PG-CIDR72 (n = 418), same as PG-CIDR54, however, TAI was performed 72 ± 2 h after CIDR removal. Estrus detection patches were applied to all heifers on Day 0 and were evaluated for activation at TAI. Pregnancy was diagnosed via transrectal ultrasonography between 30 and 47 d after TAI. The percentage of heifers exhibiting estrus between Day 0 and TAI was greater (P < 0.01) in the PG72, PG-CIDR54, and PG-CIDR72 treatments compared to the PG54 treatment (78.11, 86.59, and 91.09 vs. 31.05%, respectively). Furthermore, estrus response was greater (P < 0.01) in PG-CIDR72 heifers when compared to PG72. Pregnancy rates to TAI differed among treatments and were greater (P < 0.05) in the PG72 and PG-CIDR54 treatments when compared to PG-CIDR72 (48.8 and 50.4 vs. 38.4%, respectively), and were greater (P = 0.03) in PG-CIDR54 vs. PG54 (43.1%). Moreover, a tendency (P = 0.10) was determined on PR/AI between PG54 and PG72. In conclusion, presynchronization strategies and prolonged exposure to exogenous progesterone have the potential to alter estrus expression and improve fertility in replacement beef heifers.

Presynchronization with CIDR, with or without GnRH, prior to CO-Synch in beef heifers

Objectives were to compare ovarian responses and pregnancy per AI (P/AI) in Angus-cross beef heifers (n = 521; 4 locations) synchronized with CIDR-CO-Synch (CCOS) versus CIDR-GnRH-CO-Synch (CGCOS) protocols. Heifers were assigned a reproductive tract score (RTS: 1, immature, acyclic; 5, mature, cyclic), body condition score (BCS: 1, emaciated; 9, obese) and temperament score (0, calm, 1, excitable). Heifers in the CCOS (n = 261) group received a CIDR on Day -20 (removed on Day -13), 100 μg GnRH on Day -10, 25 mg PGF2α on Day -3 and were timed inseminated 60 h later, with concomitant GnRH (Day 0). Heifers in the CGCOS (n = 260) group received a CIDR on Day -26 (removed on Day -19), 100 μg of GnRH on days -16 and -10, 25 mg of PGF2α on Day -3 and were timed inseminated 60 h later, with concomitant GnRH (Day 0). Ovarian ultrasonography was done in a subset of heifers (n = 60; 30 in each group) to determine number and size of ovarian follicles and presence of corpus luteum (CL). There was increased (P < 0.05) percentage of heifers with CL in CGCOS group compared to heifers in CCOS group on Day -10 (82.3 vs 68.2%) and on Day -3 (88.3 vs 75.1%). Average size of the largest ovarian follicle on Day 0 was greater for heifers in CGCOS group compared to CCOS group (P < 0.05). However, P/AI did not differ between CCOS and CGCOS groups, 55.0% (143/260) and 59.8% (156/261), respectively (P > 0.1). In conclusion, CIDR presynchronization with or without GnRH (CCOS and CGCOS protocols) in beef heifers resulted in similar P/AI. Adding GnRH to presynchronization with CIDR resulted in more heifers with a CL at PGF2α and increased preovulatory follicle diameter at AI. Future studies are needed with bigger sample size and CIDR + CO-Synch treatment as control to determine economic benefit.

Cyclicity, estrus expression and pregnancy rates in beef heifers with different reproductive tract scores following progesterone supplementation

Two experiments were conducted to determine effects of progesterone (P4) on cyclicity, estrus expression rate (EER) and artificial insemination pregnancy rate (AIPR) in beef heifers with various reproductive tract scores (RTS; 1 to 5; 1, immature, acyclic; 5, mature, cyclic). In Experiment 1, Angus-cross heifers (n = 100, 20 per RTS category; mean (±SEM) age, 15 ± 0.8 mo) were randomly assigned to receive a CIDR (Days -17 to -10) or no CIDR (untreated control), with weekly blood samples and ultrasonography (Days 0-85). Among heifers with RTS 2 to 4, median interval to cyclicity were shorter (P < 0.05) for heifers in CIDR versus control. In Experiment 2, Angus-cross heifers (n = 11,098) were assigned RTS, body condition score (BCS; 1 to 9; 1, emaciated; 9, obese) and temperament score (calm versus excitable). Heifers with RTS 2-5 (n = 10,569) were allocated to CO-Synch (n = 5099) or CO-Synch + CIDR (n = 5470). Estrus was detected until AI (72 h after PGF2α), with pregnancy diagnosis ∼70 d later. Controlling for RTS (P < 0.0001), BCS (P < 0.0001), temperament (P < 0.0001), age (P < 0.0001), treatment by RTS (P < 0.01), treatment by BCS (P < 0.01), and treatment by age, EER differed between CO-Synch and CO-Synch + CIDR (71.0 vs 75.9%, respectively, P < 0.0001). Accounting for RTS (P < 0.0001), BCS (P < 0.0001), temperament (P < 0.0001), age (P < 0.0001), heifers detected in estrus (P < 0.0001), RTS by treatment (P < 0.01), BCS by treatment (P < 0.01), and age by treatment, AIPR differed between CO-Synch versus CO-Synch + CIDR (55.3 vs 61.0%, P < 0.0001). In conclusion, exogenous P4 hastened cyclicity in pre- and peri-pubertal beef heifers. Further, it increased EER and AIPR. However, RTS, BCS and age influenced EER and AIPR. Among RTS 4 and 5, EER was greater for CO-Synch + CIDR vs CO-Synch. Among RTS 3 to 5, AIPR was greater for CO-Synch + CIDR versus CO-Synch. Progesterone status or supplementation at onset of synchronization protocols was critical to pregnancy outcomes, emphasizing heifer development for early puberty or progesterone supplementation.

Altering duration of the presynchronization period in a long-term progestin-based estrus synchronization protocol for timed artificial insemination of beef heifers

An experiment was designed to evaluate the effect of extending duration of the presynchronization treatment in a long-term progestin-based estrus synchronization protocol. Heifers were assigned to either an 18 d (Day 0-18) or 14 d (Day 4 to Day 18) CIDR^® treatment (1.38 g progesterone controlled internal drug release insert; Zoetis, Madison, NJ), with prostaglandin F_2α (PG; 250 μg im cloprostenol sodium) administered 16 d after CIDR^® removal (Day 34). Heifers at two locations (location one, n = 193; location two, n = 649) were assigned to treatment based on reproductive tract score (RTS; Scale 1-5) and body weight. Heifers that were assigned RTS 1 were not retained for the trial (n = 6). Estrus detection aids (Estrotect^®) were applied at PG. Split-time artificial insemination (STAI) was utilized and AI performed based on expression of estrus at 66 h. Expression of estrus was defined as removal of ≥50% of the grey coating from the Estrotect^® patch. Heifers that expressed estrus at 66 h were inseminated then and heifers that had not expressed estrus were inseminated at 90 h. Only heifers that failed to express estrus by 90 h received gonadotropin-releasing hormone (GnRH; 100 μg im gonadorelin acetate) at the time of AI. At location one, blood samples were collected at PG and AI (66 h or 90 h) from all heifers to determine E₂ concentration by radioimmunoassay, and transrectal ovarian ultrasound was performed to detail ovarian structures on a subset of heifers (n = 73) at both time points. The proportion of heifers expressing estrus did not differ between treatments, either by 66 h (60%) or in total by 90 h (84%) after PG. Pregnancy rate to STAI did not differ between treatments (P = 0.3; 52%, 14-d CIDR^®-PG; 50%, 18-d CIDR^®-PG), or at the end of the 60 d breeding season (P = 0.2; 86%, 14-d CIDR^®-PG; 82%, 18-d CIDR^®-PG). No differences were detected in mean diameter of the dominant follicle at PG (P = 0.6; 10.9 ± 0.4 mm, 14-d CIDR^®-PG; 11.0 ± 0.4 mm, 18-d CIDR^®-PG) or at STAI (P = 0.3; 12.6 ± 0.4 mm, 14-d CIDR^®-PG; 13.2 ± 0.4 mm, 18-d CIDR^®-PG), nor were any differences observed between treatments in concentrations of E₂ at PG (P = 0.8; 1.1 ± 0.19 pg/ml, 14-d CIDR^®-PG; 1.1 ± 0.19 pg/ml, 18-d CIDR^®-PG) or STAI (P = 0.6; 3.8 ± 0.19 pg/ml, 14-d CIDR^®-PG; 3.6 ± 0.19 pg/ml, 18-d CIDR^®-PG). These data indicate that duration of CIDR^® treatment can be extended from 14 to 18 d, thus providing flexibility in scheduling without compromising reproductive outcomes.

Evaluation of split-time artificial insemination following administration of a long or short-term progestin-based estrus synchronization protocol in beef heifers

Fixed-time and split-time AI were compared following the melengestrol acetate (MGA^®) prostaglandin F_2α (Experiment 1) and 7-d CO-Synch + controlled internal drug release (CIDR^®) protocols (Experiment 2). Heifers in Experiments 1 (n = 524) and 2 (n = 456) were assigned within pen to balanced treatments based on weight and reproductive tract score (RTS; Scale 1-5). In Experiment 1, MGA^® (0.5 mg∙animal^-1∙d^-1) was fed for 14 d, and prostaglandin F_2α (PG; 250 μg im cloprostenol sodium) was administered 19 d after MGA^® withdrawal. In Experiment 2, gonadotropin-releasing hormone (GnRH; 100 μg gonadorelin acetate) was administered coincident with CIDR^® (1.38 g progesterone [P₄]) insertion. Inserts were removed after 7 d, and PG (250 μg im cloprostenol sodium) was administered at CIDR^® removal. In both experiments, estrus detection aids (Estrotect^®) were applied at the time of PG administration. Estrous status was recorded at FTAI or STAI. Estrus was defined as removal of ≥ 50% of the grey coating from the Estrotect^® patch. Heifers assigned to FTAI treatments received GnRH and were artificially inseminated at the standard time for FTAI for each protocol: 72 or 54 h after PG administration for the MGA-PG or 7-d CO-Synch + CIDR^® protocol, respectively. In the STAI treatments, only heifers that expressed estrus prior to the standard time of FTAI were artificially inseminated at that time. For heifers failing to express estrus, AI was postponed 24 h. Only heifers that failed to exhibit estrus by the delayed time received GnRH concurrent with AI. In both experiments, estrous response prior to the standard time of FTAI did not differ between treatments. Total estrous response was increased (P < 0.01) among heifers assigned to STAI in Experiment 1 (88%, STAI; 72%, FTAI) and 2 (74%, STAI; 47%, FTAI). In Experiment 1, pregnancy rates resulting from AI were greater (P < 0.04) for heifers assigned to STAI compared with FTAI (55% vs 46%, respectively). In Experiment 2, pregnancy rates resulting from AI were similar between treatments (48% and 46%, respectively; P = 0.6). In summary, when compared with FTAI, STAI resulted in greater estrous response following both the MGA^®-PG and 7-d CO-Synch + CIDR^® protocols. The increased estrous response through use of STAI was associated with a corresponding increase in pregnancy rates to AI following the MGA^®-PG protocol; however, a similar improvement in pregnancy rates was not observed following the 7-d CO-Synch + CIDR^® protocol.

Prostaglandin F2α 7 d prior to initiation of the 7-d CO-synch + CIDR protocol failed to enhance estrus response and pregnancy rates in beef heifers

To determine the effects of administration of 25 mg of PGF2α 7 d prior to the initiation of the 7-d CO-Synch + controlled internal drug release (CIDR) fixed-time AI (TAI) protocol, 985 Bos taurus beef heifers were enrolled in a completely randomized design at 9 locations from April to July of 2016. Within location, all heifers were randomly assigned to 1 of 2 treatments: 1) CONTROL (n = 496); 100 µg injection of GnRH and a CIDR insert for 7 d [day 7], administration of 25 mg of PGF2α at CIDR removal [day 0], followed by a second injection of GnRH and TAI 54 ± 2 h later; or 2) PRESYNCH (n = 489); same as CONTROL but heifers received an additional injection of 25 mg of PGF2α 7 d prior [day 14] to CIDR insertion. Estrous detection patches were applied to all heifers on day 14 and were evaluated for estrual activity on day 7. Similarly, estrus alert patches were placed on all heifers on day 0 and evaluated for estrual activity at the time of TAI. Pregnancy was diagnosed via transrectal ultrasonography between 35 and 55 d after TAI. The percentage of heifers exhibiting estrus between days 14 and 7 was greater (P < 0.001) for the PRESYNCH (70.1 ± 2.4%) than the CONTROL (41.1 ± 2.3%) treatment, whereas the percentage of heifers exhibiting estrus between day 0 and TAI was greater (P < 0.001) for the CONTROL (55.6 ± 2.4%) than the PRESYNCH (39.7 ± 2.5%) treatment. Estrus response rates differed (P < 0.001) among locations. Pregnancy rates to TAI differed (P = 0.023) among locations; however, they did not differ (P = 0.739) between CONTROL and PRESYNCH treatments (45.4 ± 2.5 vs. 43.2 ± 2.5%, respectively). Final breeding season pregnancy rates did not differ (P = 0.811) between treatments. Therefore, an injection of PGF2α 7 d prior to initiation of the 7-d CO-Synch + CIDR protocol failed to improve pregnancy rates to TAI in replacement beef heifers.

Effect of protein supplementation and forage allowance on the growth and reproduction of beef heifers grazing stockpiled tall fescue

Stockpiled tall fescue can provide adequate winter forage for beef cattle, although unsupplemented replacement heifers may display marginal performance before breeding. The objective of this study was to determine if protein supplementation and/or additional forage improves growth and reproductive performance of replacement heifers grazing stockpiled fescue. Cattle averaging 272 ± 1.59 kg were stratified by BW and then randomly assigned to 1 of 4 plots within a pasture replication. Treatment combinations were assigned in a 2 × 2 factorial arrangement and included 1) a conservative forage allocation ("normal," targeting 85% forage use) and mineral supplement (normal forage allocation with mineral supplement [FM]), 2) normal forage allocation with protein tub (FT), 3) more liberal forage allocation ("extra," targeting 70% forage use) and mineral supplement (extra forage allocation with mineral supplement [EM]), and 4) "extra forage allocation with protein tub (ET). Treatments were administered for 8 wk from early November to early January. Heifers were fed fescue hay for 1 wk before breeding in late January. Heifers were synchronized with the 7-d CO-Synch + controlled internal drug release device protocol and inseminated in late January. Heifers were checked for pregnancy by ultrasonography at 35 and 90 d after AI. Main and interaction effects between the 2 treatments were determined. Total supplement intake was greater for protein tub than mineral supplement (0.36 vs. 0.11 kg·heifer·d, respectively; < 0.0001), and the additional dietary protein in the tub groups resulted in greater serum urea N concentrations ( < 0.0001; 8.15 vs. 10.4 mg/dL for mineral and protein tub, respectively). Forage utilization efficiency was greater for normal than extra forage allocation (74.7 vs. 65.8%, respectively; < 0.0001). Main effects of both treatments on ADG were significant ( < 0.0001; 0.28, 0.43, 0.43, and 0.51 kg·heifer·d for FM, FT, EM, and ET, respectively). There was an interaction effect of the 2 treatments on change in BCS ( < 0.05; 0.12, 0.10, 0.18, and 0.31 for FM, FT, EM, and ET, respectively). Reproductive tract scores, pelvic area, and AI pregnancy rates were not different between treatments ( > 0.05). Overall, feeding a protein supplement or providing extra forage increased gain and interacted to increase BCS but did not have an effect on reproductive performance. Supplementing with protein and providing extra forage are strategies that can increase gain in heifers, which could aid heifers in reaching puberty before estrous synchronization.

Site of PGF2α injection does not alter effectiveness of the Select Synch + controlled internal drug release and timed artificial insemination protocol

Beef Quality Assurance programs have contributed to significant improvements in the wholesomeness of beef available for consumption. Injection site blemishes in the round have declined since the promotion of administering intramuscular injections in the neck. Unfortunately, many producers continue to administer estrus synchronization (ES) drugs in the rump. The objective of this study was to compare the effectiveness of injection site of PGF_2α, in ES protocols, on steroid hormone concentrations and pregnancy rates. A Select Synch + 7-day controlled internal drug release ES protocol was conducted with the site of PGF_2α injection alternated between neck and rump in beef cattle (n = 312) at the Ohio State University Agricultural Technical Institute and North Carolina State University. Blood samples (n = 75) were collected at controlled internal drug release insertion and at the time of artificial insemination (AI) to determine if progesterone (P4) and estrogen (E2) concentrations varied due to PGF_2α injection site. All cattle were confirmed pregnant by ultrasonography at approximately 30 and 90 days after insemination in North Carolina and approximately 70 days after insemination in Ohio. Data were analyzed as randomized complete block designs in PROC GLIMMIX with animal as the experimental unit. Differences were declared significant at P < 0.05. Site of PGF_2α injection, in either the neck or rump, did not affect (P > 0.05) overall conception rates in response to AI (58.4% and 55.6%, respectively). Altering PGF_2α injection site did not impact P4, E2 concentrations, or the P4:E2 ratio at AI (P > 0.05). However, cattle inseminated after displaying estrus had greater (P < 0.05) pregnancy rates than timed AI (67.8 vs. 47.5%, respectively). First service conception rates and pregnancy rates were consistent with previous reports. Overall, altering the location of the PGF_2α injection during ES did not change circulating hormone concentrations at AI or pregnancy rates; therefore, cattle producers should follow Beef Quality Assurance guidelines when administering ES protocols.

Comparing strategies to synchronize estrus before fixed-time artificial insemination in primiparous 2-year-old beef cows

Two experiments evaluated controlled internal drug release (CIDR)-based protocols to synchronize estrus in primiparous 2-year-old beef cows. In each experiment, treatments were balanced according to body condition score and days postpartum. Experiment 1 compared the 14-day CIDR-PG (14-d) and 7-day CO-Synch + CIDR (7-d) protocols on the basis of estrous response, pregnancy rates after fixed-time artificial insemination (FTAI), and final pregnancy rate. Cows assigned to 14-d (n = 355) received a CIDR insert on Day 0 with removal on Day 14. Cows assigned to 7-d (n = 349) received gonadotropin releasing hormone (GnRH) and a CIDR insert on Day 23. On Day 30, CIDRs were removed from 7-d cows, and PGF_2α was administered to all cows in each treatment. On Day 33, GnRH was administered concurrent with FTAI at 66 and 72 hours after PGF_2α for 7-d and 14-d treated cows, respectively. Estrous response before FTAI was higher for 7-d compared with 14-d cows (74% vs. 43%, respectively; P < 0.0001); however, pregnancy rates resulting from FTAI were similar (14-d 63%; 7-d 64%; P = 0.52). Ovarian follicular dynamics and serum estradiol-17β concentrations were evaluated among a subset of cows assigned to each protocol. Dominant follicle diameter was smaller at PGF_2α (P = 0.04) and FTAI (P = 0.002) among 14-d cows compared with 7-d cows; however, estradiol-17β at PGF_2α (P = 0.06) and FTAI (P = 0.001) was greater for 14-d versus 7-d treated cows. Experiment 2 compared estrous response and pregnancy rates in 2-year-old beef cows after FTAI- or split-time artificial insemination (STAI) following synchronization of estrus with the 14-day protocol. Cows assigned to FTAI (n = 266) were inseminated at a fixed time concurrent with GnRH at 72 hours after PGF_2α regardless of estrus expression, whereas cows assigned to STAI (n = 257) were inseminated based on estrus expression as determined by activation of an estrus detection aid. Cows assigned to STAI that exhibited estrus by 72 hours were inseminated; however, AI was delayed until 24 hours after GnRH (96 hours after PGF_2α) for nonestrous cows. Total estrous response was increased for STAI- versus FTAI-treated cows (STAI 64%; FTAI 42%; P < 0.0001); pregnancy rates resulting from AI were similar (STAI 55%; FTAI 56%; P = 0.60). In summary, the 14-day CIDR-PG and 7-day CO-Synch + CIDR protocols can be used effectively to synchronize estrus before FTAI in primiparous 2-year-old beef cows. Although expression of estrus was increased using STAI in conjunction with the 14-day protocol, this approach did not increase pregnancy rates compared with FTAI.

Synchronization and Artificial Insemination Strategies in Beef Cattle

Utilization of estrus or ovulation synchronization and fixed-timed artificial insemination (TAI) has facilitated the widespread utilization of artificial insemination (AI) and can greatly impact the economic viability of cow-calf systems by enhancing weaning weights. Implementation of TAI programs by beef producers results in limited frequency of handling cattle and elimination of the need to detect estrus. Continued use of intensive reproductive management tools such as estrus synchronization and AI will result positive changes to calving distribution, pregnancy rates, and subsequent calf value.

The 9-day CIDR-PG protocol: Incorporation of PGF2α pretreatment into a long-term progestin-based estrus synchronization protocol for postpartum beef cows

A pilot experiment was designed to test the hypothesis that administration of PGF2α before progestin treatment would allow for a reduced duration of progestin treatment in a long-term progestin-based estrus synchronization protocol. A modified presynchronization treatment was compared with 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 artificial insemination (TAI) in postpartum beef cows. Estrous was synchronized for 85 cows, with cows assigned to one of two treatments based on age, days postpartum, and body condition score. Cows assigned to the 14-day CIDR-PG protocol received a CIDR insert (1.38 g progesterone) on Day 0, CIDR removal on Day 14, and administration of PGF2α (25 mg im) on Day 30. Cows assigned to the 9-day CIDR-PG protocol received PGF2α concurrent with CIDR insertion on Day 5, PGF2α concurrent with CIDR removal on Day 14, and administration of PGF2α on Day 30. In both treatments, split-time AI was performed based on estrous response. At 72 hours after PGF2α (Day 33), cows having expressed estrus received TAI; cows that failed to express estrus by 72 hours received TAI 24 hours later (96 hours after PGF2α on Day 34), with GnRH (100 μg im) administered to nonestrous cows. Estrus-detection transmitters were used from CIDR removal until AI to determine onset time of estrus expression both after CIDR removal and after PGF2α. Ovarian ultrasonography was performed at CIDR removal on Day 14, PGF2α on Day 30, and AI on Days 33 or 34. At CIDR removal on Day 14, diameter of the largest follicle present on the ovary was similar between treatments. The proportion of cows expressing estrus after CIDR removal tended to be higher (P = 0.09) among cows assigned to the 9-day CIDR-PG treatment (93%; 40 of 43) than among cows assigned to the 14-day CIDR-PG treatment (81%; 34 of 42). After PGF2α, a significantly higher proportion (P = 0.02) of cows expressed estrus after synchronization with the 9-day CIDR-PG treatment (91%; 39 of 43) than the 14-day CIDR-PG treatment (69%; 29 of 42). Consequently, pregnancy rate to TAI tended to be increased (P = 0.09) among the 9-day CIDR-PG treatment (76.7%; 33 of 43) compared with the 14-day CIDR-PG treatment (59.5%; 25 of 42). In summary, a long-term CIDR-based estrous synchronization protocol for postpartum beef cows was enhanced through administration of PGF2α at CIDR insertion and CIDR removal.

Comparison of pregnancy rates in beef cattle after a fixed-time AI with once- or twice-used controlled internal drug release devices

The use of fixed-time artificial insemination (FTAI) provides producers with numerous benefits including the use of superior genetics, shorter breeding and calving seasons, and a more uniform calf crop. However, the cost of implementing FTAI protocols is one of the several drawbacks hindering their use in the beef industry. Potential injection-site lesions from intramuscular injections of the hormones necessary for estrus synchronization are also a cause of concern for carcass quality. The objectives of this experiment were to (1) determine whether or not a twice-used controlled internal drug release (CIDR) device would be effective in an FTAI protocol without adversely affecting pregnancy rate and (2) whether or not the subcutaneous administration of PGF2α affects pregnancy rate. Nulliparous females (n = 99) between 13 and 27 months of age and multiparous cows (n = 43) between 48 and 74 months of age were synchronized for estrus using the 7-day CO-Synch + CIDR protocol. The females were randomly assigned to one of the two treatments: (1) a once-used CIDR (control) or (2) a twice-used CIDR device (treatment) incorporated into their synchronization protocol. The females were also randomly assigned to have their injection of PGF2α administered either intramuscularly or subcutaneously. Blood was taken in a random subset of nulliparous females (n = 52) just before device removal and assayed for concentration of progesterone. The concentration of progesterone was higher (P = 0.01) in the animals that received once-used CIDR devices than that in those received twice-used CIDR devices (3.4 ± 0.5 and 1.4 ± 0.5 ng/mL, respectively). There was no significant effect of parity (P = 0.82), artificial insemination technician (P = 0.60), PGF2α administration (P = 0.83), or treatment (P = 0.67) on pregnancy rates to artificial insemination which were 75.4 ± 6.0% and 71.7 ± 6.4%, for animals that received once- and twice-used CIDR devices, respectively. This study provides evidence that although concentration of progesterone is decreased in animals treated with a twice-used CIDR device, there is still a sufficient release of progesterone from the device to effectively synchronize estrus without adversely affecting the fertility of a herd.

Comparison of three CIDR-based fixed-time AI protocols in beef heifers

Several effective fixed-time AI (FTAI) protocols have been developed to facilitate AI in beef heifers that circumvent the need for estrus detection. Among these are the 5-d CO-Synch + controlled intravaginal progesterone insert (CIDR) protocol (5dCO), PGF2α (PG) 6-d CIDR protocol (PG-6dCIDR), and 14-d CIDR-PG protocol (14dCIDR-PG). Although each of these protocols varies in duration and approach to synchronizing estrus and ovulation, each has been reported as an effective method to facilitate FTAI in beef heifers. Therefore, the objective of this study was to compare FTAI pregnancy rates in beef heifers synchronized with these 3 CIDR-based protocols. Virgin beef heifers (n = 801) at 4 locations were synchronized with 1 of 3 protocols: 1) 5dCO, an injection of GnRH (100 μg) and insertion of a CIDR on d -5, PG (25 mg) and CIDR removal on d 0 with a second injection of PG (>4 h after CIDR removal) on d 0 and FTAI at 72 h after CIDR removal, 2) PG-6dCIDR, PG (25 mg) on d -9, GnRH (100 μg) and insertion of a CIDR on d -6, PG and CIDR removal on d 0, and FTAI at 66 h after CIDR removal, or 3) 14dCIDR-PG, a 14-d CIDR insert from d -30 to -16, PG (25 mg) on d 0, and FTAI at 66 h after PG. All heifers received an injection of GnRH (100 μg) concurrent with FTAI. Timing of treatment initiation was offset to allow all heifers to receive FTAI concomitantly and at random. Pregnancy success was determined between 35 and 40 d after FTAI by transrectal ultrasonography. Blood samples were collected before the beginning of each protocol and at the initiation of each protocol to determine estrous cycling status (77%). Data were analyzed using the GLIMMIX procedures of SAS. As expected, because of the duration of protocols, fewer heifers in the 14dCIDR-PG treatment were pubertal at initiation of synchronization than in the 5dCO (P < 0.05) and PG-6dCIDR (P = 0.10) treatments. Fixed-time AI pregnancy success did not differ between treatments (P = 0.14; 62.6%, 56.9%, and 53.3% for 5dCO, PG-6dCIDR, and 14dCIDR-PG, respectively). However, heifers that had reached puberty by initiation of synchronization had greater (P < 0.01) pregnancy success compared to heifers that were prepubertal (60.7% and 47.3%, respectively). In summary, all 3 protocols had similar FTAI pregnancy success, and puberty status had the greatest impact on pregnancy success.

Luteal function, largest follicle, and fertility in postpartum dairy cows treated with 14dCIDR-PGF2α versus 2xPGF2α-Ovsynch for timed AI

A method for timed artificial insemination (AI) that is used for beef cows, beef heifers, and dairy heifers employs progesterone-releasing inserts, such as the controlled internal drug release (CIDR; Zoetis, New York, NY, USA) that are left in place for 14 days. The 14-day CIDR treatment is a method of presynchronization that ensures that cattle are in the late luteal phase of the estrous cycle when PGF2α is administered before timed AI. The objective of this study was to test the effectiveness of the 14dCIDR-PGF2α program in postpartum dairy cows by comparing it with the traditional "Presynch-Ovsynch" (2xPGF2α-Ovsynch) program. The 14dCIDR-PGF2α cows (n = 132) were treated with a CIDR insert on Day 0 for 14 days. At 19 days after CIDR removal (Day 33), the cows were treated with a luteolytic dose of PGF2α, 56 hours later were treated with an ovulatory dose of GnRH (Day 35), and 16 hours later were inseminated. The 2xPGF2α-Ovsynch cows were treated with a luteolytic dose of PGF2α on Day 0 and again on Day 14. At 12 days after the second PGF2α treatment (Day 26), the cows were treated with GnRH. At 7 days after GnRH, the cows were treated with PGF2α (Day 33), then 56 hours later treated with GnRH (Day 35), and then 16 hours later were inseminated. There was no effect of treatment or treatment by parity interaction on pregnancies per AI (P/AI) when pregnancy diagnosis was performed on Day 32 (115/263; 43.7%) or Days 60 to 90 (99/263; 37.6%) after insemination. There was an effect of parity (P < 0.05) on P/AI because primiparous cows had lesser P/AI (35/98; 35.7%) than multiparous cows (80/165; 48.5%) on Day 32. Cows observed in estrus after the presynchronization step (within 5 days after CIDR removal or within 5 days after the second PGF2α treatment) had greater P/AI than those not observed in estrus (55/103; 53.4% vs. 60/160; 37.5%; observed vs. not observed; P < 0.01; d 32 pregnancy diagnosis). When progesterone data were examined in a subset of cows (n = 208), 55.3% of cows had a "prototypical" response to treatment (i.e., the cow had an estrous cycle that was synchronized by the presynchronization treatment and then the cow responded appropriately to the subsequent PGF2α and GnRH treatments before timed AI). Collectively, cows with a prototypical response to either treatment had 52.2% P/AI that was greater (P < 0.001) than the P/AI for cows that had a nonprototypical response (19%) (P/AI determined at 60-90 days of pregnancy). In conclusion, we did not detect a difference in P/AI when postpartum dairy cows were treated with 14dCIDR-PGF2α or 2xPGF2α-Ovsynch before timed AI. The primary limitation to the success of either program was the failure of the cow to respond appropriately to the sequence of treatments.

Hot topic: Comparison of sex-sorted and conventional semen within a fixed-time artificial insemination protocol designed for dairy heifers

The objective was to compare pregnancy per AI (P/AI) with conventional (CON) or sex-sorted (SS) semen from a single sire within a fixed-time AI (FTAI) program designed for dairy heifers. Holstein heifers (n=240) were assigned to treatment (CON or SS) according to body weight and reproductive tract score. All heifers underwent FTAI by using the "Show-Me-Synch" protocol [controlled internal drug release (CIDR) insert from d 0 to 14 followed by PGF(2α) (25mg i.m.) 16d after insert removal (d 30) with GnRH (100 µg i.m.) and FTAI at 66 h after PGF(2α)]. A single professional technician performed the FTAI. Heifers were fitted with heat detection patches at PGF(2α) to characterize estrous response. Estrous response did not differ between CON (63/120; 53%) and SS (70/120; 58%) treatments. The CON heifers, however, achieved greater FTAI P/AI (82/120; 68%) compared with SS (45/120; 38%) heifers. The P/AI did not differ for CON heifers that exhibited or failed to exhibit estrus before FTAI [44/63 (70%) vs. 38/57(67%), respectively]. For SS heifers, however, those that exhibited estrus had greater P/AI compared with those that failed to exhibit estrus [32/70 (46%) vs. 13/50 (26%)]. Pregnancy per AI resulting from FTAI was greater for heifers that were inseminated with CON semen compared with those that received SS semen. The expression of estrus before FTAI did not affect P/AI when CON semen was used, whereas the P/AI with SS semen was greater for heifers detected in estrus. Further studies are required to develop strategies for using sex-sorted semen when inseminating heifers at predetermined fixed times on the basis of expression of estrus before FTAI.

Evaluation of the 5-day versus a modified 7-day CIDR breeding program in dairy heifers

Dairy heifers were used to compared the effects of two timed AI + controlled internal drug release (CIDR) protocols (5-day vs. a modified 7-day) on: (1) luteal regression to initiate a new ovarian follicular wave; (2) ovarian response to the initial GnRH injection; and (3) pregnancy outcomes. Holstein heifers (N = 543) were assigned randomly to two treatments: (1) 25 mg PGF(2α) (im) and a CIDR insert on Day -7 followed by 100 μg of GnRH (GnRH-1) on Day -5 and 25 mg PGF(2α) (im) at CIDR insert removal (7-day [7D]) on Day 0; or (2) 100 μg GnRH (GnRH-1) and insertion of a CIDR on Day -5 and 25 mg PGF(2α) (im) at CIDR removal (5-day [5D]) on Day 0. Insemination with frozen-thawed conventional or gender-biased semen occurred after detected estrus from Days 0 to 2 or by appointment at 72 h after PGF(2α) when a second 100-μg dose of GnRH was given. Blood was collected on Days -7, -5, 0, and 3 to determine concentrations of progesterone and incidence of luteolysis. Ovaries were scanned on Days -5 and 0. Luteolysis in the 7D treatment by 48 h after the initial PGF(2α) was greater (P < 0.01) than what occurred spontaneously in the 5D treatment (36.2% vs. 19.7%, respectively). Incidence of ovulation after GnRH-1 on Day -5 was greater (P < 0.05) for 7D than for 5D heifers, but the proportion of heifers with an induced CL on Day 0 did not differ between treatments. Heifers inseminated after detected estrus (166/543, 30.6%) on Days 0, 1, and 2 had greater (P < 0.05) pregnancy per AI (P/AI) at 32 days post AI than after timed AI (38.2% vs. 28.3%) on Day 3. Pregnancy P/AI, however, was greater (P < 0.05) for 7D heifers inseminated at estrus (46.5%) than for 7D heifers receiving the timed AI (26.8%) and differed (P < 0.05) from all 5D heifers regardless of insemination time at estrus (30.5%) or at timed AI at 72 h (29.9%). At the Florida location in which conventional and sexed semen were used during two breeding clusters, P/AI using sexed semen (43.9%, N = 56) did not differ from that of conventional semen (21.2%, N = 50). Remaining replicates of sexed semen produced similar P/AI at the other two locations (sexed = 27.6%, N = 71; and sexed = 31.9%, N = 215). We concluded that the modified 7-day CO-Synch + CIDR program produced more P/AI in heifers inseminated at estrus than a standard 5-day CO-Synch + CIDR program, but when timed AI occurred at 72 h after PGF(2α) and CIDR insert removal, P/AI did not differ between programs.

Physiology and Endocrinology Symposium: Harnessing basic knowledge of factors controlling puberty to improve synchronization of estrus and fertility in heifers

The development of replacement heifers is a major economic investment for all beef and dairy operations. The costs associated with heifer development cannot be recovered if heifers do not conceive and remain productive in the herd; therefore, heifers need to conceive early in the breeding season or risk being culled. Previous research has reported up to a 21% increase in fertility from pubertal estrus to the third estrus of a heifer. The use of reproductive tract scores to determine pubertal status has demonstrated that peripubertal and pubertal heifers have increased pregnancy success to estrous synchronization compared with heifers that were prepubertal. The development of RIA has allowed accurate measurement of peripheral blood hormone concentrations associated with the pubertal process and luteal formation. This basic knowledge has increased our understanding of the mechanisms that control puberty in heifers. In addition, understanding the hormonal changes that occur during the estrous cycle has allowed for the development of estrous synchronization protocols that result in increased control of follicular growth, regression of luteal tissue, and ovulation. Transrectal ultrasonography has increased our understanding of follicular waves; this understanding led to research investigating the endocrine regulation of follicular waves and development of methods to synchronize follicular waves for purposes of fixed-time AI. Current topics of research include the effect of antral follicle count on fertility and the effect of maternal nutrition (on the fetus in utero) on subsequent reproductive potential of a heifer (i.e., fetal programming). Advancements in genomic technologies will likely provide a powerful tool for selecting heifers at birth that will have a greater probability of being reproductively successful if managed correctly. Therefore, knowledge gained through basic research on factors that control puberty has improved and will continue to improve heifer development and fertility.

Influence of inducing luteal regression before a modified controlled internal drug-releasing device treatment on control of follicular development

At the initiation of most controlled internal drug-releasing (CIDR) device protocols, GnRH has been used to induce ovulation and reset follicular waves; however, its ability to initiate a new follicular wave is variable and dependent on stage of the estrous cycle. The objectives of the current studies were to determine 1) if inducing luteal regression before the injection of GnRH at time of insertion of a CIDR resulted in increased control of follicular development, and 2) if removing endogenous progesterone by inducing luteal regression before insertion of the CIDR decreased variation in LH pulse frequency. In Exp. 1 and 2, Angus-cross cycling beef heifers (n = 22 and 38, respectively) were allotted to 1 of 2 treatments: 1) heifers received an injection of PGF(2α) on d -3, an injection of GnRH and insertion of a CIDR on d 0, and a PGF(2α) injection and CIDR removal on d 6 (PG-CIDR) or 2) an injection of GnRH and insertion of a CIDR on d 0 and on d 7 an injection of PGF(2α) and removal of CIDR (Select Synch + CIDR). In Exp. 3, Angus-cross beef heifers (n = 15) were assigned to 1 of 3 treatments: 1) PG-CIDR; 2) PGF(2α) on d -3, GnRH on d 0, and PGF(2α) on d 6 (PG-No CIDR); or 3) Select Synch + CIDR. Follicular development and ovulatory response were determined by transrectal ultrasonography. Across all experiments, more (P = 0.02) heifers treated with PG before GnRH initiated a new follicular wave after the injection of GnRH compared with Select Synch + CIDR-treated heifers. In Exp. 1, after CIDR removal, interval to estrus did not differ (P = 0.18) between treatments; however, the variance for the interval to estrus was reduced (P < 0.01) in PG-CIDR heifers compared with Select Synch + CIDR heifers. In Exp. 3, there was a tendency (P = 0.09) for LH pulse frequency to be greater among PG-CIDR and PG-No CIDR compared with the Select Synch + CIDR, but area under the curve, mean LH concentrations, and mean amplitude did not differ (P > 0.76). In summary, induction of luteal regression before an injection of GnRH increased the percentage of heifers initiating a new follicular wave. Removal of endogenous progesterone tended to increase LH pulse frequency, and the modified treatment increased the synchrony of estrus after CIDR removal.

Comparison of long-term progestin-based protocols to synchronize estrus before fixed-time artificial insemination in beef heifers

Two experiments were conducted to compare pregnancy rates resulting from fixed-time AI (FTAI) after administration of 1 of 2 long-term controlled internal drug release (CIDR)-based protocols. Heifers were assigned to treatment by age, BW, and pubertal status. The CIDR Select-treated heifers (Exp. 1, n = 37; Exp. 2, n = 192) received a CIDR (1.38 g of progesterone) from d 0 to 14, followed by 100 µg of GnRH, intramuscularly (i.m.) 9 d after CIDR removal (d 23) and PGF(2α) (25 mg, i.m.) 7 d after GnRH treatment (d 30). Heifers assigned to the Show-Me-Synch protocol (Exp. 1, n = 40; Exp. 2, n = 200) received a CIDR from d 0 to 14, followed by PGF(2α) 16 d later (d 30). Artificial insemination was performed at 72 or 66 h after PGF(2α) treatment for the CIDR Select- and Show-Me-Synch-treated heifers, respectively, and each heifer was given GnRH (100 µg, i.m.) at the time of AI. In Exp. 1, ovaries of each heifer were examined by transrectal ultrasonography on d 23 and 30 to characterize follicular dynamics. Follicles ≥5 mm and the presence of corpora lutea were recorded. On d 25, ovaries of each heifer were examined to characterize the status of dominant follicles recorded on d 23. Heifers were fitted with HeatWatch (DDx Inc., Denver, CO) estrus-detection transmitters at PGF(2α) to characterize estrus distribution up to FTAI. The diameter of dominant follicles on d 23 at PGF(2α) and on d 30, and the estrous response after PGF(2α) treatment up to the point of FTAI did not differ between CIDR Select- and Show-Me-Synch-treated heifers. Concentrations of progesterone in serum at PGF(2α) were greater (P = 0.07) in Show-Me-Synch- than CIDR Select-treated heifers (6.0 vs. 4.8 ng/mL, respectively). Pregnancy rates of heifers resulting from FTAI did not differ (P = 0.33) between CIDR Select- and Show-Me-Synch-treated heifers (CIDR Select, 59%; Show-Me-Synch, 70%). In Exp. 2, FTAI pregnancy rates tended (P = 0.07) to be greater in Show-Me-Synch-treated (62%) than in CIDR Select-treated (51%) heifers. Pregnancy rates at the end of the breeding season did not differ (P = 0.72; CIDR Select, 85%; Show-Me-Synch, 83%) between treatments. In summary, pregnancy rates resulting from FTAI were comparable for heifers assigned to each of the 2 long-term progestin-based protocols. The reduced treatment cost and animal handling associated with administration of the Show-Me-Synch protocol offer distinct advantages over the CIDR Select protocol despite similarities in pregnancy rates resulting from FTAI.

Control of the estrous cycle to improve fertility for fixed-time artificial insemination in beef cattle: a review

Early estrus-synchronization protocols focused on regressing the corpus luteum (CL) with an injection of PGF(2alpha) followed by detection of estrus or involved the use of exogenous progestins that prevent estrus from occurring. Later, protocols combining the use of PGF(2alpha) and exogenous progestins were developed. Gonadotropin-releasing hormone was utilized to control follicular waves, synchronize ovulation, or to luteinize large dominant follicles. Our research aimed to develop reliable protocols that 1) relied solely on fixed-timed AI (TAI); 2) required a maximum of 3 animal handlings, and 3) were successful in estrous-cycling and noncycling females. In cows, insertion of an intravaginal progesterone insert during the 7-d interval between the initial GnRH and PGF(2alpha) injections enhanced pregnancy rates by 9 to 10%. In a multi-location study, a TAI protocol yielded pregnancy rates similar to a protocol involving detection of estrus plus a fixed-time clean-up AI for females not detected in estrus (54 vs. 58%, respectively, for cows and 53 vs. 57%, respectively, for heifers). Initiation of estrous cycles in noncycling cows is likely the primary manner in which beef producers may improve fertility in response to estrus synchronization and TAI protocols. Treatment of noncycling females with progesterone and GnRH increases the percentage of cycling females and improves fertility to a TAI, but inducing cyclicity with hCG failed to enhance fertility in TAI protocols. Supplementing progesterone after TAI failed to increase pregnancy rates in beef cattle. In contrast, administration of hCG 7 d after TAI induced an accessory CL, increased progesterone, and tended to enhance pregnancy rates. Development of TAI protocols that reduce the hassle factors associated with ovulation synchronization and AI provide cattle producers efficient and effective tools for capturing selective genetic traits of economic consequences. Location variables, however, which may include differences in pasture and diet, breed composition, body condition, postpartum interval, climate, and geographic location, affect the success of TAI protocols.

Comparison of controlled internal drug release insert-based protocols to synchronize estrus in prepubertal and estrous-cycling beef heifers

The objective of the experiment was to examine the necessity of adding a GnRH injection to a 14-d controlled internal drug release (CIDR)-based protocol for synchronization of estrus in beef heifers that were prepubertal or estrous-cycling at the initiation of treatment. The hypothesis tested was that the addition of GnRH in a CIDR-based estrus synchronization protocol would increase the synchrony of estrus after PGF(2alpha) (PG). Beef heifers (n = 285) were assigned to 1 of 2 treatments within reproductive tract scores (2 or 3 = prepubertal; 4 or 5 = estrous-cycling) by age and BW. Heifers assigned to CIDR Select received a CIDR insert (1.38 g of progesterone) from d 0 to 14 followed by GnRH (100 microg, intramuscularly) on d 23 and PG (25 mg intramuscularly) on d 30. Heifers assigned to CIDR-PG received a CIDR insert from d 0 to 14 and PG on d 30. Heifers were fitted with a HeatWatch estrus detection system transmitter at the time of PG administration for continuous estrus detection during the synchronized period (0 to 144 h after PG); AI was performed 12 h after estrus onset. Estrous response did not differ (P = 0.43) between treatments (94% CIDR Select, 98% CIDR-PG). Mean interval to estrus after PG was 7 h shorter (P = 0.01) and variance for interval to estrus was reduced (P < 0.01) among CIDR-PG-treated compared with CIDR Select-treated heifers. Conception rate to AI tended (P = 0.09) to be greater for CIDR-PG heifers (67%) compared with CIDR Select heifers (58%), and AI pregnancy rate was greater (P = 0.05) for CIDR-PG heifers (66%) compared with CIDR Select heifers (55%). Final pregnancy rate at the end of the breeding season was similar for the 2 treatments (81% for both; P = 0.94). We conclude that the administration of GnRH 9 d after CIDR removal in the CIDR Select protocol is not required to facilitate an improvement in the synchrony of estrus in beef heifers.