J. Anim Sci. 2008. 86:1808-1818. doi:10.2527/jas.2008-0970
© 2008 American Society of Animal Science
ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Comparison of protocols to synchronize estrus and ovulation in estrous-cycling and prepubertal beef heifers1
N. R. Leitman*,
D. C. Busch*,
J. F. Bader
,
D. A. Mallory*,
D. J. Wilson*,
M. C. Lucy*,
M. R. Ellersieck
,
M. F. Smith* and
D. J. Patterson*,2
* Division of Animal Science, S132 ASRC, and
and
Agricultural Experiment Station, University of Missouri, Columbia 65211; and
Merial Limited, Fulton, MO 65251
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Abstract
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The objective of the experiment was to compare follicular dynamics, ovulatory response to GnRH, and synchrony of estrus and ovulation among estrous-cycling and prepubertal beef heifers synchronized with a controlled internal drug-release (CIDR)- based or GnRH-PGF2
(PG) protocol. Estrous-cycling beef heifers were randomly assigned to 1 of 4 treatments (C1, C2, C3, C4), and prepubertal beef heifers were randomly assigned to 1 of 2 treatments (P1, P2) by age and BW. Blood samples were taken 10 and 1 d before treatment to confirm estrous cyclicity status (progesterone 
0.5 ng/mL estrous cycling). The CIDR Select (C1, n = 12; P1, n = 14)-treated heifers received a CIDR insert (1.38 g of progesterone) from d 0 to 14, GnRH (100 µg, i.m.) on d 23, and PG (25 mg, i.m.) on d 30. Select Synch + CIDR (C2, n = 12; P2, n = 11)-treated heifers received a CIDR insert and GnRH on d 23 and PG at CIDR removal on d 30. The CIDR-PG (C3, n = 12)-treated heifers received a CIDR insert on d 23 and PG at CIDR removal on d 30. Select Synch (C4, n = 12)-treated heifers received GnRH on d 23 and PG on d 30. HeatWatch transmitters were fitted at CIDR removal (C1, C2, C3, P1, and P2) or at GnRH administration (C4) for estrus detection. Ultrasound was used to determine the response to GnRH and the timing of ovulation after estrus. Among the estrous-cycling heifers, ovulatory response to GnRH and estrous response did not differ (P > 0.05). Among the prepubertal heifers, more (P = 0.02) P1 heifers responded to GnRH than P2 heifers, but estrous response did not differ (P > 0.05). Among the estrous-cycling heifers, variance for interval to estrus after PG was reduced (P < 0.05) for C1 compared with each of the other treatments, and C2 was reduced (P < 0.05) compared with C3. Variance for interval to ovulation after PG was reduced (P < 0.05) for C1 compared with each of the other treatments. Among the prepubertal heifers, there was no difference (P > 0.05) in variance for interval to estrus or ovulation. Results from C1 and P1 (T1) and C2 and P2 (T2) were combined to compare T1 and T2 among mixed groups of estrous-cycling and prepubertal heifers. Response to GnRH was greater (P < 0.01; 81% T1 and 39% T2), and variances for interval to estrus and ovulation for T1 were reduced (P < 0.01) compared with T2. In summary, CIDR Select improved (P < 0.01) the synchrony of estrus and ovulation compared with Select Synch + CIDR.
Key Words: beef heifer • controlled internal drug-release insert • estrus synchronization • ovulation
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INTRODUCTION
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Although acceptable pregnancy rates to fixed-time AI (FTAI) are attainable in beef cows (Bader et al., 2005
; Larson et al., 2006), FTAI estrus synchronization protocols in beef heifers have yielded less desirable results (Dahlen et al., 2003; Lamb et al., 2006). Lower FTAI pregnancy rates in heifers are largely attributed to an inability to synchronize follicular waves with the same degree of success that is achievable in cows (Lamb et al., 2006). Research indicates that ovulatory response to GnRH in heifers is influenced by the day of the estrous cycle at GnRH administration and that presynchronization with a progestin [controlled internal drug-release (CIDR) insert] before a GnRH and PGF2 (PG) regimen may increase the proportion of dominant follicles that respond to GnRH and thereby improve the synchronization of follicular waves (Kojima et al., 2004; Schafer et al., 2006; Atkins et al., 2008). Other studies indicate that the addition of GnRH at CIDR insertion may be of limited value (Lamb et al., 2006). Recently, Busch et al. (2007) reported an increased estrous response, improved synchrony of estrus, and greater FTAI pregnancy rates in beef heifers that were presynchronized with a long-term (14-d) CIDR protocol compared with a short-term (7-d) CIDR protocol.
To date, no studies have been conducted in beef heifers to compare estrus and ovulatory responses to long- and short-term CIDR-based protocols and on the basis of their potential for facilitating the successful use of FTAI. The hypothesis tested was that presynchronization with a progestin before GnRH and PG would facilitate an improvement in synchrony of estrus and ovulation in beef heifers compared with a short-term CIDR-based or GnRH-PG estrus synchronization protocol. The objective of this experiment was to compare follicular dynamics, ovulatory response to GnRH, and synchrony of estrus and ovulation among estrous-cycling and prepubertal beef heifers synchronized with CIDR-based or GnRH-PG protocols.
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MATERIALS AND METHODS
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The experimental procedures were approved by the University of Missouri-Columbia Animal Care and Use Committee.
Experimental Design
Crossbred estrous-cycling (n = 48) beef heifers were randomly assigned to 1 of 4 treatments, and crossbred prepubertal (n = 25) beef heifers were randomly assigned to 1 of 2 treatments by age and BW. Heifers assigned to the CIDR Select protocol (C1, estrous-cycling heifers, n = 12; P1, prepubertal heifers, n = 14) received an Eazi-Breed CIDR insert (1.38 g of progesterone; Pfizer Animal Health, New York, NY) from d 0 to 14, followed by GnRH (100 µg, i.m.; Cystorelin, Merial, Athens, GA) on d 23 and PG (25 mg, i.m.; Lutalyse, Pfizer Animal Health) on d 30. Select Synch + CIDR (C2, estrous-cycling heifers, n = 12; P2, prepubertal heifers, n = 11)-treated heifers received a CIDR insert and GnRH on d 23 and PG at CIDR removal on d 30. The CIDR-PG (C3, estrous-cycling heifers, n = 12)-treated heifers received a CIDR insert on d 23 and PG at CIDR removal on d 30. Heifers assigned to Select Synch (C4, estrous-cycling heifers, n = 12) received GnRH on d 23 and PG on d 30 (Figure 1). Results from C1 and P1 (T1) and C2 and P2 (T2) were combined and analyzed to compare T1 and T2 among mixed groups of estrous-cycling and prepubertal beef heifers.
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Figure 1. Treatment schedule for heifers assigned to the controlled internal drug-release (CIDR) Select (C1, P1; T1 denotes combined results from C1 and P1 where "C" denotes estrous-cycling and "P" denotes prepubertal heifers), Select Synch + CIDR (C2, P2; T2 denotes combined results from C2 and P2), CIDR-PG (C3), and Select Synch (C4) protocols. Heifers assigned to C1 or P1 were equipped with an Eazi-Breed CIDR insert (1.38 g of progesterone) insert from d 0 to 14, GnRH (Cystorelin, Merial, Athens, GA; 100 µg, i.m.) on d 23, and PGF2 (PG; 25 mg, i.m.; Lutalyse, Pfizer Animal Health, New York, NY) on d 30. Heifers assigned to C2 or P2 were administered GnRH and received a CIDR insert on d 23 and PG at CIDR removal on d 30. Heifers assigned to C3 received a CIDR insert on d 23 and PG at CIDR removal on d 30. Heifers assigned to C4 received GnRH on d 23 and PG on d 30.
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Estrus Detection
Each heifer was fitted with a HeatWatch estrus detection transmitter (DDx Inc., Denver, CO) at the time of CIDR removal (C1, C2, C3, P1, and P2) or at GnRH administration (C4) for continuous estrus detection. Estrus was defined as heifers receiving 3 mounts, each of which was 2 s in duration, within a 4-h period. The synchronized period was designated as 0 to 144 h after PG, and transmitters were maintained on the heifers until ovulation was confirmed.
Ultrasonography
Ovaries were scanned by transrectal ultrasonography (Aloka 500V equipped with a 7.5-MHz linear array transducer, Aloka, Wallingford, CT) on d 22 and 23 to characterize follicular dynamics. Follicles 5 mm and the presence of corpora lutea were recorded. On d 25, ovaries were scanned to determine the response to GnRH (defined as disappearance of the dominant follicle). Ovaries were scanned daily from d 30 to estrus to monitor follicular dynamics. Ultrasonography was performed every 4 h until ovulation was confirmed (designated as disappearance of the dominant follicle) beginning 20 h after the onset of estrus. Time of ovulation was considered to be the mean time of ultrasound scans when the dominant follicle was last observed and no longer present.
Blood Collection and RIA
Initial treatment assignments were based on blood samples collected via jugular venipuncture that were obtained 10 and 1 d before treatment initiation to con- firm estrous cyclicity status. Prepubertal heifers assigned to the Select-Synch + CIDR protocol were resampled on treatment d 7, 14, 21, and 23 to reconfirm estrous-cyclicity status. Heifers were considered to be estrous cycling when progesterone concentrations were 0.5 ng/mL at any one of the sampling times. The authors acknowledge the potential for misclassification of heifers by cyclicity determined from 2 blood samples before treatment initiation and the use of progesterone values 0.5 ng/mL to confirm cyclicity. However, the potential for committing a type II error is greatly minimized, if not negated, in describing heifers as prepubertal by using a 0.5 ng/mL cutoff. Additionally, blood samples were collected daily from PG through estrus. Blood samples were allowed to clot and were stored at 4°C for 24 h. Serum was collected by centrifugation and was stored at -20°C until hormone assays were performed. Serum concentrations of estradiol 17-β (E2) and progesterone (P4) were determined by RIA. Serum E2 concentrations were determined by validated extraction assay (Kirby et al., 1997). Intra- and interassay coefficients of variation were 12.5 and 10.5%, respectively, with an assay sensitivity of 0.83 pg/mL. Serum P4 concentrations were determined with a Coat- A-Count kit (Diagnostic Products Corp., Los Angeles, CA; Kirby et al., 1997) with intra- and interassay coef- ficients of variation of 2.3 and 8.8%, respectively, and an assay sensitivity of 0.1 ng/mL.
Statistical Analyses
Differences in age, BW, intervals to estrus and ovulation, follicle size at GnRH and 20 h after estrus, and serum P4 concentrations among treatments were analyzed by one-way ANOVA (PROC GLM; SAS Inst. Inc., Cary, NC). Variances associated with intervals to estrus and ovulation were compared by performing an F-test (the greater variance divided by the smaller variance; Snedecor and Cochran, 1989). These variances were calculated to provide comparisons of the degree of synchrony of estrus and ovulation after treatment. Response to GnRH was analyzed by using the generalized linear models procedure of SAS (PROC GENMOD of SAS). Because of select treatments containing all positive or negative values, estrous responses were analyzed by using a contingency 
2 analysis (PROC FREQ of SAS). Follicle size and serum E2 concentrations at the time of PG injection and at 24 and 48 h after PG injection were analyzed by using repeated measures over time with the mixed model procedures of SAS (PROC MIXED) as outlined by Littell et al. (1998). Because variances for serum E2 concentrations were heterogeneous among treatments, a log10 transformation was done for testing differences. However, all tables and figures present actual values.
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RESULTS
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The number, age, and BW of heifers before the initiation of treatments are shown in Table 1. There was no difference among treatments for age or BW among the estrous-cycling heifers and the prepubertal heifers. However, the prepubertal heifers weighed less (P < 0.02) than the estrous-cycling heifers.
Synchrony of Estrus After CIDR Removal
There was no difference in estrous response after CIDR removal for the estrous-cycling or prepubertal heifers treated with the CIDR Select protocol (P = 0.09; Figure 2). The mean interval to estrus was shorter for the estrous-cycling heifers compared with the prepubertal heifers (P = 0.02); however, there was no difference in the variance for interval to estrus (P > 0.10; Table 2).
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Figure 2. Percentage of estrous-cycling (C) and prepubertal (P) heifers assigned to the controlled internal drug-release (CIDR) Select treatment in estrus after Eazi-Breed CIDR insert removal: C1 (black bar) and P1 (gray bar); NR = no response (no estrous response). See Figure 1 for a description of treatment protocols.
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Table 2. Estrous response after CIDR insert1 removal for estrous-cycling (C) and prepubertal (P) heifers treated with the CIDR Select protocol
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Follicular Dynamics
Among the estrous-cycling heifers, there was no difference (P > 0.05) in response to GnRH, but more P1 heifers responded to GnRH than P2 heifers (P = 0.02; Table 3). Mean follicle diameter did not differ (P > 0.10) among treatments at GnRH, PG, 24 h after PG, 48 h after PG, or 20 h after the onset of estrus. Table 4 summarizes the response to GnRH based on the day of the estrous cycle when GnRH was administered for C1 and P1. The majority of heifers were on d 8 of the estrous cycle at GnRH administration.
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Table 3. Response to GnRH and diameter of the dominant follicle (mean ± SE) among estrous-cycling (C) and prepubertal (P) heifers assigned to the various treatments
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Table 4. Response to GnRH based on day of the estrous cycle for estrous-cycling (C) and prepubertal (P) heifers assigned to the CIDR Select treatment
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Synchrony of Estrus and Ovulation After PG
There was no difference among treatments in the proportion of heifers exhibiting estrus during the synchronized period (P > 0.05) or in the mean intervals to estrus or ovulation (P > 0.20). Variance for the interval to estrus was reduced for C1 compared with C2 (P < 0.01), C3 (P < 0.05), and C4 (P < 0.05). Variance for the interval to estrus was also reduced for C3 compared with C2 (P < 0.05) and tended to be less for P2 compared with C2 (P = 0.06). Variance for the interval to ovulation was reduced for C1 compared with C2 (P < 0.01), C3 (P < 0.05), and C4 (P < 0.01). Variance for the interval to ovulation was also reduced for P2 compared with C2 (P < 0.05; Table 5).
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Table 5. Estrous response and interval from PGF2 to estrus and ovulation among estrous-cycling (C) and prepubertal (P) heifers assigned to the various treatments
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Serum Steroid Hormone Concentrations
Serum concentrations of P4 at GnRH administration were less for P2 compared with all other treatments (P < 0.02). This was expected, because heifers assigned to P2 were prepubertal and had no exogenous exposure to P4 before GnRH administration. At PG administration, P2 heifers had lower serum P4 concentrations than did P1 (P < 0.01), C1 (P < 0.02), C2 (P < 0.01), and C3 (P < 0.03) heifers. Heifers assigned to C4 had lower serum P4 concentrations than did C2 (P < 0.04) heifers at PG. There was a significant treatment x day interaction (P < 0.05) for serum E2 concentrations. At PG administration, P2-treated heifers tended to have greater (P = 0.06; Table 6) serum E2 concentrations than did P1 heifers. Forty-eight hours after PG administration, P1- treated heifers had greater (P < 0.02; Table 6) serum E2 concentrations than did P2 heifers. Serum E2 concentrations did not differ among the C1, C2, C3, and C4 treatments at PG or at 24 or 48 h after PG (Table 6).
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Table 6. Serum steroid hormone concentrations among estrous-cycling (C) and prepubertal (P) heifers assigned to the various treatments
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Combined Results for CIDR Select and Select Synch + CIDR
Results from C1 and P1 (T1) and C2 and P2 (T2) were combined and analyzed collectively to compare the CIDR Select and Select-Synch + CIDR protocols among mixed groups of estrous-cycling and prepubertal heifers. More T1 heifers ovulated in response to GnRH than did T2 heifers (P < 0.01; Table 7). There was no difference (P > 0.10) between treatments in estrous response during the synchronized period or in mean intervals to estrus or ovulation (Table 7). Figures 3 and 4 illustrate the distribution of estrus and ovulation after PG administration, respectively. Variances for intervals to estrus (P < 0.01) and ovulation (P < 0.01) were less for T1 compared with T2 (Table 7). It is important to note that there were no within-treatment differences between estrous-cycling and prepubertal heifers in the variance for interval to estrus or ovulation for T1 (P > 0.10; Table 5), but within-treatment differences in the variance for intervals to estrus (P = 0.06; Table 5) and ovulation (P < 0.05; Table 5) were detected for T2. Serum concentrations of P4 were greater for T1 than T2 at GnRH (P < 0.01) and PG injections (P < 0.05; Table 7). There were no significant differences in follicle diameter between the 2 treatments (P > 0.10; Figure 5). It is important to note that despite a lack of difference in follicle diameter between treatments, there were differences in serum concentrations of E2 between the 2 treatments, as indicated by the treatment x day interaction (P < 0.05; Figure 6). Serum concentrations of E2 tended to be greater (P = 0.06) for T2 than for T1 at PG. However, 48 h after PG, T1 had greater (P < 0.01) serum E2 concentrations than T2.
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Table 7. Ovulatory response to GnRH, serum progesterone concentrations, estrous response, interval from PGF2 to estrus and ovulation for estrous-cycling and prepubertal heifers assigned to the controlled internal drug-release (CIDR) Select (T1) and Select Synch + CIDR (T2) treatments
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DISCUSSION
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Estrus synchronization and AI are reproductive management techniques that have been commercially available to beef producers for more than 30 yr. However, US beef producers have been reluctant to implement these technologies in their operations despite the potential benefits of shortening the calving season, increasing calf uniformity, and incorporating proven superior genetics into the cow herd. The development of economical methods to synchronize estrus and ovulation that reduce time and labor inputs with resulting high fertility may increase the adoption of these technologies in US beef herds (Patterson et al., 2003). Recent research has revisited the concept of estrus synchronization in the development of systems that facilitate FTAI, thereby eliminating the time and labor associated with estrus detection (Larson et al., 2006; Busch et al., 2007; Schafer et al., 2007).
Recently, progesterone intravaginal inserts were approved by the US Food and Drug Administration (2002). Formal approved uses include for synchronization of estrus in suckled beef cows and replacement beef and dairy heifers, for advancement of first postpartum estrus in suckled beef cows, and for advancement of first pubertal estrus in replacement beef heifers (US Food and Drug Administration, 2002). Estrus synchronization protocols for beef heifers involving short-term CIDR treatment have produced inconsistent results when used in conjunction with FTAI (Lamb et al., 2006). This inconsistency is widely attributed to the inability to synchronize follicular waves successfully after administration of GnRH. Reports indicate that only 43 to 60% of beef and dairy heifers ovulated in response to GnRH (Macmillan and Thatcher, 1991; Pursley et al., 1995; Moreira et al., 2000; Atkins et al., 2008), whereas 64 to 75% of beef and dairy cows ovulated in response to a similar treatment (Geary et al., 1998; Thompson et al., 1999; El-Zarkouny et al., 2004). Lucy and Stevenson (1986) reported a reduced magnitude of GnRH-induced LH release in heifers compared with cows; however, among heifers on d 15 and 18 of the estrous cycle, the magnitude of GnRH-induced LH secretion was not associated with ovulatory response (Atkins et al., 2008).
In experiments conducted by New Zealand researchers, heifers received a 7-d CIDR with PG administered at CIDR removal, a 14-d CIDR, or a 21-d CIDR (Macmillan and Peterson, 1993). Although pregnancy rates differed based on the length of CIDR treatment, no attempt was made in these studies to use FTAI (Macmillan and Peterson, 1993). More recently, studies in beef heifers have shown improvements in synchrony of estrus after treatment with CIDR compared with melengestrol acetate-based estrus synchronization protocols (Kojima et al., 2004; Tauck et al., 2007). These improvements were attributed to the ability of the CIDR insert to deliver a known, constant dose of P4.
This experiment was conducted to compare estrus and ovulatory response to long- and short-term CIDRbased protocols to carefully evaluate their potential to facilitate FTAI in beef heifers. We hypothesized that presynchronization with a progestin before GnRH and PG administration would be more effective in successfully synchronizing estrus and ovulation compared with short-term CIDR-based or GnRH-PG estrus synchronization protocols. In the present study, a greater proportion of prepubertal heifers that were presynchronized with P1 (86%) ovulated in response to GnRH compared with the prepubertal heifers synchronized with P2 (36%). Furthermore, the combined results of T1 and T2 indicate that presynchronization with the long-term CIDR resulted in a greater ovulatory response (81% T1; 39% T2) among mixed groups of estrous-cycling and prepubertal heifers.
The greater response to GnRH among heifers treated with T1 may be explained by the greater degree of synchrony that resulted from presynchronization with P4 before the GnRH injection. Atkins et al. (2008) reported a relationship between response to GnRH and day of the estrous cycle, with the greatest response observed when heifers were administered GnRH on d 5 of the estrous cycle, followed by d 15, 10, 18, and 2. Schafer et al. (2006) reported similar results in that the greatest ovulatory response was observed when GnRH was administered on d 5 to 8 of the estrous cycle when heifers were presynchronized with a 14-d CIDR insert. Eighty-eight percent of T1-treated heifers were on d 7 or 8 of the estrous cycle at the time of the GnRH injection, with an overall 78% response.
Characterization of follicular diameters and steroid hormone profiles revealed no differences in diameter of the dominant follicle among the individual treatments at the time PG was administered or 24 or 48 h after PG. This is reflected in similar serum E2 concentrations among treatments at PG and 24 h after PG administration. Although there was no difference in follicle diameter 48 h after PG administration between P1 and P2, the P1-treated heifers had higher concentrations of E2 than did P2 heifers at this time. This difference may be attributed to differences in synchrony of estrus. Although not significantly different, heifers assigned to P2 displayed estrus 9.5 h earlier and had a greater variance associated with the interval from PG to estrus compared with those in P1, indicating a reduction in the degree of synchrony of estrus among short- versus long-term CIDR-treated heifers. Therefore, more P2 heifers may have undergone the LH surge sooner, and E2 production may have declined in comparison with P1 heifers.
When considering the results for T1 and T2, there was no difference in diameter of the dominant follicle at PG, but T2 tended to have greater serum E2 and lower serum P4 concentrations than T1. We assume that follicles in T2 grow and develop under low-circulating P4 concentrations and thus create greater circulating concentrations of E2. The lower P4 concentrations at the time of the GnRH and PG injections for T2 may have allowed for a high pulse frequency and low pulse amplitude release of LH. Increased LH pulse frequency reportedly stimulates E2 secretion by the follicle (Schallenberger et al., 1984; Walters et al., 1984). An important consideration in relation to serum E2 concentrations at PG is that a greater variance for interval to estrus was associated with the T2 heifers, and, furthermore, fewer T2 heifers ovulated in response to GnRH compared with T1 heifers (39 vs. 81%, respectively). Perhaps more of the follicles that failed to ovulate to GnRH among the T2 heifers continued to produce E2. At 48 h after the injection of PG, follicle diameters were similar for T1 and T2, but T1 had greater serum E2 concentrations. Stegner et al. (2004) also reported differences in E2 concentrations when follicular diameters were similar and suggested that the maturation rate of follicles depends on the hormonal milieu under which they develop, independent of measurable differences in follicle diameter.
Estrus synchronization protocols that facilitate FTAI must be capable of synchronizing follicular development and luteal regression, resulting in a highly synchronized estrus and ovulation. Although mean intervals to estrus and ovulation after PG administration did not differ among treatments, differences in the variances for intervals to estrus and ovulation were detected. Heifers treated with C1 had less variance associated with the intervals from PG to estrus and ovulation compared with C2, C3, and C4, suggesting that C1 may facilitate FTAI more effectively among estrous-cycling beef heifers. Less variance was associated with the interval to estrus for C3 than C2, but no difference in the variance for interval to ovulation was detected.
Although no difference in the variance for interval to estrus during the synchronized period was detected between C4 and the 2 other short-term protocols in the present study, C4 is generally not used to facilitate FTAI because of unacceptable pregnancy rates. Dahlen et al. (2003) reported a 22% pregnancy rate to FTAI in beef heifers when using a 6-d GnRH-PG regimen. In the present study, 25% (3/12) of heifers synchronized with C4 displayed estrus before PG. Approximately 5 to 15% of cows and heifers that are in the late luteal phase of the estrous cycle when GnRH is administered will display estrus before PG (Downing et al., 1998; Kojima et al., 2000; Atkins et al., 2008), which precludes the use of C4 in FTAI systems. In the present study, 1 of the 3 heifers that displayed estrus before PG ovulated in response to GnRH. Atkins et al. (2008) recently reported that a proportion of heifers that were on d 15 of the estrous cycle ovulated and formed an accessory corpus luteum in response to GnRH but displayed estrus before PG. This may have occurred as a result of inadequate P4 production by the accessory corpus luteum, premature regression of the accessory corpus luteum, or both (Keisler and Keisler, 1989; Atkins et al., 2008).
Busch et al. (2007) reported a greater synchrony of estrus and greater FTAI pregnancy rates for heifers synchronized with CIDR Select compared with CO-Synch + CIDR. The combined results of estrous-cycling and prepubertal heifers in T1 and T2 in the current experiment may help to explain these results. The improved synchrony of estrus and ovulation for T1-treated heifers appears to be associated with an increased response to GnRH (81%, T1; 39%, T2) and more effective control of the emerging follicular wave resulting from presynchronization with long-term CIDR treatment before GnRH and PG. Measurable differences in the resulting synchrony of estrus after treatment suggest that differences in pregnancy rates resulting from FTAI (Busch et al., 2007) occur as a consequence of these improvements in synchrony of estrus and ovulation when comparing CIDR Select-treated with CO-Synch + CIDR-treated heifers.
Perhaps the most important finding of the current experiment is that the variances for intervals to estrus and ovulation did not differ between the estrous-cycling and prepubertal heifers assigned to T1, but within-treatment differences in variances for interval to estrus were detected for T2. These results support the concept that presynchronization with a progestin before GnRH and PG is more effective in successfully synchronizing estrus and ovulation among mixed groups of estrous-cycling and prepubertal heifers.
In summary, the CIDR Select protocol improved the synchrony of estrus and ovulation compared with the Select Synch + CIDR, CIDR-PG, and Select Synch protocols among estrous-cycling beef heifers. More variance was associated with the intervals from PG to estrus and ovulation between prepubertal and estrous-cycling beef heifers synchronized with the Select Synch + CIDR protocol compared with the CIDR Select protocol. These data provide a possible explanation for differences in synchrony of estrus after treatment administration based on long- versus short-term CIDR-based protocols. Furthermore, these data support the findings of Busch et al. (2007), which clearly demonstrate improvements in FTAI pregnancy rates in beef heifers resulting from presynchronization with progesterone before GnRH and PG compared with the 7-d CO-Synch + CIDR protocol.
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Footnotes
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1 Contribution from the Missouri Agricultural Experiment Station. This project was supported by National Research Initiative Competitive Grant number 2005-55203-15750 from the USDA Cooperative State Research, Education, and Extension Service. The authors gratefully acknowledge Pfizer Animal Health (New York, NY) for providing the Lutalyse sterile solution and Eazi-Breed CIDR cattle inserts; Merial (Athens, GA) for providing the Cystorelin; and D. S. McAtee and J. J. D. Schreffler (University of Missouri Thompson Farm, Spickard, MO), R. L. Smoot (University of Missouri Greenley Research Center, Novelty, MO), and University of Missouri South Farm (Columbia, MO) for their dedicated support of this research. 
2 Corresponding author: pattersond{at}missouri.edu
Received for publication February 19, 2008.
Accepted for publication April 2, 2008.
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LITERATURE CITED
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Abstract
INTRODUCTION
