J. Anim Sci. 2007. 85:151-162. doi:10.2527/jas.2006-335
© 2007 American Society of Animal Science
ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Ovarian, hormonal, and reproductive events associated with synchronization of ovulation and timed appointment breeding of Bos indicus-influenced cattle using intravaginal progesterone, gonadotropin-releasing hormone, and prostaglandin F2
1
J. P. Saldarriaga*,
D. A. Cooper*,
J. A. Cartmill*,
J. F. Zuluaga*,
R. L. Stanko*,
and
G. L. Williams*,2
* Animal Reproduction Laboratory, Texas A&M University Agricultural Research Station, Beeville 78102; and
and
Department of Animal and Wildlife Sciences, Texas A&M University, Kingsville 78363
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Abstract
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The objectives of this study were to 1) compare cumulative pregnancy rates in a traditional management (TM) scheme with those using a synchronization of ovulation protocol (CO-Synch + CIDR) for timed AI (TAI) in Bos indicus-influenced cattle; 2) evaluate ovarian and hormonal events associated with CO-Synch + CIDR and CO-Synch without CIDR; and 3) determine estrual and ovulatory distributions in cattle synchronized with Select-Synch + CIDR. The CO-Synch + CIDR regimen included insertion of a controlled internal drug-releasing device (CIDR) and an injection of GnRH (GnRH-1) on d 0, removal of the CIDR and injection of PGF2
(PGF) on d 7, and injection of GnRH (GnRH-2) and TAI 48 h later. For Exp. 1, predominantly Brahman x Hereford (F1) and Brangus females (n = 335) were stratified by BCS, parity, and day postpartum (parous females) before random assignment to CO-Synch + CIDR or TM. To maximize the number of observations related to TAI conception rate (n = 266), an additional 96 females in which TM controls were not available for comparison also received CO-Synch + CIDR. Conception rates to TAI averaged 39 ± 3% and were not affected by location, year, parity, AI sire, or AI technician. Cumulative pregnancy rates were greater (P < 0.05) at 30 and 60 d of the breeding season in CO-Synch + CIDR (74.1 and 95.9%) compared with TM (61.8 and 89.7%). In Exp. 2, postpartum Brahman x Hereford (F1) cows (n = 100) were stratified as in Exp. 1 and divided into 4 replicates of 25. Within each replicate, approximately one-half (12 to 13) received CO-Synch + CIDR, and the other half received CO-Synch only (no CIDR). No differences were observed between treatments, and the data were pooled. Percentages of cows ovulating to GnRH-1, developing a synchronized follicular wave, exhibiting luteal regression to PGF, and ovulating to GnRH-2 were 40 ± 5, 60 ± 5, 93 ± 2, and 72 ± 4%, respectively. In Exp. 3, primiparous Brahman x Hereford, (F1) heifers (n = 32) and pluriparous cows (n = 18) received the Select Synch + CIDR synchronization regimen (no GnRH-2 or TAI). Mean intervals from CIDR removal to estrus and ovulation, and from estrus to ovulation were 70 ± 2.9, 99 ± 2.8, and 29 ± 2.2 h, respectively. These results indicate that the relatively low TAI conception rate observed with CO-Synch + CIDR in these studies was attributable primarily to failure of 40% of the cattle to develop a synchronized follicular wave after GnRH-1 and also to inappropriate timing of TAI/GnRH-2.
Key Words: artificial insemination • beef cattle • Bos indicus • estrus synchronization
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INTRODUCTION
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Several pharmacological protocols, developed primarily in Bos taurus females, are available currently for synchronization of ovulation in beef cattle (Patterson et al., 2003
; Stevenson et al., 2003a). The CO-Synch protocol (Geary and Whittier, 1998), which involves the use of GnRH and PGF2 (PGF) to synchronize ovulation, has been coupled with an exogenous source of progesterone, the controlled internal drug-releasing device (CIDR). Recently, this combination (CO-Synch + CIDR) has been reported to produce timed AI (TAI) conception rates consistently averaging 
50% (Lamb et al., 2001; Larson et al., 2004) in Bos taurus females.
These rates are greater than those that have been reported previously using other synchronization methods (Stevenson et al., 2003a). Improved outcomes have been linked to the ability of pretreatment with exogenous progesterone to facilitate induction of ovulation in a high proportion of anestrous cows (Stevenson et al., 2000) and to synchronize ovarian follicular waves (DeJarnette et al., 2001; Martinez et al., 2002). However, in environments that are predominantly subtropical to tropical, the need to utilize breed types with a Bos indicus-influence may reduce the efficiency of synchronization and TAI compared with Bos taurus (Lemaster et al., 2001; Hiers et al., 2003). This may occur as a result of greater excitability and susceptibility to stress in Bos indicus-influenced cattle when subjected to intense management or subtle differences in timing of ovarian events such as follicular wave emergence and preovulatory follicle diameters (Bo et al., 2003), or both.
The objectives of this study were to: 1) compare cumulative pregnancy rates using traditional management (TM) without synchronization and AI with those using a synchronization of ovulation protocol (CO-Synch + CIDR) for TAI in Bos indicus-influenced cattle; 2) evaluate specific ovarian, hormonal, and estrual events associated with CO-Synch + CIDR and CO-Synch without CIDR; and 3) determine estrual and ovulatory distributions in cattle synchronized with Select-Synch + CIDR.
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MATERIALS AND METHODS
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The Institutional Agricultural Animal Care and Use Committee of the Texas A&M University approved all procedures involving animals that were used in these studies.
Experiment 1: Pregnancy Rates in Bos indicus-Influenced Cattle Synchronized with CO-Synch + CIDR or Managed Traditionally
The specific objectives of this experiment were to determine reproductive performance after TAI in females synchronized with CO-Synch + CIDR vs. a TM scheme. All cattle (n = 431) were required to have a minimum BCS of 5 (1 to 9 scale, with 1 = emaciated and 9 = obese; Herd and Sprott, 1998) and if suckled, to be at least 50 d postpartum. Predominantly Brahman x Hereford (F1) and Brangus females (n = 335) were stratified by BCS, parity, and day postpartum (primiparous and pluriparous females only) at each location and assigned randomly in groups of not less than 25 to CO-Synch + CIDR or TM groups. Nulliparous heifers were 14.5 to 16 mo of age and parous females ranged from 2 (primiparous) to 16 yr of age. An additional 96 females that were similar but for which TM controls were not available for comparison were also treated with CO-Synch + CIDR to increase the number of observations for TAI conception in this group. However, the data from these cows were not used in comparing the COSynch + CIDR group to TM.
The CO-Synch + CIDR regimen included the insertion of a CIDR (Pfizer Animal Health, New York, NY) and an i.m. injection of GnRH (GnRH-1; 100 µg of Cystorelin, Merial, Athens, GA) on d 0, removal of the CIDR and i.m. injection of PGF (25 mg of Lutalyse; Pfizer Animal Health) on d 7, and injection of GnRH (GnRH-2) and TAI on d 9. Timing of AI in this study was based on previous reports indicating that AI at 48 h after CIDR removal was appropriate and would yield acceptable conception rates in the range of 50% or greater (Geary and Whittier, 1998; Lamb et al., 2001; Stevenson et al., 2003a).
Synchronized cows were placed with fertile bulls for 85 d beginning 5 to 7 d after TAI, whereas TM cows were placed with bulls on d 0 for 90 d. Bulls used for natural service had successfully passed a standard breeding soundness examination approximately 2 wk before the onset of breeding. In the CO-Synch + CIDR group, conception rates to TAI were determined by transrectal ultrasonography (Concept/MCV equipped with a 5/7.5 MHz linear probe, Dynamic Imaging, Livingston, UK) 30 d after TAI. The presence of both uterine fluid accumulation and an embryonic vesicle with a heartbeat were used as determinants of pregnancy. The final pregnancy rates were assessed by palpation per rectum 45 d after the end of the breeding season in both synchronized and TM groups to retrospectively estimate the cumulative pregnancy rate at 30 and 60 d of the breeding season.
Experiment 2. Follicular, Luteal, and Hormonal Characteristics of CO-Synch and CO-Synch + CIDR Synchronization Protocols in Bos indicus-Influenced Females
The specific objectives of this experiment were to a) characterize ovarian events associated with synchronization of ovulation using the CO-Synch treatment with (CO-Synch + CIDR) and without the CIDR; and b) describe the pattern of progesterone secretion and pituitary release of LH in response to GnRH-1 and -2. Pluriparous, postpartum Brahman x Hereford (F1) cows (n = 100) in 4 replicates of 25 females each were utilized. Criteria for inclusion in the study and stratification procedures were similar to those of Exp. 1. Cows were placed in pens measuring 25.6 x 9.6 m at 8 d before the onset of the treatments, with 5 cow-calf pairs per pen, and were fed according to National Research Council (NRC, 1996) recommendations for lactating beef cows. One-half of the cows within each replicate (n = 12 to 13) were allocated randomly to receive the CO-Synch + CIDR treatment as in Exp. 1, and the other half received the CO-Synch treatment only. Cows were placed with bulls after TAI, as described for Exp. 1.
Transrectal ultrasonography was performed every other day from d –8 to d 0, and then daily from d 0 until ovulation or d 12, whichever occurred first. All ultrasound examinations were performed by the same operator. Follicles greater than 6 mm in dia. as well as luteal structures were measured, and a picture of the dorsal and lateral view of each ovary was then obtained for further analysis. The dominant follicle was defined as the follicle that reached the largest diameter (Sirois and Fortune, 1988). Ovulation was defined as the sudden disappearance of a follicle within 2 consecutive ultrasound examinations and confirmation by the subsequent appearance of a morphologically identifiable corpus luteum (CL). Follicular regression was defined as the gradual reduction of follicular size until disappearance (Ginther et al., 1989a). Emergence of a follicular wave was determined retrospectively as the day the dominant follicle reached 4 to 5 mm; if the follicle was not detected until it was 6 to 7 mm, then the day before was considered as the day of emergence (Ginther et al., 1989b). Follicular growth rate was measured from the day of follicular wave emergence to the day of PGF injection. A synchronized follicular wave was considered to have occurred if it emerged between 1 and 4 d after GnRH-1. Follicular wave emergence occurring outside of this period was considered to be spontaneous. Luteal regression was defined as the progressive reduction in size of the CL until its disappearance.
Blood sampling followed the same time course as for transrectal ultrasonography. Samples were placed on ice immediately after collection until arrival at the laboratory. After arrival, the samples were removed from the ice and allowed to stand at room temperature for approximately 1 h before centrifugation. Samples were centrifuged at 1,854 x g for 30 min within the first 4 h of collection. Serum was collected and stored at –20°C until hormone analyses.
Concentrations of progesterone in serum were determined with a solid phase RIA using the Coat-A-Count assay kit (Diagnostic Products Corporation, Los Angeles, CA) as reported previously by this laboratory (Fajersson et al., 1999). Intra- and interassay CV for the progesterone assay were 10.6 and 10.8%, respectively (n = 7 assays), and the sensitivity was 0.05 ng/mL. Concentrations of LH were determined, as validated previously by McVey and Williams (1991), in blood samples collected during the first replicate at 0, 30, 60, and 120 min relative to GnRH injections on d 0 (GnRH-1) and d 9 (GnRH-2). Intra- and interassay CV for the LH assay were 4.28 and 6.51%, respectively (n = 2 assays), with an assay sensitivity of 0.1 ng/mL. All hormone determinations for a given animal were performed within the same assay.
Cows were observed for estrus 3 times daily from d 0 until ovulation or d 12, whichever occurred first, with the aid of androgenized cows. On d 12, all cows were returned to their pasture with clean-up bulls for a 90-d breeding period. Pregnancy evaluation was performed by transrectal ultrasonography at 30 to 32 d postAI, and pregnancy was reconfirmed by palpation per rectum 45 d after the bulls were removed.
Experiment 3. Distribution of Estrus and Ovulation in Cows Synchronized with the Select Synch + CIDR Protocol in Bos indicus-Influenced Females
The specific objectives of this experiment were to characterize follicular events and the distribution of estrus and ovulation after treatment with the Select Synch + CIDR protocol. The Select-Synch + CIDR protocol is similar to CO-Synch + CIDR but does not utilize GnRH-2 or TAI, but rather AI is performed relative to estrus. Fifty postpartum, suckled Brahman x Hereford (F-1) females were used. Criteria for inclusion were the same as for Exp. 1 and 2. Females in the study were primiparous heifers (n = 32) and pluriparous cows (n = 18). Females were placed in pens as in Exp. 2, with 8 dam-calf pairs per pen, and fed according to NRC recommendations for lactating, 2-yr-old or older females.
Transrectal ultrasonography to assess ovarian morphology was conducted at the time of CIDR removal and every 12 h until ovulation or d 12, whichever occurred first. The technique of ultrasonography was the same as that used in Exp. 2, but the main focus was on the development of the preovulatory follicle and CL regression. Observations for estrus were performed by visual observation every 3 h from CIDR removal on d 7 through 12 and was based on homosexual behavior among herdmates. Artificial insemination was performed 12 h after detected estrus. Estrous behavior was characterized as standing estrus (cows that stood to be mounted for at least 4 s and repeated at least 3 times in a 6-h period), nonstanding estrous behavior (mounting activity, hyperactivity, and vocalization, but not observed to stand to be mounted), or no estrous behavior.
Blood samples were collected on d –21, –11, 0 (CIDR insertion), 7, 8, and 9 using the same procedures as described in Exp. 2. To retrospectively estimate cyclicity and luteal regression during the study, serum was assayed for progesterone in all samples by RIA as described previously. Cattle were considered to be cyclic at the onset of the study if they exhibited serum concentrations of progesterone 1 ng/mL for 2 consecutive samples and had a visible CL at ultrasound within the 10 d immediately before onset of the study (d 0).
Statistical Analyses
Experiment 1.
The effects of BCS, day postpartum, year, location, parity, AI sire, AI technician, and their respective interactions on TAI conception rates within the CO-Synch + CIDR group were examined using the CATMOD procedure (SAS Inst. Inc., Cary, NC). To examine treatment (CO-Synch + CIDR vs. TM) effects on cumulative pregnancy rates after 30 and 60 d of breeding, a model that included treatment, year, location, and their respective interactions was utilized.
Experiment 2.
The effects of treatment (CO-Synch vs. CO-Synch + CIDR) on categorical data (ovulatory responses to GnRH-1 and -2, occurrence of a synchronized follicular wave, CL regression after PGF, and TAI conception rates) were evaluated by 
2 analysis (PROC FREQ of SAS). Day of follicular wave emergence, mean follicular size on d 7, 9, and 10, as well as follicular growth rate, were evaluated using PROC GLM of SAS to examine the effects of treatment, ovarian status (i.e., ovulated or not after GnRH-1), and their interaction. Treatment effects on mean concentrations of progesterone and LH were analyzed by using PROC MIXED, with treatment, ovarian cyclic status, and day included in the model. Cow was included as a random effect, and time was included as the repeated variable.
Experiment 3.
Mean follicular size was evaluated on d 7, 9, and before ovulation by using PROC GLM, with a model that included ovarian status (cyclic vs. noncyclic), type of estrous behavior, and their interaction. Effects of ovarian status on mean intervals from CIDR removal to standing estrus and ovulation, and from standing estrus to ovulation, were also evaluated by using PROC GLM.
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RESULTS
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Experiment 1
Mean age, BCS, BW, and day postpartum averaged (±SEM) 4.7 ± 0.2 yr, 5.1 ± 0.03, 468 ± 7.1 kg, and 70 ± 1.1 d, respectively. Timed AI conception rates in all females synchronized with CO-Synch + CIDR are summarized in Table 1. Conception rates to TAI averaged 39 ± 3% overall and did not vary by location (n = 4, P = 0.47), year (n = 2, P = 0.53), BCS (P = 0.94), day postpartum (P = 0.81), parity (P = 0.88), sire (n = 6, P = 0.95), or AI technician (n = 3, P = 0.74). Table 2 summarizes cumulative pregnancy rates at 30 and 60 d of the breeding season for CO-Synch + CIDR and TM groups. Cumulative pregnancy rates were greater (P < 0.05) in CO-Synch + CIDR at both 30 and 60 d of the breeding season compared with the TM group.
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Table 1. Timed AI (TAI) pregnancy rates in nulliparous heifers, postpartum primiparous heifers, and pluriparous cows synchronized with CO-Synch + CIDR1 (Exp. 1)
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Table 2. Cumulative pregnancy rates after 30 and 60 d of breeding in nulliparous heifers, primiparous heifers, and pluriparous suckled cows synchronized with CO-Synch + CIDR1 followed by timed AI (TAI) or managed using traditional methods (TM; Exp. 1)
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Experiment 2
Ovarian and reproductive variables are summarized in Table 3. Mean age, BCS, BW, and day postpartum averaged (±SEM) 8.8 ± 0.3 yr, 5.3 ± 0.07 (range 4 to 8), 543 ± 7.4 kg, and 77 ± 0.66 d, respectively. No differences in the major ovarian and reproductive endpoints were observed between CO-Synch + CIDR and CO-Synch. Therefore, data for both treatments are presented as pooled means relative to cyclic status at the onset of treatments (Table 3). The percentage of noncyclic cows ovulating after GnRH-1 was greater (P < 0.01) than for cyclic cows. The percentage ovulating in response to GnRH-2 also differed between cyclic and noncyclic cows; however, in this case, cyclic cows had the greater (P < 0.05) response. Mean follicular diameters are presented in Table 4. Noncyclic cows had greater (P < 0.05) mean follicular size at PGF than cyclic cows and therefore a greater (P < 0.05) follicular growth rate. Follicular sizes were not different at subsequent stages.
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Table 3. Ovarian and reproductive outcomes in postpartum, suckled cows synchronized with CO-Synch + CIDR1 or CO-Synch,2 and for cycling and noncycling cows (Exp. 2)
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Table 4. Mean follicular diameters in postpartum suckled cows synchronized with CO-Synch + CIDR1 or CO-Synch,2 measured at different stages of the experiment (Exp. 2)
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Data were also summarized relative to presence or absence of ovulation after GnRH-1 to evaluate its effects on subsequent ovarian responses (Table 5). More (P < 0.01) cows that ovulated after GnRH-1 developed a synchronized follicular wave compared with cows that did not ovulate. Moreover, there was a trend (P = 0.15) for ovulation rates after GnRH-2 to be greater in cows that ovulated in response to GnRH-1 than cows that did not. Also, ovulation and TAI pregnancy rates after GnRH-2 were increased (P < 0.01) in cows that developed a synchronized follicular wave after GnRH-1 compared with cows that did not develop a new wave (Table 6).
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Table 5. Effects of the response to the first GnRH injection (GnRH-1) on subsequent ovarian and reproductive outcomes in cows synchronized with CO-Synch + CIDR1 or CO-Synch2 in Exp. 2
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Table 6. Effects of synchronized follicular wave emergence after GnRH-1 on subsequent ovarian and reproductive outcomes in cows synchronized with CO-Synch + CIDR1 or CO-Synch2 in Exp. 2
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Mean serum concentrations of progesterone are illustrated in Figure 1. As expected, concentrations of progesterone from d –8 to 0 relative to GnRH-1 differed (P < 0.01) between cyclic and noncyclic cows. After CIDR insertion (d 0), serum progesterone increased (P < 0.001) acutely for cyclic and noncyclic cows that received the CO-Synch + CIDR treatment. Serum concentrations of progesterone on d 1 were greatest (P < 0.05) for cyclic cows receiving CO-Synch + CIDR compared with all other groups. Mean concentrations of progesterone did not differ between cyclic cows treated with CO-Synch and noncyclic cows treated with CO-Synch + CIDR.
Mean concentrations of progesterone were least (P < 0.01) for the noncyclic CO-Synch-treated group compared with all others, and mean serum concentrations of progesterone never exceeded 1 ng/mL during the treatment period. After injection of PGF and CIDR removal (d 7), progesterone decreased below 1 ng/mL within 24 h in all groups and remained low until d 12, when mean progesterone exhibited a slight increase (P = 0.09) in cyclic, CO-Synch-treated cows. The latter was caused by 2 cows that ovulated asynchronously before d 9.
Release of LH after GnRH was considered to have occurred when an increment in the concentration of LH of at least 2 SD above the baseline was observed. Two cows apparently had an endogenous surge of LH before GnRH-2 (concentrations declining from values elevated well above baseline during the sampling period) and were excluded from further analyses in relation to this variable. All other cows (n = 23) examined in replicate 1 exhibited increases (P < 0.01; Figure 2) in LH after GnRH-1 and -2. Magnitude of release did not differ between treatments (CO-Synch + CIDR vs. CO-Synch). Noncyclic cows had an induced LH release greater (P < 0.05; Figure 3) than cyclic cows after GnRH-1, but concentrations of LH did not differ between cyclic and noncyclic cows after GnRH-2. A time x cyclic status interaction (P < 0.05) associated with GnRH-induced LH release was observed after GnRH-2. Also, overall mean concentrations of LH (ng/mL) were greater (P < 0.01) after GnRH-2 than after GnRH-1 (7.2 ± 0.71 vs. 4.3 ± 1.1, respectively).
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Figure 3. Interval from controlled internal drug-releasing device (CIDR) removal to visual estrus (n = 27) and ovulation (n = 28) of early, postpartum, suckled cows treated with Select Synch + CIDR. Select Synch + CIDR included insertion of a CIDR and injection of GnRH on d 0, removal of the CIDR and injection of PGF on d 7, and AI approximately 12 h after detected estrus.
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Experiment 3
Neither ovarian cyclic status (cyclic 60%; noncyclic 40%) nor parity affected the number of cows exhibiting estrus or ovulating. Mean age (±SEM), BCS, BW, and day postpartum were 5.81 ± 0.5 yr, 5.6 ± 0.1 (range 4 to 8), 565 ± 10.2 kg, and 60 ± 1.1 d, respectively. On d 7, 72% of cows had a visible CL at ultrasound, and of those, 97% exhibited luteal regression after PGF, as evidenced by a reduction in ultrasonographic size and morphology of the CL and a reduction in serum concentrations of progesterone to less than 1 ng/mL. No cows were observed in estrus during the first 48 h after CIDR removal. The majority (75%) of estrual events was observed between 60 and 82 h after CIDR removal (Figure 3
). Mean size of the largest follicle at CIDR removal and 48 h after removal were 9.45 ± 0.26 and 11.65 ± 0.26 mm, respectively (Table 7). Follicular diameter was greater for cows showing standing estrus than for cows showing only nonstanding estrous behavior or no estrous behavior at CIDR removal (P < 0.05) and 48 h after removal (P < 0.01). A greater (P < 0.01) percentage of cows exhibiting standing estrus ovulated than cows not standing. Mean intervals from CIDR removal to estrus and ovulation are shown in Table 7.
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Table 7. Estrual, follicular, and ovulatory characteristics of postpartum, suckled cows programmed with Select Synch + CIDR1 (Exp. 3)
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DISCUSSION
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Two keys that are essential for increasing the use of AI in commercial beef operations are the development of a synchronization system that is easily applicable and the ability to consistently achieve TAI conception rates greater than 50% (Johnson and Jones, 2005). Based on published reports, the CO-Synch + CIDR synchronization regimen appears to have the potential to achieve these goals, at least in Bos taurus females (Lamb et al., 2001; Larson et al., 2004). Therefore, we were encouraged to test whether the CO-Synch + CIDR regimen could be economically employed in south Texas commercial beef operations in which Bos indicus-influenced cattle are commonly used due to their subtropical adaptation. In a recent study conducted in our laboratory, we obtained and overall TAI conception rate of 42.2% using the Ovsynch procedure (Williams et al., 2002), which is more complex than CO-Synch + CIDR (handling animals 4 times vs. 3 times, respectively). Thus, initial goals were to compare management systems involving CO-Synch + CIDR with traditional management under the assumption that TAI conception rates >50% could be achieved consistently following synchronization. Overall TAI conception rates in our studies (39%) were substantially less than those observed in Bos taurus females (Lamb et al., 2001; Larson et al., 2004) and below targets set nationally for protocols of this type (NAHMS, 1997; Johnson and Jones, 2005). Thus, in spite of an overall increase in 60-d pregnancy rates of approximately 6% in the CO-Synch + CIDR compared with the TM group in Exp. 1, it is not likely that this degree of benefit would be adequate to make the protocol economically viable for use in commercial beef operations in south Texas. Therefore, efforts to examine economic feasibilities of the protocol as employed in Exp. 1 were abandoned in favor of additional studies designed to determine the cause of suboptimal outcomes associated with the regimen in Bos indicus-influenced cattle in this region. Attempts to account for the lower pregnancy rates in Bos indicus-influenced vs. straightbred Bos taurus cattle often lead to conjecture about differences in overall fertility of these 2 breed types. However, it is apparent that the fertility of the Braford and Brangus cattle used in Exp. 1 was quite high (90% pregnancy rates in 60 d) and cannot be used to account for the poor TAI conception rates.
The proportion of cows that are anovulatory at the time of synchronization is an important contributor to reduced rates of success in synchronization regimens and TAI (Stevenson et al., 2003a). Anovulatory conditions are greatly influenced by BCS (Short et al., 1990) and day postpartum (Williams, 1990). Therefore, a general approach for most investigators to minimize the effect of the anovulatory state has been to require a BCS of at least 5, with cows a minimum of 50 d postpartum at the time of TAI (Lamb et al., 2001; Williams et al., 2002). In the current studies, these conditions were fulfilled; moreover, TAI conception rates using CO-Synch + CIDR were not influenced by BCS or day postpartum in Exp. 1 or 2. Even though anestrous females can negatively influence outcomes of a synchronization program, an increase in TAI conception rates in anestrous females has been observed with the addition of an exogenous source of progesterone (Thompson et al., 1999; Stevenson et al., 2003b). Lamb et al. (2001) increased TAI conception rates in postpartum anestrous females by 25% with the addition of progesterone to the CO-Synch synchronization regimen. Low-level increases in progesterone occur during the natural resumption of ovulatory cycles postpartum (Arije et al., 1974; Williams and Ray, 1980) and the use of exogenous sources of progesterone can increase the frequency of LH pulses in postpartum cows to hasten first ovulation (Williams et al., 1983). Although some differences were observed in follicular dynamics between cyclic and noncyclic cows in Exp. 2, TAI conception rates did not differ between the 2 groups when treated with CO-Synch + CIDR. Therefore, surprisingly, ovarian status did not account for reduced efficiency in this study comparison, perhaps due to an inadequate number of females for making the comparison.
In Exp. 2 and 3, hypotheses were that CO-Synch + CIDR failed in one or more areas to adequately control the time-course of ovarian physiological events necessary to optimize TAI conception rates. These could potentially include failure to 1) optimize the frequency of ovulation or regression of follicles after GnRH-1 on d 0, 2) cause optimally timed emergence of a new follicular wave between d 1 and 4, 3) efficiently regress the CL at the time of PGF, or 4) produce an optimally receptive preovulatory follicle at the time of the second GnRH injection.
Frequency of ovulation after GnRH-1 (40%) clearly accounted for a significant proportion of synchronization failure in Exp. 1 and 2. Timing of administration of GnRH-1 is random relative to different follicular stages. Pursley et al. (1995) reported that only 60 to 80% of females with a large follicle present at the time of the first GnRH injection ovulate in response to that injection. Vasconcelos et al. (1999) reported ovulation rates exceeding 70% when GnRH was given between d 5 to 9 and 17 to 21 of the estrous cycle but were less than 50% when the treatment was given on d 1 to 4 or 10 to 16 of the cycle. In beef heifers, Martinez et al. (1999) administered GnRH on d 3, 6, and 9 of the cycle and obtained ovulation rates of 89, 56, and 22%, respectively. Ovulatory rates obtained in the current experiments were lower than those in the foregoing reports.
After exogenous administration of GnRH, circulating concentrations of LH and FSH increase within 30 min, reach peak concentrations at 120 to 150 min, then decrease to basal concentrations between 4 and 5 h after injection (Zolman et al., 1974). Results of Exp. 2 confirmed the expected pattern of LH release. However, 4 individual females exhibited unexplained, relatively small increases in LH after GnRH-1 and -2 in which peak concentrations of LH did not exceed 2 ng/mL. Increased hypothalamo-pituitary-adrenal activity is involved in the suppression of gonadotropin secretion by stressors (Dobson and Smith, 1995). Treatment with ACTH delays or abolishes estradiol-induced LH surges in anestrous sheep (Dobson et al., 1988) and also suppresses and delays LH responses to exogenous GnRH in vivo and in vitro (Matteri et al., 1986; Phogat et al., 1997). Whether the failure to detect more robust increases in LH after GnRH in some cows was caused by a physiological alteration such as stress is uncertain; however, with the exception of 1 cow, the low response was observed in 1 (GnRH-1 or 2) of the 2 sampling periods but not in both. Also, ovulation was not observed in cows with a low release of LH after GnRH-1, whereas cows exhibiting low responses after GnRH-2 did ovulate. It is possible that an endogenous release of LH occurred before or after the sampling period in the latter cows or that what appeared to be very minor increases in LH were adequate to cause ovulation in receptive follicles.
Although a complete replenishment of pituitary LH stores occurs between d 15 and 20 after parturition in cows (Lamming et al., 1981), we observed a greater release of LH after GnRH-1 in noncyclic females than in cyclic females. Similarly, Williams et al. (1982) observed that suckled cows had a greater magnitude of LH release after GnRH injection than nonsuckled cows. Pituitary stores of gonadotropins, particularly LH, in noncyclic cows continue to build as the postpartum period progresses and greater amounts are released after GnRH treatment compared with earlier postpartum and to cyclic cows (Williams et al., 1982). The greater amount of LH released after GnRH-2 compared with GnRH-1 is likely attributable to the increased sensitivity of the anterior pituitary to GnRH caused by endogenous estradiol in a low progesterone environment at the time of GnRH-2 (Kesner and Convey, 1982). Overall, results indicate that GnRH-induced release of LH release was not a major factor affecting the final pregnancy outcomes using CO-Synch + CIDR.
The stimulation of new follicle growth after GnRH has been attributed to either the acute release of FSH directly associated with the GnRH injection (Chenault, 1990) or by the subsequent endogenous increase in release of FSH caused by the disappearance of the dominant follicle (Twagiramungu et al., 1994). Bodensteiner et al. (1996) demonstrated in dairy cows that new follicular wave emergence after GnRH is initiated 21.3 ± 1.7 h after treatment and occurs immediately after the FSH peak. In the current study, induction of a synchronized follicular wave between d 1 and 4 after GnRH-1 occurred in 60% of the cows, with 31% developing a follicular wave outside of this range. Kim et al. (2005) demonstrated in lactating dairy cows that GnRH induced a new follicular wave in 95% of cows within 7 d; however, follicular waves occurring after d 4 occur too late to represent true follicular wave synchronization. The proportion of cows that developed a synchronized follicular wave in the present work was inadequate to ensure a high rate of success for a TAI protocol. The importance of developing a synchronized follicular wave after GnRH-1 is accentuated when one examines ovulation and TAI conception rates in the current experiments.
Induction of a new follicular wave was closely related to ovulation after GnRH-1. A greater proportion of cows that ovulated after GnRH-1 were induced to develop a new follicular wave compared with cows that only exhibited follicular regression or that did not respond in any manner. This is in contrast to a report by Twagiramungu et al. (1995) in which ovulation and follicular regression after GnRH-1 were equally effective for inducing new follicular wave emergence. However, in support of the current findings, Martinez et al. (2000) demonstrated that the synchrony of an induced follicular wave was less variable in heifers that ovulated after GnRH-1 than in heifers that did not ovulate.
As described earlier, a relationship between stage of the cycle and response to GnRH injection has been demonstrated; hence, presynchronization schemes have been developed (Peters and Pursley, 2002) that aim to place a larger number of females into a predetermined stage of the cycle just before the initiation of the synchronization procedure, thus increasing synchronization rates. These presynchronization schemes have not been extensively tested in Bos indicus-influenced cattle but should be evaluated in the future. Alternatively, estradiol-17ß (E2) and its esters have been effectively used to synchronize follicular wave emergence. Both E2 and GnRH have been used to induce a synchronized follicular wave; however, less variability has been demonstrated in timing of emergence of the synchronized follicular wave in females treated with E2 than in females treated with GnRH (Martinez et al., 1999). The mechanism by which E2 synchronizes follicular wave emergence has been primarily attributed to its ability to suppress FSH (Kesner and Convey, 1982) and consequently to induce follicular atresia (Bo et al., 1993). Several reports are available that demonstrate the value of this alternative in Bos indicus (Bo et al., 2003) and Bos taurus cattle (Martinez et al., 2005).
Incomplete luteolysis has been demonstrated to reduce the rates of efficiency in GnRH-based regimens. Twagiramungu et al. (1995) reported that incomplete luteolysis alters estrus rates and induces the formation of persistent follicles. Lemaster et al. (2001) suggested that a possible cause of the low rate of estrus in the Select-Synch synchronization protocol in Bos indicus-influenced cows was due to an inadequate regression of the CL; however, Hiers et al. (2003) found no differences in pregnancy rates when different PGF treatments were employed in Bos indicus-influenced females. Results of Exp. 2 and 3 of the current work indicate that the rate of luteal regression after PGF on d 7 was relatively high (93%). Concentrations of progesterone exhibit a marked decrease after PGF on d 7 (Twagiramungu et al., 1994). Studies herein supported this observation, even in females with a recently induced CL in which only a small increase in progesterone was observed before PGF injection. Therefore, failure of luteal regression did not account for a significant proportion of synchronization failure in the current studies.
Although concentrations of progesterone decreased after PGF injection below 1 ng/mL within 24 h in all groups and remained low until d 12, a few (4%) animals in the CO-Synch group exhibited premature estrus and ovulation which caused a small increase in overall mean concentrations of progesterone in this group after d 12. In heifers, there is a relatively high incidence of estrus (6 to 28%) before d 7 (d of PGF injection) in GnRH/PGF protocols and these animals are unlikely to conceive to a fixed TAI (Martinez et al., 2002; Williams et al., 2002). In beef cows, the incidence of premature estrus during the Ovsynch and similar protocols has been reported to be approximately 8 to 10% (Geary et al., 2000; DeJarnette et al., 2001). Premature estrus was observed only in the CO-Synch group in which a source of progesterone was not present.
Ovulation rate after GnRH-2 (72%) in the current studies also accounted for reduced efficiency. Vasconcelos et al. (1999, 2001) reported ovulation rates after GnRH-2 in lactating dairy cows of 87 and 91.3%, respectively. Similarly, Pursley et al. (1995) obtained ovulation rates of 100% within 32 h after GnRH-2 in dairy cows, and in beef cows Thompson et al. (1999) reported an ovulation rate of 84.6%. Even though the overall ovulation rate in our experiment was less than expected, cows that developed a synchronized follicular wave after GnRH-1 exhibited ovulation rates after GnRH-2 (85%) similar to that reported previously. Therefore, the ability to increase ovulation rates after GnRH-1 will increase ovulation rates after GnRH-2, and attainment of this objective should increase conception rates.
At the time of GnRH-2 (Exp. 2), mean diameter of the largest follicle was 11.1 ± 0.2 mm (range 6.0 to 15.4). In Bos taurus cattle, mean follicular diameter has been reported as greater than 13 mm at the time of GnRH-2 (Thompson et al., 1999). Follicles acquire ovulatory capacity immediately after deviation (Sartori et al., 2001), which occurs at approximately 8.5 mm (Ginther et al., 1996); however, ovulatory capacity is dependent upon both the amount of LH released and follicular diameter (Sartori et al., 2001). In Holstein cows treated with GnRH on d 0 to synchronize a follicular wave, 4 mg of an exogenous LH preparation on d 7 was insufficient to cause ovulation of 10-mm follicles, although many of these follicles had undergone physiological deviation. However, all growing follicles greater than 12 mm and 17% of all 11-mm follicles ovulated to the same LH treatment (Sartori et al., 2001). In the same experiment, a greater dose of LH (24 or 40 mg) resulted in ovulation of 70 to 80% of 10-mm follicles, and those 10-mm follicles that failed to ovulate had generally not yet undergone physiological deviation at the time of treatment. Thus, follicles that had undergone deviation and had reached a diameter of 10 mm had acquired ovulatory capacity and ovulated in response to a relatively large dose of LH but not to a lower dose. This indicates that some of the follicles present at the time of GnRH-2 injection in our studies had not yet reached ovulatory capacity, thus reducing the proportion of cows ovulating after GnRH-2. Additionally, although we obtained an 85% ovulation rate within 48 h after GnRH-2 and TAI in cows that developed a synchronized follicular wave, only 45% became pregnant. This suggests that a significant proportion of the follicles that ovulated were immature and infertile. Perry et al. (2005) reported that in beef cows synchronized with CO-Synch, follicles ovulating at sizes of 12.1 mm or less in diameter are less likely to support a pregnancy to d 25 after insemination compared with cows that ovulate follicles of 14.7 mm. In our experiment, mean size of the ovulatory follicle was 11.6 ± 0.2 mm (range 8.1 to 15.4) which, based on the report by Perry et al. (2005), would substantially reduce the likelihood of a viable pregnancy. However, that report was derived from experiments with straightbred Bos taurus cattle and the corresponding value in Bos indicus or Bos indicus crossbred cattle could differ.
In Exp. 3, we examined the distribution of estrus and ovulation in cows treated with the Select-Synch protocol (observe for estrus; no GnRH-2) to determine whether TAI and GnRH at 48 h in CO-Synch + CIDR were too early to optimize fertile ovulations and pregnancy. Mean size of the ovulatory follicle in Exp. 3 was greater than observed in Exp. 2 at the time of GnRH-2 and TAI. The larger follicle size should increase the likelihood that oocytes from those follicles would yield a viable pregnancy (Perry et al., 2005). In support of the assumption that follicular maturity and timing of GnRH administration are keys for the success of these protocols, Peters and Pursley (2003) evaluated the effect of administering the second GnRH injection at different times after PGF injection in the Ovsynch protocol in dairy cows. Follicle diameter was measured and correlated with TAI conception rates. A linear relationship was observed between the time of GnRH injection, follicular size, and TAI conception rate. The greatest TAI conception rate was obtained at 36 h after PGF with a mean follicle size of 14.6 ± 0.4. In a similar experiment, Vasconcelos et al. (2001), using dairy cows and the Ov-synch protocol, aspirated all follicles greater than 4 mm on d 4 after GnRH injection to initiate a delayed follicular wave. The aim was to reduce the size of the preovulatory follicle at the time of second GnRH injection so as to test whether a reduction in follicular size would reduce subsequent luteal size, progesterone concentrations, and thus conception rates. It was concluded that ovulation of follicles smaller than 11.5 mm results in reduced fertility, possibly because of development of smaller CL and decreased circulating concentrations of progesterone.
In Exp. 3, mean intervals from CIDR removal to standing estrus (70 ± 2.9 h) and to ovulation (99 ± 2.8 h) were very similar to results reported by Lemaster et al. (1999) using Bos indicus-influenced cattle. However, in Bos taurus beef females, the mean interval from progesterone removal/PGF injection to estrus has been reported to be 55 ± 4 h (Geary and Whittier, 1998; Martinez et al., 2000; Lamb et al., 2004), indicating that Bos taurus females are more likely to become pregnant after GnRH-2 and TAI at 48 h compared with Bos indicus-influenced females. Initial reports in Bos taurus females revealed no statistical differences in conception rates when TAI at 48 h was compared with later times (Stevenson et al., 2003a; Bremer et al., 2004). However, more recent studies indicate that increasing the time of GnRH-2 and TAI to 66 h also improves TAI pregnancy rates in Bos taurus females compared with 48 h (Schafer et al., 2004; Walker et al., 2005). There are no data to determine whether the same assumption would hold for Bos indicus-influenced females. As noted earlier, Bos taurus females appear to exhibit a shorter interval from CIDR removal to estrus than Bos indicus-influenced females. Therefore, changing the timing of GnRH-2 from 48 to 66 or 72 h may increase TAI conception rates, but this hypothesis remains to be tested.
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IMPLICATIONS
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The use of a controlled internal drug release device containing progesterone in combination with gonadotropin-releasing hormone, prostaglandin F2 and timed artificial insemination as described herein, is a relatively convenient and systematically functional synchronization protocol that is designed to maximize the ability to achieve high timed artificial insemination conception rates. However, in order to successfully utilize this or similar methodology in breeds or breed types with Bos indicus genetic influence, further modifications will be required. Adjustments in the timing of insemination need to be evaluated extensively. Other approaches for inducing a greater proportion of ovulations after the first injection of gonadotropin-releasing hormone, and thereby increasing the frequency of a synchronized follicular wave, must also be examined.
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Footnotes
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1 Supported by Pfizer Animal Health and the Texas Agricultural Experiment Station. 
2 Corresponding author: glwilliams{at}tamu.edu
Received for publication May 23, 2006.
Accepted for publication August 30, 2006.
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LITERATURE CITED
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Top
Abstract
INTRODUCTION
; n = 11) cows treated with CO-Synch + controlled internal drug-releasing device (CIDR), and cyclic (
; n = 39) and noncyclic (
; n = 11) cows treated with CO-Synch only (Exp. 2). The CO-Synch + CIDR included insertion of a CIDR and injection of GnRH on d 0, removal of the CIDR and injection of PGF on d 7, and injection of GnRH and timed AI beginning 48 h later. The CO-Synch is the identical treatment but without the CIDR.