J. Anim. Sci. 2002. 80:1746-1751
© 2002 American Society of Animal Science
The use of a progesterone-releasing device (CIDR-B) or melengestrol acetate with GnRH, LH, or estradiol benzoate for fixed-time AI in beef heifers1
M. F. Martinez*,
J. P. Kastelic
,
G. P. Adams* and
R. J. Mapletoft*,2
* Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B4 and
and
Research Centre, Agriculture and Agri-Food Canada, Lethbridge, Alberta, Canada T1J 4B1
2 Correspondence:
Dept. of Large Anim. Clinical Sci., 52 Campus Dr. (phone: 306-996-7149; fax: 306-966-7159; E-mail:
reuben.mapletoft{at}usask.ca).
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Abstract
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The objective of this experiment was to compare two progestins and three treatments for synchronizing follicular wave emergence and ovulation in protocols for fixed-time AI in beef heifers. On d 0 (beginning of the experiment), Angus and Angus-Simmental cross beef heifers at random stages of the estrous cycle either received a CIDR-B device (n = 257) or were started on 0.5 mg•animal-1•d-1 melengestrol acetate (MGA; n = 246) and were randomly assigned to receive i.m. injections of 100 µg GnRH, 12.5 mg porcine LH (pLH), or 2 mg estradiol benzoate (EB) and 50 mg progesterone (P4). The last feeding of MGA was given on d 6 and on d 7, CIDR-B devices were removed and all heifers received 500 µg cloprostenol (PG). Consistent with their treatment groups on d 0, heifers were given either 100 µg GnRH or 12.5 mg pLH 48 h after PG (and were concurrently inseminated) or 1 mg EB 24 h after PG and were inseminated 28 h later (52 h after PGF). Estrus rate (combined for both progestins) in heifers receiving EB (92.0%) was greater (P < 0.05) than that in heifers receiving GnRH and pLH (combined) and a CIDR-B device (62.9%) or MGA (34.3%). Although the mean interval from PG treatment to estrus did not differ among groups (overall, 47.8 h; P = 0.85), it was less variable (P < 0.01) in MGA-fed heifers (SD = 2.5 h) than in CIDR-B-treated heifers (SD = 8.1 h). Pregnancy rates (determined by ultrasonography approximately 30 d after AI) did not differ (P = 0.30) among the six treatment groups (average, 58.0%; range, 52.5 to 65.0%). Although fixed-time AI was done, pregnancy rates were greater in heifers detected in estrus than in those not detected in estrus (62.6 vs 51.9%; P < 0.05). In conclusion, GnRH, pLH, or EB treatment in combination with a CIDR-B device or MGA effectively synchronized ovulation for fixed-time AI, resulting in acceptable pregnancy rates in beef heifers.
Key Words: Estradiol • Fixed-Time AI • GnRH • LH • Progestins • Synchronization
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Introduction
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Current approaches to fixed-time AI in cattle involve a source of progestin, and synchronization of follicular wave emergence, the preovulatory LH surge, and ovulation (De Rensis and Peters, 1999). One protocol (Ovsynch) consists of two injections of GnRH (8 or 9 d apart), PG 30 to 48 h before the second GnRH, and timed AI, 0 to 24 h after the second GnRH (Risco et al., 1998). The first GnRH is to synchronize follicular wave emergence and the second is to induce the preovulatory LH surge after ovulation. However, this protocol has not resulted in acceptable pregnancy rates in heifers (Pursley et al., 1997; Risco et al., 1998; Martinez et al., 2002). Failure of ovulation to the first GnRH (Martinez et al., 1999) and asynchronous onset of estrus relative to the second GnRH may account for the reduced fertility in some heifers (Risco et al., 1998). The insertion of a progesterone-releasing intravaginal device (CIDR-B) or feeding melengestrol acetate (MGA) provides the basis for many fixed-time AI programs. The addition of a CIDR-B to a Co-synch program improved pregnancy rates in heifers (68.0 vs 39.1%; Martinez et al., 2002). Porcine LH (pLH) has also been used in lieu of GnRH in an Ovsynch program (Martinez et al., 2002). Estrogens have also been shown to effectively synchronize follicle wave emergence (Bo et al., 1995, 1996; Caccia and Bo, 1998) and a preovulatory LH surge (Lammoglia et al., 1998), and have been used in synchronization programs with a CIDR-B (Martinez et al., 2000a) or MGA (Kastelic et al., 1997). Estradiol resulted in a greater pregnancy rate than GnRH in a CIDR-B-based fixed-time AI program (Martinez et al., 2000b), with no difference when MGA was used (Martinez et al., 2001).
The objective of this study was to compare the two progestins (CIDR-B and MGA) and three treatments that we have used previously to synchronize follicular wave emergence and ovulation (GnRH, pLH and estradiol benzoate) in a fixed-time AI program in beef heifers.
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Materials and Methods
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Angus (Black and Red) and Angus-Simmental cross heifers, approximately 15 mo of age and weighing from 275 to 350 kg (body condition score approximately 3.25/5), were used in this study. Heifers had been in a feedlot for approximately 7 mo and were fed approximately 5 kg barley silage and 1 kg of rolled barley•animal-1•d-1. Heifers were housed outdoors in feedlot pens (approximately 50 or 100 heifers per pen, 50 m2/animal).
Transrectal ultrasound examinations were conducted to determine the presence of a corpus luteum (CL) in order to confirm cyclicity and that heifers were nonpregnant and free of morphologic abnormalities of the reproductive tract. Heifers were randomly assigned to one of six groups in a 2 x 3 factorial design (Figure 1
). On d 0, 257 heifers had a CIDR-B device containing 1.9 g of progesterone (Vetrepharm Canada Inc, Belleville, ON, Canada) placed in the vagina. The tails of the CIDR-B devices were cut even with the vulva to prevent pen mates from pulling them out. The remaining heifers (n = 246) were given 0.5 mg•animal-1•d-1 MGA in their feed (Pharmacia and Upjohn, Orangeville, ON, Canada) from d 0 to 6. On d 0, heifers were given i.m. injections of 100 µg GnRH (Cystorelin, Merial Canada Inc, Victoriaville, PQ, Canada), 12.5 mg pLH (Lutropin-V, Vetrepharm Canada Inc), or 2 mg estradiol benzoate (EB, Sigma-Aldrich Can Ltd, Oakville, ON, Canada) and 50 mg progesterone (Sigma-Aldrich) in 2 mL of canola oil. On d 7, CIDR-B devices were removed and all heifers received an i.m. injection of 500 µg cloprostenol (PG; Estrumate, Schering-Plough Animal Health, Pointe-Claire, PQ, Canada). Consistent with their treatment groups on d 0 (estradiol, GnRH, or pLH), heifers were given either 100 µg GnRH or 12.5 mg pLH 48 h after PG and concurrently inseminated or 1 mg EB in 2 mL of canola oil i.m. (Sigma-Aldrich) 24 h after PG and inseminated, approximately 28 h later (52 h after PGF; the LH peak is expected to occur approximately 18 h after treatment with estradiol [Bo et al., 1994]). Frozen-thawed semen from a single sire was used and the same operator performed all inseminations. Although observations for behavioral estrus were conducted at least twice daily from 24 to 72 h after PG treatment to determine estrus synchrony, all heifers were fixed-time inseminated.
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Figure 1. Schematic outline of the experiment (arrows indicate times of administration of treatments). The CIDR-B devices were removed after 7 d and MGA was fed once daily from d 0 to 6. Heifers were treated with GnRH, pLH, or estradiol benzoate (EB) and progesterone on d 0 and all received PG on d 7. Heifers that received GnRH or pLH received a second injection of the same hormone 48 h after PG (d 9) and were inseminated concurrently, while those in the EB group received EB 24 h after PG (d 8) and were inseminated 28 h later (52 h after PG; d 9).
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Heifers were retained in the feedlot and exposed to bulls for 17 d, starting approximately 20 d after AI. Pregnancy was diagnosed by transrectal ultrasound examination approximately 30 d after AI (Curran et al., 1986). Following bull exposure, heifers were sent to pasture. Fall pregnancy rate was determined by rectal palpation, conducted approximately 150 d after fixed-time AI. A heifer diagnosed as pregnant at the first examination but subsequently diagnosed nonpregnant in the fall was defined as having a pregnancy loss.
A two-way analysis of variance was used to determine the effect of progestin, synchronizing treatment, and their interaction on the interval from PG treatment to estrus. Bartlett’s test was used to compare the variances in the interval from PG treatment to estrus among the six treatment groups, between the two progestins, and among the three synchronizing treatments. Logistic regression analysis was used to compare the effect of progestin, treatment and progestin x treatment interaction on estrus and pregnancy rates. Embryonic loss rate was compared among the six treatment groups, between the two progestins, and among the three synchronizing treatments by chi-square analysis. Chi-square was also used to compare the pregnancy rate between heifers that showed estrus and those that did not. All statistical analyses, except logistic regression (SPSS, version 10.05, 1999, SPSS Inc., Chicago, IL, USA), were conducted using SAS (SAS Inst. Inc., Cary, NC).
The University of Saskatchewan Animal Care Committee has approved protocol No. 19970080 for these experiments.
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Results
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The distribution of estrus is shown in Table 1 and Figure 2. Reproductive performance according to the presence or absence of detected estrus is shown in Table 1. The proportion of heifers detected in estrus was greatest in EB groups (combined for both progestins, 92.0%) and least in GnRH or pLH groups fed MGA (35.6 and 33.0%, respectively). Estrus was detected in a greater (P < 0.0001) proportion of heifers given a CIDR-B device than in those fed MGA (68.9 vs 45.5%). The mean interval from PG treatment to estrus was not different among groups (overall, 47.8 h; P = 0.85) and the modal interval from PG to estrus was 48 h. However, the interval from PG treatment to estrus was less variable (P < 0.01) in heifers fed MGA (SD = 2.5 h) than in those treated with a CIDR-B (SD = 8.1 h). There was an effect of progestin (P < 0.001), but there was no effect of treatment (P = 0.72) or progestin x treatment interaction (P = 0.11) on estrus rate (overall mean, 58.0%). There was no effect of progestin (P = 0.11), treatment (P = 0.26), or progestin x treatment interaction (P = 0.22) on pregnancy rate to fixed-time AI (overall mean, 58.0%). Combined for all treatments, pregnancy rate was greater (P < 0.05) in heifers detected in estrus (62.6%) than in those not detected in estrus (51.9%). Fall pregnancy rate was not different (P = 0.86) among groups (overall mean, 77.1%). Pregnancy losses were not different among groups and ranged from 5.3% in CIDR-B-treated heifers that were synchronized with pLH to 13.0% in MGA-treated heifers synchronized with pLH (overall mean, 9.3%).
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Table 1. Estrus rates, interval from PG to estrus, and pregnancy rates in beef heifers receiving a CIDR-B device or fed MGA and given GnRH, pLH, or EB for the synchronization of follicular wave emergence and ovulation for fixed-time AIa
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Figure 2. Distribution of estrus after PG administration in heifers that received a CIDR-B device (A) or were fed MGA (B) and treated with GnRH, pLH, or estradiol benzoate (EB) to synchronize follicular wave emergence and ovulation for fixed-time AI.
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Discussion
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The use of progestins in estrus synchronize programs has become widespread in the last 10 yr (De Rensis and Peters, 1999). In a recent report (Martinez et al., 2002), pregnancy rates to fixed-time AI were greater in cattle that received a CIDR-B than in those fed MGA. Because pregnancy rates to fixed-time AI in the present study were not significantly different among groups (overall mean, 58.0%), factors other than pregnancy rate (e.g., costs and management) may determine the program selected. For example, CIDR-B devices are more expensive than MGA, but they can be used in both confined cattle and those at pasture.
Although MGA is marketed for the suppression of estrus in feedlot heifers, it has also been used in estrus synchronization programs (Patterson et al. 1989; Odde, 1990). The feeding of MGA for 14 to 17 d effectively synchronizes estrus but results in low fertility (Odde, 1990). However, feeding MGA for shorter periods of time (e.g., 7 d) has resulted in improved fertility when feeding was started early in the estrous cycle, but not when MGA was started late in the cycle (Beal et al., 1988). This is probably due to the development of persistent dominant follicles (Custer et al. 1994; Kojima et al. 1995), which are associated with reduced fertility (Patterson et al., 1989; Custer et al., 1994; Ahmad et al., 1995). This problem may be overcome by strategically synchronizing follicular wave emergence at the start of MGA treatment.
It has been shown that estradiol synchronizes emergence of a new follicular wave by suppressing FSH (Bo et al., 1995). Estradiol and progesterone given at the beginning of a CIDR-B protocol in beef heifers was followed by follicle regression and the synchronous emergence of a new follicular wave 3.4 ± 0.1 d later (Martinez et al., 2000a). Therefore, in a 7-d protocol (as used in the present study), the dominant follicle would be on approximately d 3 or 4 of its growing phase (9 to 10 mm) at the time of CIDR-B removal or discontinuation of MGA.
There has been continuing development of CIDR-B-based programs in the last several years. Initially, EB was delivered by a gelatin capsule placed in the vagina along with the CIDR-B device (Macmillan et al., 1991). Subsequent work showed that the i.m. administration of EB resulted in more consistent and synchronous wave emergence (Bo et al., 1996). Estradiol-17ß and progesterone have been incorporated into a 7-d MGA feeding program, resulting in acceptable fertility after estrus detection and AI (Kastelic et al., 1996). Estrogens and GnRH have been compared in CIDR-B based programs for synchronization of wave emergence and ovulation (Martinez et al., 2001). In one study, estrus and pregnancy rates to fixed-time AI were 100 and 76%, respectively, in CIDR-B-treated heifers given EB, compared with 55 and 48% in heifers given GnRH (Martinez et al., 2000b). In another study, beef cows were given MGA for 7 d (Martinez et al., 2001); estradiol or GnRH were given to synchronize follicular wave emergence and ovulation, resulting in pregnancy rates to fixed-time AI of 55.7 and 47.5%, respectively. The results of the present study suggest that there is likely to be no difference in pregnancy rates to fixed-time AI between the use of CIDR-B or MGA and one of the three methods of synchronizing follicular wave emergence and ovulation.
We have previously shown that the administration of estradiol when circulating levels of progesterone are low will induce LH release, incomplete suppression of the dominant follicle, and a delay in the emergence of the next follicular wave (Bo et al., 1994). Therefore, we included progesterone with the first estradiol treatment in the present study. However, the inclusion of progesterone with EB at the time of CIDR-B insertion has been reported to have no effect on pregnancy rate (Bo et al., 2000). Plasma progesterone concentrations have been reported to increase by 2 h after CIDR-B insertion (Burke et al., 1999) and may prevent an estrogen-induced LH surge in cattle without a functional CL. However, progestin levels may increase more slowly and be less suppressive with an oral preparation such as MGA (Kojima et al., 1995). In addition, progesterone along with estradiol may improve its efficacy in inducing regression of larger antral follicles (Anderson and Day, 1994, 1998; McDowell et al., 1998).
Estrogen treatment has also been given after PG administration to increase the proportion of cattle in estrus and the synchrony of estrus behavior (Dailey et al., 1983, 1986). Bo et al. (1994) reported that an LH surge occurred 16 to 18 h after an injection of estradiol in animals without a functional CL. The administration of EB following CIDR-B removal has been shown to result in a greater estrus rate than that in control heifers (Hanlon et al., 1996) and an LH surge and ovulation (Lammoglia et al., 1998). In the present study, EB appears to have been very effective in synchronizing both the emergence of an ovulatory wave and ovulation of the dominant follicle of that wave.
Treatment with GnRH induces ovulation of large antral follicles, with a new follicular wave emerging approximately 2 d later (Twagiramungu et al., 1994, 1995). However, synchronous emergence of a new follicular wave occurs only when treatment causes ovulation. In an experiment in which GnRH or pLH was administered at various phases of the first follicular wave, ovulation of the dominant follicle was induced in 56 or 78% of heifers, respectively (Martinez et al., 1999). Heifers that failed to ovulate following the second GnRH treatment in an Ovsynch protocol were reported to be in metestrus or early diestrus at the first GnRH treatment (Pursley et al., 1995). Therefore, ovulation following PG treatment may be poorly synchronized if the first GnRH treatment does not induce ovulation of the dominant follicle and thereby fails to synchronize wave emergence.
The Ovsynch protocol has yielded acceptable pregnancy rates in both dairy (Pursley et al., 1997) and beef (Geary et al., 2001) cows, but pregnancy rates in heifers have been unacceptably low (Pursley et al., 1997). The addition of a CIDR-B device to a GnRH- or pLH-based Ovsynch program resulted in improved pregnancy rates in beef heifers (68 vs 39% or 65 vs 38%, respectively; Martinez et al., 2002). Therefore, progestins (CIDR-B or MGA) were used in the period between the first GnRH or pLH treatment and the administration of PG in the present study; pregnancy rates to fixed-time AI were highly acceptable.
In the present study, the use of EB for synchronization of estrus and ovulation required one additional handling compared with GnRH or pLH treatment groups, but there was no difference in pregnancy rates. However, insemination 8 to 24 h after GnRH treatment has been reported to result in numerically the highest pregnancy rates (Pursley et al., 1998). Insemination a few hours after GnRH or pLH treatment may have improved pregnancy rates in the present study, but it would have required an additional handling. Although stress may affect pregnancy rates, estradiol treatment with an additional handling resulted in pregnancy rates that were at least as high as that of groups requiring less handling.
Heifers were fixed-time inseminated in the present study, but they were also observed for estrus. Although there was no effect of treatment group, estrus rate was numerically greater in EB-treated heifers (92.0%) than in those given GnRH (50.7%) or pLH (47.2%) and was greater in heifers with a CIDR-B device (68.9%) than in those fed MGA (45.5%). High estrus rates have been previously reported for estrogen-treated cattle (Hanlon et al., 1996; Ryan et al., 1996; Day et al., 2000) but were much lower in cattle given GnRH in an Ovsynch program (Stevenson et al., 1996). Obviously, many factors influence the expression and detection of estrus (Loeffler et al., 1999), and in the present experiment, the use of a CIDR-B device significantly increased the expression of estrus. However, synchronization of follicle wave emergence and the preovulatory LH surge (and ovulation) in fixed-time AI programs preclude the need for estrus detection. Nevertheless, pregnancy rates were greater in GnRH- or pLH treated heifers that were detected in estrus, but pregnancy rates were not different in EB-treated heifers.
Because pregnancy rate is the product of estrus rate and conception rate, pregnancy rate will be low if either is low, or if both are modest. Fixed-time AI eliminates variability associated with estrus detection and it will be optimized if ovulation is synchronous; this can be facilitated by synchronizing wave emergence and the preovulatory LH surge. The programs used in the present experiment were successful in achieving this objective.
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Implications
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The use of GnRH, porcine LH, or estradiol benzoate in combination with an intravaginal progesterone-impregnated device or feeding of melengestrol acetate apparently synchronized ovulation (and follicular wave emergence), facilitating fixed-time AI in beef heifers. Pregnancy rates to a single fixed-time insemination were highly acceptable in all groups (overall average, 58%). Factors including product cost and availability, animal confinement, and handling will influence the program that will be used.
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Footnotes
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1 Financial support was provided by Canada-Alberta Beef Industry Development Fund; Agriculture and Agri-Food Canada Matching Investment Initiative; Saskatchewan Agriculture Development Fund—Strategic Research Program (ADF-SRP); Univ. of Saskatchewan; and Agriculture and Agri-Food Canada. We thank Schering-Plough Animal Health, Merial Canada Inc., and Vetrepharm Canada Inc. for donating pharmaceuticals, our collaborating cattle producer (Bernie Loman) for his cooperation and support, and Margaret Fisher and Heng Wang for technical assistance. Portions of these data were previously reported at the annual meeting of the International Embryo Transfer Society in January 1999. 
Received for publication August 2, 2001.
Accepted for publication February 4, 2002.
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Literature Cited
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Ahmad, N., F. N. Shrick, R. I. Butcher, and E. K. Inskeep. 1995. Effect of persistent follicles on early embryonic losses in beef cows. Biol. Reprod 52: 1129–1135.[Abstract]
Anderson, L. H., and M. L. Day. 1994. Acute progesterone administration regresses persistent dominant follicles and improves fertility of cattle in which estrus was synchronized with melengestrol acetate. J. Anim. Sci. 72: 2955–2961.[Abstract]
Anderson, L. H., and M. L. Day. 1998. Development of a progestin-based estrus synchronization program: I. Reproductive response of cows fed melengestrol acetate for 20 days with an injection of progesterone. J. Anim. Sci. 76: 1267–1272.[Abstract/Free Full Text]
Beal, W. E., J. R. Chenault, M. L. Day, and L. R. Corah. 1988. Variation in conception rates following synchronization of estrus with melengestrol acetate and prostaglandin F2
. J. Anim. Sci. 66: 599–602.[Abstract/Free Full Text]
Bo, G. A., G. P. Adams, R. A. Pierson, M. Caccia, H. Tribulo, and R. J. Mapletoft. 1994. Follicular wave dynamics after estradiol-17ß treatment of heifers with or without a progestagen implant. Theriogenology 41:1555–1569.
Bo, G. A., G. P. Adams, R. A. Pierson, and R. J. Mapletoft. 1995. Exogenous control of follicular wave emergence in cattle. Theriogenology 43:31–40.
Bo, G. A., M. Caccia, M. F. Martinez, and R. J. Mapletoft. 1996. Follicle wave emergence after treatment with estradiol benzoate and CIDR-B vaginal devices in beef cattle. In: Proc. Int. Cong. Anim. Reprod., Sydney, NSW, Australia. 7:22. (Abstr.).
Bo, G. A., M. Medina, J. C. Tegli, A. Costamagna, and G. M. Brogliatti. 2000. Fixed-time artificial insemination in CIDR-B treated cows induced to ovulate with estradiol benzoate or GnRH. In: Proc. Int. Cong. of Anim. Reprod., Stockholm, Sweden. 14:14. (Abstr.).
Burke, C. R., M. P. Boland, and K. L. Macmillan. 1999. Ovarian responses to progesterone and oestradiol benzoate administered intravaginally during diestrus in cattle. Anim. Reprod. Sci. 55:23–33.[Medline]
Caccia M., and G. A. Bo. 1998. Follicle wave emergence following treatment of CIDR-B implanted beef cows with estradiol benzoate and progesterone. Theriogenology 49:341 (Abstr.).
Curran, S., R. A. Pierson, and O. J. Ginther. 1986. Ultrasonographic appearance of the bovine conceptus from Days 20 through 60. J. Am. Vet. Med. Assoc. 189:1295–1302.[Medline]
Custer, E. E., W. E. Beal, Wilson, S. J. Meadows, A. W. Berardinelli, J. G., and R. Adair. 1994. Effect of melengestrol acetate (MGA) or progesterone-releasing intravaginal device (PRID) on follicular development, concentrations of estradiol-17ß and progesterone and luteinizing hormone release during an artificially lengthened bovine estrous cycle. J. Anim. Sci. 72:1282–1289.[Abstract]
Dailey, R. A., R. E. James, E. K. Inskeep, and S. P. Washburn. 1983. Synchronization of estrus in dairy heifers with prostaglandin F2 with or without estradiol benzoate. J. Dairy Sci. 66:881–886.[Abstract/Free Full Text]
Dailey, R. A., J. C. Price, K. R. Simmons, E. M. Meisterling, P. A. Quinn, and S. P. Washburn 1986. Synchronization of estrus in dairy cows with prostaglandin F2 and estradiol benzoate. J. Dairy Sci. 69:1110–1114.[Abstract/Free Full Text]
Day, M. L., C. R. Burke, V. K. Taufa, A. M. Day, and K. L. Macmillan 2000. The strategic use of estradiol to enhance fertility and submission rates of progestin-based estrus synchronization programs in dairy herds. J. Anim. Sci. 78:523–529.[Abstract/Free Full Text]
De Rensis, F., and A. R. Peters. 1999. The control of follicular dynamics by PGF2, GnRH, hCG and estrus synchronization in cattle. Reprod. Domest. Anim. 34:49–59.
Geary T. W., J. C. Whittier, D. M. Hallford, and M. D. MacNeil. 2001. Effect of calf removal on pregnancy rates of cows synchronized with the Ovsynch or Co-Synch protocol. J. Anim. Sci. 79:1–4.[Abstract/Free Full Text]
Hanlon, D. W., N. B. Williamson, J. J. Wichtel, I. J. Steffert, A. L. Craigie, and D. U. Pfeiffer. 1996. The effect of estradiol benzoate administration on estrous response and synchronized pregnancy rate in dairy heifers after treatment with exogenous progesterone. Theriogenology 45:775–785.[Medline]
Kastelic, J. P., D. H. McCartney, A. Garcia, W. Olson, and R. J. Mapletoft. 1996. Estrus synchronization in cattle using estradiol, melengestrol acetate and PGF. Theriogenology 46:1295–1304.[Medline]
Kastelic, J. P., D. H. McCartney, W. O. Olson, A. D. Barth, and R. J. Mapletoft. 1997. Melengestrol acetate (MGA) and estradiol-17ß for synchronizing estrus and improving reproductive performance in beef cattle. Can. J. Anim. Sci. 77:683–688.
Kojima, F. N., J. R. Chenault, M. E. Wehrman, E. G. Bergfeld, A. S. Cupp, L. A. Werth, V. Mariscal, T. Sanchez, R. J. Kittok, and J. E. Kinder. 1995. Melengestrol acetate at greater doses than typically used for estrus synchrony in bovine females does not mimic endogenous progesterone in regulation of secretion of luteinizing hormone and estradiol-17ß. Biol. Reprod. 52:455–463.[Abstract]
Lammoglia, M. A., R. E. Short, R. E. Bellows, S. E. Bellows, M. D. MacNeill, and H. D. Hafs. 1998. Induced and synchronized estrus in cattle: Dose titration of estradiol benzoate in prepubertal heifers and postpartum cows after treatment with and intravaginal progesterone-releasing insert and prostaglandinF2. J. Anim. Sci. 76:1662–1670.[Abstract/Free Full Text]
Loeffler, S. H., M. J. de Vries, Y. H. Schukken, A. C. de Zeeuw, A. A. Dijkhuizen, F. M. de Graaf, and A. Brand. 1999. Use of AI technician scores for body condition, uterine tone and uterine discharge in a model with disease and milk production parameters to predict pregnancy risk at first AI in Holstein dairy cows. Theriogenology 51:1267–1284.[Medline]
Macmillan, K. L., V. K. Taufa, D. R. Barnes and A. M. Day. 1991. Plasma progesterone concentrations in heifers with a new intravaginal device. Anim. Reprod. Sci. 26:25–40.
Martinez, M. F., G. P. Adams, J. P. Kastelic, D. Bergfelt and R. J. Mapletoft. 2000a. Induction of follicular wave emergence for estrus synchronization and artificial insemination in heifers. Theriogenology 54:757–769.[Medline]
Martinez M. F., D. Bergfelt, G. Adams, J. Kastelic, and R. Mapletoft. 1999. Effect of LH or GnRH on the dominant follicle of the first follicular wave in beef heifers. Anim. Reprod. Sci. 57:23–33.[Medline]
Martinez, M. F., J. P. Kastelic, G. P. Adams, R. B. Cook, W. O. Olson, and R. J. Mapletoft. 2002. The use of progestins in regimens for fixed-time artificial insemination in beef cattle. Theriogenology 57:1049–1059.[Medline]
Martinez, M. F., J. P. Kastelic, G. P. Adams, E. Janzen, D. H. McCartney, and R. J. Mapletoft 2000b. Estrus synchronization and pregnancy rates in beef cattle given CIDR-B, prostaglandin and estradiol or GnRH. Can. Vet. J. 41:786–790.[Medline]
Martinez, M. F., J. P. Kastelic, G. P Adams, and R. J. Mapletoft 2001. The use of GnRH or estradiol to facilitate timed insemination in an MGA-based synchronization regimen in beef cattle. Anim. Reprod. Sci. 67:221–229.[Medline]
McDowell, C. M., L. H. Anderson, R. P. Lemenager, D. A. Mangione, and M. L. Day. 1998. Development of a progestin-based estrus synchronization program: I. Reproductive response of cows fed melengestrol acetate for 14 days with injections of progesterone and prostaglandin F2. J. Anim. Sci. 76:1273–1279.[Abstract/Free Full Text]
Odde, K. G. 1990. A review of synchronization of estrus in postpartum cattle. J. Anim. Sci. 68:817–830.[Abstract]
Patterson, D. J., G. H. Kiracofe, J. S. Stevenson, and L. R. Corah. 1989. Control of the bovine estrous cycle with melengestrol acetate (MGA): A review. J. Anim. Sci. 67:1895–1906.[Abstract/Free Full Text]
Pursley, J. R., M. O. Mee, and M. C. Wiltbank. 1995. Synchronization of ovulation in dairy cows using PGF2 and GnRH. Theriogenology 44:915–923.[Medline]
Pursley, J. R., R. W. Silcox, and M. C. Wiltbank. 1998. Effect of time of artificial insemination on pregnancy rates, calving rates, pregnancy loss, and gender ratio after synchronization of ovulation in lactating dairy cows. J. Dairy Sci. 81:2139–2144.[Abstract]
Pursley, J. R., M. C. Wiltbank, J. S. Stevenson, J. S. Ottobre, H. A. Garverick, and L. L. Anderson. 1997. Pregnancy rates per artificial insemination for cows and heifers inseminated at a synchronized ovulation or synchronized estrus. J. Dairy Sci. 80:295–300.[Abstract]
Risco C. A., Drost M., Archbald L., F. Moreira, R. L. de la Sota, J. Burke and W. Thatcher. 1998. Timed artificial insemination in dairy cattle - Part I. Compend. Continuing Ed. Med. Manage. 20: 280–287.
Ryan D. P., J. A. Galvin, and K. J. O’Farrell. 1996. Effect of various oestrus synchronization programs applied to lactating dairy cows on oestrus synchrony and pregnancy rates. In: Proc. Int. Cong. Anim. Reprod., Sydney, NSW, Australia. 19:10 (Abstr.).
Stevenson J. S., Y. Kobayashi, M. P. Shipka, and K. C. Rauchholz. 1996. Altering conception of dairy cattle by goandotrphin-releasing hormone preceding luteolysis induced by prostaglandin F2. J. Dairy Sci. 79:402–410.[Abstract]
Twagiramungu, H. L., L. A. Guilbault, and J. J. Dufour. 1995. Synchronization of ovarian follicular waves with a gonadotropin-releasing hormone agonist to increase the precision of estrus in cattle: A review. J. Anim. Sci. 73:3141–3151.[Abstract]
Twagiramungu, H. L., L. A. Guilbault, J. Proulx, and J. J. Dufour. 1994. Influence of corpus luteum and induced ovulation on ovarian follicular dynamics in postpartum cyclic cows treated with buserelin and cloprostenol. J. Anim. Sci. 72:1796–1805.[Abstract]
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Abstract
Introduction
