J. Anim. Sci. 2003. 81:E166-E177
© 2003 American Society of Animal Science
A review of methods to synchronize estrus in replacement beef heifers and postpartum cows1
D. J. Patterson2,
F. N. Kojima and
M. F. Smith
Department of Animal Science, University of Missouri, Columbia 65211
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Abstract
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This review considers methods currently available to control estrous cycles of postpartum beef cows and replacement beef heifers. Development of methods to control the estrous cycle of the cow has occurred in six distinct phases. The physiological basis for estrus synchronization followed the discovery that progesterone inhibited preovulatory follicular maturation and ovulation. Regulation of estrous cycles was believed to be associated with control of the corpus luteum, the life span and secretory activity of which are regulated by trophic and lytic mechanisms. Phase I (Progesterone Phase) included efforts to prolong the luteal phase of the estrous cycle or to establish an artificial luteal phase by administering exogenous progestins. Later, progestational agents were combined with estrogens or gonadotropins in Phase II (Progesterone-Estrogen Phase), whereas Phase III (PG Phase) involved prostaglandin F2
(PG) and its analogs as luteolytic agents. Treatments that combined progestational agents with PG characterized Phase IV (Progestogen-PG Phase). Precise monitoring of ovarian follicles and corpora lutea over time by transrectal ultrasonography expanded our understanding of the bovine estrous cycle and particularly the change that occurs during a follicular wave. We now know (Phase V, GnRH-PG Phase) that precise control of estrous cycles requires the manipulation of both follicular waves and luteal lifespan. This review includes specific discussion of progestins, PG, GnRH, and various combinations of these hormones or their analogs used to more precisely control the interval and timing of estrus following treatment (Phase VI, Progestogen-GnRH-PG Phase). The review also addresses the potential benefits of these treatments in eliciting a response from peripubertal heifers and anestrous cows, and points to the flexibility in matching specific protocols with the particular beef management system involved. Recent advances in the development of methods of artificially inseminating beef cows and heifers at a fixed time with high fertility are discussed, which should potentially result in a dramatic increase in the adoption of AI in beef herds.
Key Words: Artificial Insemination • Beef Cattle • Estrous Cycle • Synchronization
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Introduction
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The percentage of beef cattle inseminated artificially is predicted to increase substantially with the advent of sexed semen (Seidel, 1998
). Currently, however, surveys indicate that fewer than 5% of the beef cows in the United States are bred by AI, and only half of the cattlemen that practice AI use any form of estrus synchronization to facilitate their AI programs (Corah and Kiracofe, 1989; NAHMS, 1994). The inability to predict time of estrus for individual females in a group often makes it impractical to use AI because of the labor required for estrus detection (Britt, 1987). The development of methods of artificially inseminating beef cows and heifers at a fixed time with high fertility should result in a dramatic increase in the adoption of AI in beef herds.
Expanded use of AI and/or adoption of emerging reproductive technologies for beef cows and heifers requires precise methods of estrous cycle control. Effective control of the estrous cycle requires the synchronization of both luteal and follicular functions. Efforts to develop more effective estrus synchronization protocols have focused recently on synchronizing follicular waves by injecting GnRH, followed 7 d later by injection of PG (Ovsynch, CO-Synch, and Select Synch). A factor contributing to reduced synchronized pregnancy rates in beef cows treated with the preceding protocols is that 5 to 15% of estrous cycling cows show estrus on or before PG injection (Kojima et al., 2000). New protocols for inducing and synchronizing a fertile estrus in postpartum beef cows and replacement heifers in which the GnRH-PG protocol is preceded by either short- or long-term progestin treatment offer significant potential to enhance response to estrus synchronization, increase pregnancy rate to AI during the synchronized period, and facilitate insemination at a fixed time (Kojima et al., 2000; Wood et al., 2001; Perry et al., 2002).
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Definitions, Protocols, and Terms
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The following definitions, protocols, and terms referred to throughout this manuscript are defined below.
Protocols
PG: Prostaglandin F2 (Lutalyse Sterile Solution, Pharmacia Animal Health, Kalamazoo, MI; Estrumate, Bayer Corp., Shawnee Mission, KS; ProstaMate, Phoenix Scientific, Inc., St. Joseph, MO; In Synch, Agri Laboratories, Ltd., St. Joseph, MO). MGA-PG: Melengestrol acetate (MGA; 0.5 mg/animal per day) is fed for a period of 14 d with PG administered 17 or 19 d after MGA withdrawal. GnRH-PG, Select Synch: Gonadotropin-releasing hormone injection (Cystorelin, Merial Ltd., Iselin, NJ; Factrel, Fort Dodge Animal Health, Overland Park, KS; Fertagyl, Intervet, Inc., Millsboro, DE) followed after 7 d with an injection of PG. MGA Select: MGA is fed for 14 d, GnRH is administered 10 or 12 d after MGA withdrawal, and PG is administered 7 d after GnRH. 7-11 Synch: MGA is fed for 7 d, PG is administered on the last day MGA is fed, GnRH is administered 4 d after the cessation of MGA, and a second injection of PG is administered 11 d after MGA withdrawal.
Terms
Estrous response: The number of females that exhibit estrus during a synchronized period. Synchronized period: The period of time during which estrus is expressed after treatment. Synchronized conception rate: The proportion of females that become pregnant of those exhibiting estrus and inseminated during the synchronized period. Synchronized pregnancy rate: The proportion of females that become pregnant of the total number treated.
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Development of Methods to Synchronize Estrus
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The development of methods to control the estrous cycle of the cow has occurred in six distinct phases. The physiological basis for estrus synchronization followed the discovery that progesterone inhibited ovulation (Ulberg et al., 1951) and preovulatory follicular maturation (Nellor and Cole, 1956; Hansel et al., 1961; Lamond, 1964). Regulation of estrous cycles was believed to be associated with control of the corpus luteum, the life span and secretory activity of which are regulated by trophic and lytic mechanisms (Thimonier et al., 1975). The Progesterone Phase included efforts to prolong the luteal phase of the estrous cycle or to establish an artificial luteal phase by administering exogenous progesterone. Later, progestational agents were combined with estrogens or gonadotropins in the Progesterone-Estrogen Phase. Prostaglandin F2 and its analogs were reported in 1972 to be luteolytic in the bovine (Lauderdale, 1972; Liehr et al., 1972; Rowson et al., 1972; Lauderdale et al., 1974) and ushered in the PG Phase. Treatments that combined progestational agents with PG characterized the Progestogen-PG Phase. All of these protocols addressed control of the luteal phase of the estrous cycle since follicular waves were not recognized at the time.
Precise monitoring of ovarian follicles and corpora lutea over time by transrectal ultrasonography expanded our understanding of the bovine estrous cycle and particularly the change that occurs during a follicular wave (Fortune et al., 1988). Growth of follicles in cattle occurs in distinct wave-like patterns, with new follicular waves occurring approximately every 10 d (6 to 15 d range). We now know that precise control of estrous cycles requires the manipulation of both follicular waves and luteal lifespan (GnRH-PG Phase).
A single injection of GnRH to cows at random stages of their estrous cycles causes release of LH, which leads to synchronized ovulation or luteinization of most large, dominant follicles (
10 mm; Garverick et al., 1980; Bao and Garverick, 1998; Sartori et al., 2001). Consequently, a new follicular wave is initiated in all cows within 2 to 3 d of GnRH administration. Luteal tissue that forms after GnRH administration is capable of undergoing PG-induced luteolysis 6 or 7 d later (Twagiramungu et al., 1995). The GnRH-PG protocol increased estrus synchronization rate in beef (Twagiramungu et al., 1992a,b) and dairy (Thatcher et al., 1993) cattle. A drawback of this method, however, is that approximately 5 to 15% of the cows are detected in estrus on or before the day of PG injection, thus reducing the proportion of females that are detected in estrus and inseminated during the synchronized period (Kojima et al., 2000). This information stimulated research in the Progestogen-GnRH-PG Phase.
Synchronization of Estrus and Ovulation with the GnRH-PG-GnRH Protocol
Administration of PG alone is commonly utilized to synchronize an ovulatory estrus in estrous cycling cows. However, this method is ineffective in anestrous females, and variation among animals in the stage of the follicular wave at the time of PG injection directly contributes to the variation in onset of estrus during the synchronized period (Macmillan and Henderson, 1984; Sirois and Fortune, 1988). Consequently, the GnRH-PG-GnRH protocol was developed to synchronize follicular waves and timing of ovulation. The GnRH-PG-GnRH protocol (Figure 1) for fixed-time AI results in the development of a preovulatory follicle that ovulates in response to a second GnRH-induced LH surge 48 h after PG injection (Ovsynch; Pursely et al., 1995). Ovsynch was validated recently as a reliable means of synchronizing ovulation for fixed-time AI in lactating dairy cows (Pursley et al., 1995; Burke et al., 1996; Schmitt et al., 1996; Pursley et al., 1997a,b). Time of ovulation with Ovsynch occurs between 24 to 32 h after the second GnRH injection and is synchronized in 87 to 100% of lactating dairy cows (Pursley et al., 1997a). Pregnancy rates among cows that were inseminated at a fixed time following Ovsynch ranged from 32 to 45% (Pursley et al., 1997b; 1998). The Ovsynch protocol, however, did not effectively synchronize estrus and ovulation in dairy heifers (35% pregnancy rate compared with 74% in PG controls; Pursley et al., 1997b).
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Figure 1. Methods currently being used to synchronize ovulation in postpartum beef cows: Ovsynch, CO-Synch, and Select Synch.
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Protocols for fixed-time insemination were recently tested in postpartum beef cows. Pregnancy rates for Ovsynch treated beef cows were compared with those of cows synchronized and inseminated at a fixed time following treatment with Syncro-Mate-B (Geary et al., 1998a). Calves in both treatment groups were removed from their dams for a period of 48 h beginning either at the time of implant removal (Syncro-Mate-B) or at the time PG was administered (Ovsynch). Pregnancy rates following fixed-time AI after Ovsynch (54%) were higher than those for Syncro-Mate-B-treated cows (42%). One should note that on the day following timed insemination, cows were exposed to fertile bulls of the same breed; no attempt was made to determine progeny paternity. Additionally, we do not know the incidence of short cycles among cows that were anestrous prior to treatment and that perhaps returned to estrus prematurely and became pregnant to natural service.
Recently, variations of the Ovsynch protocol (CO-Synch and Select Synch) were tested in postpartum beef cows (Figure 1
). It is important to understand that treatment variations of Ovsynch currently being used in postpartum beef cows have not undergone the same validation process that Ovsynch underwent in lactating dairy cows. At this point, we do not know whether response in postpartum beef cows to the protocols outlined in Figure 1 is the same or different from lactating dairy cows due to potential differences in follicular wave patterns. Differences in specific response variables may include: a) the relative length of time to ovulation from the second GnRH injection; b) the anticipated range in timing of ovulation; and c) the degree of ovulation synchrony that occurs.
Two variations from Ovsynch being used most extensively in postpartum beef cows are currently referred to as CO-Synch and Select Synch. CO-Synch (Geary et al., 1998b) is similar to Ovsynch in that timing and sequence of injections are the same and all cows are inseminated at a fixed time. CO-Synch differs from Ovsynch, however, in that cows are inseminated when the second GnRH injection is administered, compared to the recommended 16 h after GnRH for Ovsynch treated cows. Select Synch (Geary et al., 2000) differs, too, in that cows do not receive the second injection of GnRH and are not inseminated at a fixed time. Cows synchronized with this protocol are inseminated 12 h after detected estrus. It is currently recommended that for Select Synch-treated cows, detection of estrus begin as early as 4 d after GnRH injection and continue through 6 d after PG (Kojima et al., 2000). Select Synch, similar to Ovsynch, was less effective than the MGA-PG protocol in synchronizing estrus in beef heifers (Stevenson et al., 1999).
Synchronization of Estrus with the MGA-PG Protocol
Melengestrol acetate, as a progestogen, was shown to be effective for estrus synchronization of beef cows and heifers (Zimbelman et al., 1970). Estrus synchronization programs designed for heifers and postpartum beef cows should be evaluated in relation to their effect on conception (Patterson et al., 1989; Folman et al., 1990). Until recently, there was little published evidence comparing methods of estrous cycle control that utilize PG alone to methods that utilize progesterone or progestogens in conjunction with PG. Feeding MGA for 14 d followed by PG injection 17 d after MGA feeding (MGA-PG protocol) is an effective method of estrous cycle control in heifers (Brown et al., 1988; Patterson and Corah, 1992). More recently, an increase in estrous response, synchronized conception and pregnancy rates, and fecundity in the postpartum cow was reported among cows treated with the MGA-PG protocol when compared with PG alone (Figure 2A and B; Patterson et al., 1995).
We know from work with both dairy (Britt et al., 1972) and beef cows (Zimbelman et al., 1970; Patterson et al., 1995) that the second synchronized estrus after MGA, whether spontaneous or induced with PG, may be inherently more fertile. Reported differences in conception rate for beef cows are shown in Figure 2B (Patterson et al., 1995). Treated cows in that study each received 0.5 mg of MGA or carrier without MGA for 14 d. All cows received PG 17 d after the last feeding day of MGA or carrier without MGA. Control and treated cows that failed to exhibit estrus within 6 d after the first injection of PG were reinjected with PG 11 d later (Figure 2A). Many of the cows that failed to respond to PG on d 17 after withdrawal of MGA were cows that were anestrus prior to MGA treatment. Fralix et al. (1996) reported that up to 20% of anestrous cows experience short cycles prior to PG administered on d 17 after MGA withdrawal (Figure 3). To remedy this problem, unresponsive cows may be reinjected with PG 11 d later, or on d 42 from the beginning of treatment (Figure 2).
The administration of MGA at the recommended daily rate of 0.5 mg prevents the expression of behavioral estrus, blocks the preovulatory surge of LH, and ovulation (Zimbelman and Smith, 1966; Zimbelman et al., 1970; Imwalle et al., 2002). Until recently, however, there was no evidence to suggest that MGA would induce cyclicity in peripubertal heifers (Patterson et al., 1990; Imwalle et al., 1998) or improve conception and increase ovulation rate in postpartum beef cows (Patterson et al., 1995; Fralix et al., 1996; Patterson et al., 1997).
The disadvantages of the MGA-PG system include: 1) anestrous cows that experience a short luteal phase after the period of MGA feeding, which in some cases necessitates a second PG injection (Fralix et al., 1996); 2) although twinning was not detected in an extensive study of MGA in beef cattle estrus synchronization (Zimbelman et al., 1970), the potential for an increased incidence of twinning (Patterson et al., 1995) was reported, which is undesirable in many beef production systems from a management viewpoint; 3) the overall length of the treatment period; and 4) the difficulty in some management situations of ensuring adequate intake of MGA on a daily basis.
Advantages of MGA for synchronization of estrus are ease of administration and cost. Furthermore, MGA recently received clearance from the FDA (Federal Register, 1997) for use in reproductive classes of beef cows and heifers and dairy heifers; therefore, research of methods for use in estrous cycle control involving MGA bear increased significance and marked relevance to current industry needs. These are important considerations for widespread use of any successful estrus synchronization treatment and are essential to expanded application of AI in beef cattle. The MGA-PG protocol avoids problems with reduced conception and offers advantages compared with untreated controls (Brown et al., 1988; Patterson and Corah, 1992).
Effect of the MGA-GnRH-PG Protocol on Estrus Synchronization
Twagiramungu et al. (1995) reported an increase in estrous response in postpartum beef cows over PG-treated controls when cows received an injection of GnRH 7 d before PG. Advantages of the GnRH-PG system include simplicity of administration and short duration of treatment. The major disadvantage of the GnRH-PG protocol is the percentage of cows that exhibit estrus after GnRH and before PG, which in some cases may be as high as 15% of the total number of cows treated (Kojima et al., 2000). In order to inseminate all cows that respond during the treatment period, estrus detection is required for a period of 10 d, beginning 4 d before and 6 d after PG administration.
There have been few studies designed to evaluate progestin treatment prior to administration of the GnRH-PG protocol. The addition of MGA to the GnRH-PG protocol was compared with the standard MGA-PG protocol in postpartum beef cows (Patterson et al., 1999). The design and results from that study are shown in Figures 4A and 4B. None of the cows in the MGA-GnRH-PG group exhibited estrus before PG. Synchrony of estrus was improved among MGA-GnRH-PG treated cows, with about 80% of the cows assigned to that treatment exhibiting estrus 48 to 96 h after PG (Figure 4B). Additionally, there was no difference between MGA-GnRH-PG and MGA-PG protocols in synchronized conception (78 and 83%, respectively) or pregnancy rate (65 and 67%, respectively) during the synchronized period.
The MGA-GnRH-PG protocol was also compared to the GnRH-PG protocol in postpartum beef cows. The design and results from that study are shown in Figures 5A and 5B (Patterson et al., 2000a). Synchrony of estrus was improved among MGA-GnRH-PG-treated cows compared to cows that did not receive MGA (GnRH-PG) and high pregnancy rates to AI were maintained (70 and 59%, respectively). The distribution of estrus among MGA-GnRH-PG-treated cows was similar to the distribution illustrated in Figure 4B, demonstrating the repeatability of response following this treatment.
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Using MGA to Synchronize Estrus in Heifers
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A Modified MGA Protocol for Heifers
Recent studies with heifers show that both synchrony of estrus and total estrous response improved when PG was administered 19 d after MGA withdrawal compared with heifers that were injected on d 17 after MGA withdrawal (Figure 6; Nix et al., 1998; Deutscher, 2000; Lamb et al., 2000). No difference in fertility between treatments was reported.
We evaluated a modified MGA-PG protocol for inducing and synchronizing a fertile estrus in beef heifers (Figure 7; Wood et al., 2001). The first modification changed the day of PG injection from d 31 to d 33 of treatment. The second modification was the addition of a GnRH injection on d 26 of treatment. Wood et al. (2001) found that injection of GnRH on d 26 of the MGA-PG protocol induced luteal tissue formation and initiated a new follicular wave on approximately d 28 in cycling beef heifers (Figure 8B). The proportion of heifers with synchronized follicular waves on d 33 was increased significantly compared to heifers that did not receive GnRH (Figure 8A and B; Wood et al., 2001).
Wood (2000) also reported differences in estrous response and synchrony of estrus during the synchronized period among heifers assigned to the treatments illustrated in Figure 7. This difference in estrous response and degree of synchrony was based on the percentage of heifers that were pubertal at the time treatment with MGA began. Figure 9A and B illustrate these differences (Wood, 2000).
Figure 9A shows the distribution of estrus where only 30% of the heifers were pubertal at the time treatment with MGA began, whereas Figure 9B illustrates the distribution of estrus for heifers where 56% of the heifers were pubertal at the same time. The increased cyclicity of heifers shown in Figure 9B was associated with a reduced variance in the interval to estrus among MGA-GnRH-PG-treated heifers. AI pregnancy rates remained high for both MGA-GnRH-PG- and MGA-PG-treated heifers and were not different (67 and 60%, respectively [Location 1] and 75 and 72%, respectively [Location 2]).
Collectively, results from several studies indicate that the decision to add GnRH to a 14- to 19-d MGA-PG protocol for heifers should involve careful consideration of age, weight, and pubertal status of heifers at the time treatment with MGA-PG is initiated (Wood, 2000; Kojima et al., 2001). In situations where heifers are scheduled to begin an estrus synchronization treatment with MGA, we recommend that reproductive tract scores (RTS; Anderson et al., 1991; Patterson et al., 2000b) be performed within 2 wk prior to the initiation of treatment. We further recommend that heifers are ready to begin treatment with MGA if 50% of the heifers within a group are assigned RTS of 4 or 5 (Patterson et al., 2000b). This indicates that these heifers have reached puberty and are estrous cycling. Based on the age and weight of prepubertal or peripubertal contemporaries, up to 70% of these heifers can be expected to exhibit estrus and ovulate after MGA withdrawal, so the potential estrous response during the synchronized period is up to 80%. Estrous response among heifers that were assigned scores of 2 or 3 was lower than for those assigned scores of 4 or 5. However, as RTS increased, estrous response improved (Patterson et al., 2000b; Funston et al., 2002).
Considerations Related to Long-Term Feeding of MGA to Heifers
Long-term feeding of MGA to beef heifers and associated effects on fertility may be a concern in specific production systems. It is not uncommon for heifers to be placed on MGA for extended periods of time and subsequently exposed for breeding after placement in backgrounding programs that necessitate long-term MGA administration. Zimbelman et al. (1970) reported no negative effect of either long-term or repeated intervals of feeding MGA to beef cows and heifers other than the expected reduced conception rate when cattle were bred at the synchronized estrus 3 to 7 d after the last day of MGA feeding. Patterson et al. (1993) designed a study (Figure 10) to compare estrous response and fertility during synchronized estrous periods among beef heifers that were fed MGA for 87 d (long-term, LT) or 14 d (short-term, ST) prior to PG. Heifers were stratified by age and weight to LT- or ST-MGA treatments (Table 1), and received 0.5 mg of MGA per animal, per day for 87 or 14 d. Heifers in each group were administered PG 17 d after MGA withdrawal. Heifers in both groups that failed to exhibit estrus within 6 d after the first injection of PG, were administered a second injection of PG 11 d after the first injection (Figure 10). Transrectal ultrasonography was used to examine ovaries of all heifers at the end of treatment with MGA and at the time PG was administered. Heifers that failed to exhibit estrus after the first injection of PG were reexamined prior to the second PG injection. All heifers were exposed for natural-service for an additional 45 d after the AI period.
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Figure 10. Treatment schedule for long-term and short-term feeding of melengestrol acetate (MGA; adapted from Patterson et al., 1993).
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More ST-treated heifers exhibited estrus after the first injection of PG than LT-treated heifers (Table 2; P < 0.05). Total response after the two injections of PG, however, did not differ between treatments. Furthermore, there were no significant differences between treatments in synchronized conception and pregnancy rates, or pregnancy rates at the end of the breeding period (Table 2). A higher incidence of luteinized follicular cysts (Table 3) was observed among heifers in the LT-treatment compared with heifers in the ST-treatment (LT, 11/30 [37%]; ST, 0/31 [0%]). This observation may explain differences in estrous response between treatments following the first injection of PG.
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Table 2. Estrous response and fertility of heifers treated long-term or short-term with melengestrol acetate (MGA)
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These data indicate that long-term feeding of MGA may result in a higher than normal incidence of luteinized follicular cysts and an associated reduction in estrous response after PG. The data indicate, however, that reinjection with PG resulted in satisfactory breeding performance among heifers that were fed MGA for extended periods of time.
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Using MGA to Synchronize Estrus in Postpartum Beef Cows
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Development of the MGA Select Protocol for Postpartum Cows
Patterson et al. (2002) compared the 14- to 19-d MGA-PG protocol in postpartum suckled beef cows with or without the addition of GnRH on d 12 after MGA withdrawal and 7 d before PG as described by Wood et al. (2001; Figure 5). Table 4 provides a summary of the number of cows within age group by treatment, the mean number of days postpartum on the first day of MGA feeding, the BCS of cows on the day GnRH was administered, and the percentage of cows that were estrous cycling prior to initiation of MGA treatment. Cyclicity rates of cows at the onset of MGA feeding were lower among cows 5 yr of age than cows 
4 yr of age. Mean intervals to estrus differed between treatments (P < 0.06), with longer intervals observed among cows assigned to the MGA-PG protocol. Mean intervals to estrus for MGA-GnRH-PG- and MGA-PG-treated cows were 74.4 ± 1.8 and 81.1 ± 2.3 h, respectively. Table 5 provides a summary of estrous response and the synchronized conception and pregnancy rates of cows assigned to the two treatments. Estrous response was higher (P < 0.05) among MGA-GnRH-PG-treated cows than MGA-PG-treated cows. These data demonstrate an improvement in estrous response among postpartum beef cows in which anestrous rates were high.
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Table 4. Number of cows, age, days postpartum, body condition score, and cyclicity status for cows in each treatment (mean ± SE)a
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Table 5. Estrous response, synchronized conception and pregnancy rates for cows assigned to MGA-PG or MGA-GnRH-PG treatmentsa
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There was no difference between treatments in conception rate of cows during the synchronized period. Synchronized pregnancy rate (number of cows pregnant of the total number treated), however, was higher among cows 5 yr of age assigned to the MGA-GnRH-PG treatment compared with MGA-PG-treated cows of the same age. Final pregnancy rate of cows at the end of the breeding season was the same for cows assigned to the respective two treatments (97% for both MGA-GnRH-PG- and MGA-PG-treated cows).
Perry et al. (2002) conducted an experiment to determine whether pretreatment with MGA prior to a GnRH-PG-GnRH (control) protocol would improve pregnancy rates resulting from fixed-time AI. Cows were assigned by age and days postpartum to one of two treatments. Control and MGA-treated (Figure 11) cows were fed a supplement carrier with or without MGA for 14 d. Gonadotropin-releasing growth hormone was administered to all cows 12 d after MGA or carrier withdrawal and 7 d prior to PG. All cows were administered GnRH and artificially inseminated 72 h after PG. Pregnancy rates to fixed-time AI are reported in Table 6. There was no difference between treatments at location 1 (MGA = 58% [26/45]; Control = 51% [23/45]). However, there was a difference (P < 0.03) in pregnancy rate to fixed-time AI between treatments at location 2 (MGA = 63% [44/70]; Control = 45% [30/67]). Furthermore, when results from both locations were combined, the overall difference remained significant (MGA = 70/115 [61%]; Control = 53/112 [47%]; P < 0.05). These results indicate that pregnancy rates resulting from fixed-time insemination are improved significantly when treatment with MGA precedes the GnRH-PG-GnRH protocol. This approach to estrus synchronization that involves a 14-d feeding period of MGA followed 12 d (d 26) later by an injection of GnRH, and PG on d 19 after MGA withdrawal (d 33) was recently named MGA Select (Figure 12).
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Figure 11. Treatment schedules and timing of fixed-time insemination for melengestrol acetate (MGA)- treated and Control (modified CO-Synch) cows (adapted from Perry et al., 2002).
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Figure 12. The MGA Select protocol. This treatment protocol involves a 14-d feeding period of melengestrol acetate (MGA), followed by the administration of GnRH on d 12 after MGA withdrawal and prostaglandin F2 (PG) 7 d later.
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Development of Methods to Shorten the Length of Treatment, 7-11 Synch
Kojima et al. (2000) developed an estrus synchronization protocol for beef cattle that was designed to shorten the feeding period of MGA without compromising fertility and to improve synchrony of estrus by synchronizing development and ovulation of follicles from the first wave of development (Figure 13A). This new treatment, 7-11 Synch, was compared with the GnRH-PG protocol. Synchrony of estrus during the 24-h peak response period (42 to 66 h) was significantly higher (P < 0.01) among 7-11 Synch treated cows. Furthermore, the distribution of estrus was reduced (P < 0.05) from 144 h for GnRH-PG-treated cows to 60 h for cows assigned to the 7-11 Synch treatment (Figure 13B; Kojima et al., 2000). The 7-11 Synch protocol resulted in a higher degree of estrus synchrony (91%) and greater AI pregnancy rate (68%) during a 24-h peak response period compared to the GnRH-PG protocol (69 and 47%, respectively).
The 7-11 Synch protocol has also shown significant potential for use in conjunction with fixed-time AI programs (Hixon et al., 2001; Kojima et al., 2002). These studies report high pregnancy rates resulting from fixed-time insemination of cows that were synchronized with the 7-11 Synch protocol, thereby eliminating the need to detect estrus. Further research is needed to more precisely determine the appropriate time of AI following treatment with 7-11 Synch and the necessity of administering GnRH at AI (Kojima et al., 2002).
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Summary and Conclusions
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Estrus synchronization and AI remain the most important and widely applicable reproductive biotechnologies available for cattle (Seidel, 1995). Although hormone treatment of heifers and cows to group estrous periods has been a commercial reality now for over 30 yr, producers have been slow to adopt this management practice. Perhaps this is because of past failures, which resulted when females that were placed on estrus synchronization treatments failed to reach puberty or to resume normal estrous cycles following calving, and the reality that early estrus synchronization programs failed to manage follicular waves, resulting in more days in the synchronized period and precluded timed insemination with acceptable pregnancy rates. Patterson et al. (1999) proposed the general hypothesis that progestin treatment prior to the GnRH-PG estrus synchronization protocol would successfully: 1) induce ovulation in anestrous postpartum beef cows and peripubertal beef heifers; 2) reduce the incidence of a short luteal phase among anestrous cows induced to ovulate; 3) increase estrous response, synchronized conception and pregnancy rates; and 4) increase the likelihood of successful fixed-time insemination. New methods of synchronizing estrus in beef cattle outlined in this review present the opportunity to enhance results from AI and thereby reduce the period of time required to detect estrus or eliminate the need entirely. Further research is needed to more precisely determine the appropriate timing of fixed-time insemination following administration of these protocols and their extended application to the beef cattle industry.
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Implications
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New methods of inducing and synchronizing estrus for postpartum beef cows and replacement beef heifers in which the gonadotropin-releasing hormone-prostaglandin F2 protocol is preceded by a progestin offer significant potential to more effectively synchronize estrus with resulting high fertility. These new protocols may provide significant opportunity to enhance results from fixed-time artificial insemination and offer the beef cattle industry the prospect of expanding the use of artificial insemination.
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Footnotes
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1 Contribution from the Missouri Agric. Exp. Stn. The authors gratefully acknowledge support from Select Sires, Inc., Pharmacia Animal Health, Merial Ltd., and USDA-NRI 00-35203-9175. 
2 Correspondence: S132 Animal Science Research Center (phone: 573-882-7519; fax: 573-882-4798; E-mail: pattersonD{at}missouri.edu).
Received for publication August 7, 2002.
Accepted for publication December 24, 2002.
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Literature Cited
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Abstract
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