Estradiol valerate

Abstract

Numerous procedures have been developed to manipulate the estrous cycle of heifers to maximize the number that are in estrus during a short, predefined period with normal fertility-synchronized estrus. Early procedures consisted of synthetic progestins alone [melengestrol acetate; MGA(R) (Pharmacia and Upjohn Company, Kalamazoo, MI)], prostaglandin F^sub 2[alpha]^ (PGF^sub 2[alpha]^) alone, and synthetic progestins in combination with PGF^sub 2[alpha]^ (7 d of MGA(R) with PGF^sub 2[alpha]^ on the last d) or estradiol valerate [Syncro-Mate B(R) (Merial Ltd., Iselin, NJ)]. The immediate estrus following MGA(R) treatment had suppressed fertility, and all of these early procedures resulted in either variability in the interval to estrus, poor results, or inconsistent results. During the past decade a new estrous synchronization procedure consisting of the daily feeding of MGA(R) (0.5 mg/d per head) for 14 d and an injection of PGF^sub 2[alpha]^ 19 d after the last d of MGA(R) was demonstrated to have pregnancy rates of 50 to 62%. This procedure hastens puberty and is robust; it works in most situations. The CIDR(R) (Pharmacia and Upjohn), an intravaginal insert containing progesterone, also synchronizes estrus and hastens puberty. Studies using the CIDR(R) insert for 7 d with an injection of PGF^sub 2[alpha]^ on d 6 or 7 to synchronize estrus have reported pregnancy rates similar to heifers synchronized with the MGA(R)/PGF^sub 2[alpha]^ procedure; however, this procedure has not been as extensively studied as the MGA(R)/PGF protocol and might not be optimized. Both systems have utility in managing reproduction of beef and dairy heifers.

(Key Words: Melengestrol Acetate, Prostaglandin F^sub 2[alpha]^, Progesterone, Pregnancy Rates, Estrus.)

Introduction

Methods to manipulate the estrous cycle so that all heifers are in estrus during a short, predefined period with normal fertility (synchronized estrus) has been a difficult goal to achieve; however, valuable methods have been created, and they are available to producers today. Table 1 provides a succinct history of procedures that have been used to synchronize estrus in heifers. Although estrous synchronization and AI have been cited over the past several decades to have significant impact on reproductive performance of replacement heifers (Schafer et al., 1990) and on increased profitability in beef operations (Lesmeister et al., 1973; Bellows and Short, 1990; Wiltbank, 1990), only a percentage of beef cattle operations in the US synchronize estrus of their heifers. In a summary by the National Animal Health Monitoring Systems (NAHMS) only 3.3% of beef heifers in the US were being synchronized (NAHMS, 1994). A national survey sponsored by the National Association of Animal Breeders (NAAB) revealed that only 55 to 63% of the dairy heifers in the US were being bred by AI (Erven and Arbaugh, 1987) even though the USDA Holstein Sire Summary estimated a Net Merit Dollars advantage of $72 per lactation. A survey was conducted in Illinois in 2000 in an attempt to understand why producers were not using estrous synchronization (Kesler, 2001); the results are presented in Table 2. Of the choices provided, one factor emerged as the primary reason: lack of time/labor. The second ranked reason was poor results. These should be considered as the two most important factors that must be considered in new product development and education. Products that were made available until the 1990s had limited efficacy and consistency and might have caused producers to lose interest in or question new developments.

The objectives of this paper are to provide a brief review of the systems that have been developed to synchronize estrus in heifers and explain in greater detail the newer systems that synchronize estrus far more effectively and with greater consistency than the systems introduced before the 1990s.

Results and Discussion

Synchronization of Estrus with Melengestrol Acetate (Pharmacia and Upjohn Company, Kalamazoo, MI) Alone, Prostaglandin F^sub 2[alpha]^ Alone, Short-Term Melengesterol Acetate Feeding with PGF^sub 2[alpha]^, and Syncro-Mate B(R) (Merial Ltd., Iselin, NJ). One of the first methods developed to synchronize estrus in heifers was the long-term feeding of melengestrol acetate (MGA(R); Zimbelman and Smith, 1966). Melengestrol acetate is an orally active synthetic progestin. Melengestrol acetate suppresses estrus when fed at a daily dose of 0.5 mg per heifer. Although long-term feeding of MGA(R) effectively synchronized estrus, fertility was compromised (Zimbelman and Smith, 1966; DeBois and Bierschwal, 1970). More recent research has demonstrated that although MGA(R) and other progestins effectively suppress estrus, follicles continued to develop and persist on the ovary in cyclic females during progestin administration (Anderson and Day, 1994; Perry et al., 2002). These persistent follicles, follicles with a prolonged lifespan, are capable of ovulating and developing corpora lutea that can maintain pregnancy from embryo transfer (Wehrman et al., 1997). However, fertility in heifers with persistent follicles was depressed because the ability of the fertilized oocytes to develop to the 16-cell stage was compromised (Ahmad et al., 1995).

In the 1970s, it was discovered that prostaglandin F^sub 2[alpha]^ (PGF^sub 2[alpha]^) was luteolytic in cattle, and it was subsequently made available to synchronize estrus (Lauderdale, 1972; Lauderdale et al., 1974). However, PGF^sub 2[alpha]^ had limited utility because to effect luteolysis, only heifers with corpora lutea on d > or =5 of the estrous cycle were responsive. Therefore, prepubertal heifers, heifers on d 0 to 4 of the estrous cycle, and heifers subsequent to luteolysis were not affected by PGF^sub 2[alpha]^ treatment. Although strategies were developed to overcome some of these obstacles, such as administration of two injections of PGF^sub 2[alpha]^ at a 10- to 12-d interval, there was still considerable variability in the interval from treatment to estrus. This variability was undesirable because a 5- to 7-d estrus detection and AI period was required. It has since been discovered that this variation in the interval from PGF^sub 2[alpha]^ injection to estrus is dependent on the stage of the follicular wave when PGF^sub 2[alpha]^ is administered (Lucy et al., 1992).

In the 1980s, there was considerable research evaluating the concurrent use of MGA(R) and PGF^sub 2[alpha]^ for synchronization of estrus. This program involved daily feeding of MGA(R) for 7 d and the administration of PGF^sub 2[alpha]^ on the last day of MGA(R) feeding. The strategy was used to reduce the number of days of MGA(R) feeding in attempt to overcome the compromised fertility that resulted after long-term MGA(R) feeding (Beal et al., 1988). Although this procedure effectively synchronized estrus, fertility was suppressed in comparison with a non-synchronized estrus (Chenault et al., 1990). Conception rates to AI during a 6-d synchronization period were 68% for heifers administered a single injection of PGF^sub 2[alpha]^ vs 52% for heifers administered the short-term MGA(R) and PGF^sub 2[alpha]^ protocol. The synchronized pregnancy rate for heifers administered the short-term MGA(R) and PGF^sub 2[alpha]^ protocol was 39% and not different from the synchronized pregnancy rate for heifers administered a single injection of PGF^sub 2[alpha]^ (37% pregnancy rate).

In the 1980s and 1990s, Syncro-Mate B(R) (Merial Ltd., Iselin, NJ) was available in the US to synchronize estrus in beef and dairy heifers. A major advantage of this product was that pregnancy rates were as great, if not greater, for heifers bred at a predetermined time as compared with breeding at a detected estrus (Kesler and Favero, 1996). Syncro-Mate B(R) consisted of the use of an implant containing norgestomet and the injection of norgestomet and estradiol valerate at the time of implantation. The implant was placed subcutaneously in the ear and removed 9 d later (Kesler and Favero, 1995). A dose of norgestomet adequate to suppress estrus diffused from the implant during the 9-d implantation period (Kesler et al., 1995). The injection caused a delayed regression of early developing corpora lutea so that by the end of the implantation period, estrus was suppressed in all heifers by the norgestomet from the implant (Peterson et al., 2000). Upon implant removal, estrus and ovulation occurred at a consistent time. The disadvantage of the Syncro-Mate B(R) procedure was that fertility was inconsistent and at times

Synchronization of Estrus with MGA(R) (14 d of Daily Feeding) Followed by PGF^sub 2[alpha]^ 17 to 19 d Later. Another procedure consisted of the concurrent administration of MGA(R) fed daily at a rate of 0.5 mg/d per head for 14 d and the injection of PGF^sub 2[alpha]^ 17 d after the last day of MGA(R) feeding (Brown et al., 1988). Heifers were monitored for estrus and inseminated over the 5 d subsequent to PGF^sub 2[alpha]^ treatment. Results from this study were most encouraging, particularly as the procedure was effective in heifers that were either prepubertal (40% pregnancy rate) or estrous-cycling (68% pregnancy rate). Over the next decade, numerous studies were reported with pregnancy rates of 31 to 57% (Table 3). In 2000, two studies were published with a small modification of the treatment regimen (Deutscher, 2000; Lamb et al., 2000). This modification, which involved increasing the interval between the last feeding of MGA(R) and the injection of PGF^sub 2[alpha]^, improved synchronized pregnancy rates (Deutscher, 2000). Although only a small improvement in pregnancy rates was reported by Lamb et al. (2000), 99% of the heifers detected in estrus were in estrus within 72 h of PGF^sub 2[alpha]^ treatment. This demonstrated that synchrony was more consistent when there was a 19-d interval from the last day of MGA(R) feeding to PGF^sub 2[alpha]^ than when there was a 17-d interval. This is likely related to the stage of follicular development in heifers at the time of PGF^sub 2[alpha]^ administration (Lamb et al., 2000).

Deutscher (2000) reported a statistical (P

Results from studies conducted with this procedure using either the 17- or 19-d interval are summarized in Table 3. Conception rates averaged 67%, and pregnancy rates averaged 48%, when the 17-d interval was used; rates were 70 and 56%, respectively, when the 19-d interval was used. Data from studies using various intervals from the last day of MGA(R) feeding to the injection of PGF^sub 2[alpha]^ are summarized in Table 4. Combined, these data demonstrate that pregnancy rates increased as the interval increased from 13 to 19 d. In contrast, as the interval and pregnancy rates increased, the percentage of heifers observed in estrus from 84 to 120 h after the injection of PGF^sub 2[alpha]^ (heifers with delayed estrus) decreased. Again, this suggests that follicles were of greater size (maturity) and more uniform in development at the time of PGF^sub 2[alpha]^ administration, such that synchrony was improved when there was a 19-d interval from MGA(R) to PGF^sub 2[alpha]^.

This improved synchrony of heifers synchronized with the MGA(R)/PGF^sub 2[alpha]^ (19-d interval) protocol may permit the utilization of fixed-timed AI (TAI) as was used, and considered an advantage, of Syncro-Mate B(R). Kesler et al. (2002) conducted a study using modified TAI after synchronization of estrus with the MGA(R)/PGF^sub 2[alpha]^ (19-d interval) protocol. In this study, one group of heifers was inseminated based on the detection of estrus. The second group of heifers was inseminated if observed in estrus up to 48 h after the injection of PGF^sub 2[alpha]^. All remaining heifers underwent TAI at 72 h. Results are summarized in Table 5. In the group inseminated based on the detection of estrus, 73% of the heifers were observed in estrus. This is consistent with previous studies. Estrus detection rate has consistently been a limitation of reaching maximal pregnancy rates with the MGA(R)/PGF^sub 2[alpha]^ estrous synchronization protocol. Although pregnancy rates were not statistically (P>0.10) different between the two groups (50% vs 56%), there were 3 fewer d of estrus detection using the modified TAI protocol. Furthermore, a breakout of the herds used in the study demonstrated that more producers are likely to be satisfied with their results (Table 5). Two herds with inseminations based on the detection of estrus had pregnancy rates of 30% and 83% of the herds had pregnancy rates >50%.

The following recommendations should be followed to ensure that the MGA(R)/PGF^sub 2[alpha]^ estrous synchronization protocol will effectively synchronize a fertile estrus.

* Provide adequate bunk space so that heifers have adequate opportunity to consume the MGA(R)-treated feed.

* Acclimate the heifer to MGA(R) carrier feeding before feeding the MGA(R)-treated feed.

* Feed the MGA(R)-treated feed at the same time every day; early morning is recommended.

* Provide the MGA(R)-treated feed in a relatively concentrated form (i.e., a total mixture of 0.45 kg per heifer.

* Inject the PGF^sub 2[alpha]^ intramuscularly in the neck using 18- or 20-ga needles that are 3.81 cm long and 5- to 10-cc syringes. This will permit a deep intramuscular injection of the appropriate dose as stated on the label and comply with Beef Quality Assurance guidelines.

* Prepare the schedule carefully. A scheduling procedure is available to facilitate scheduling and reduce errors:

http://www.iowabeefcenter.org/synchplanner/synchplanner.asp.

* Although heifers will exhibit estrus after the 14 d of MGA(R) feeding and before the injection of PGF^sub 2[alpha]^, do not breed at that estrus, as fertility at this estrus is low.

What is a satisfactory synchronized pregnancy rate? Based on data summarized in Table 3, maximal conception rates, assuming 100% synchrony, average 68% (61 to 73%; x + or - 1 SD). This is based on conception rates of 14 studies. Based on this maximal pregnancy rate, a Synchronization Satisfaction Assessment has been developed (Table 6). Producers should be exceptionally satisfied with pregnancy rates of > or =60% and highly satisfied with pregnancy rates of 50 to 59%, etc. It is important to note that this assessment is based on pregnancy rates (number of heifers pregnant divided by the total number of heifers treated for synchronization). Conception rates (number of heifers pregnant divided by the number detected in estrus and inseminated) will be greater; however, a percentage of heifers were never inseminated. In most cases, synchronization protocols do not improve fertility and synchronization procedures should not be viewed as fertility enhancement procedures. One exception is that progestin-based synchronization protocols will hasten puberty in heifers; however, this will be discussed later.

Using this Synchronization Satisfaction Assessment, pregnancy rates in all studies using the 19-d interval were in the high or exceptional satisfaction categories (Table 6). When the 17-d interval was used, 50% of the studies had pregnancy rates in the low satisfaction to not satisfied categories. More (P50% than did the studies using the 17-d interval; in none of the studies was there a pregnancy rate

During the past 5 yr, numerous herds in Missouri have used the MGA(R)/PGF^sub 2[alpha]^ protocol to synchronize estrus, and these data have been summarized by the Show-Me-Select Heifer Development Program (Randel et al., 2001). When final pregnancy rates were broken into quartiles, pregnancy rates to AI during the synchronized period were similar in the top three quartiles and only slightly reduced in the lowest quartile (Table 7), which demonstrates that the MGA(R)/PGF^sub 2[alpha]^ protocol is robust; it works in most situations.

There are, however, identifiable factors that will affect pregnancy rates in synchronized heifers. One factor is synchrony. Even in the optimized MGA(R)/PGF^sub 2[alpha]^ protocol (with a 19-d interval from the last day of MGA(R) feeding to the injection of PGF^sub 2[alpha]^), synchrony is not as precise as could be desired. In a study we conducted, 8% of the heifers were detected in estrus 97 to 120 h after the injection of PGF^sub 2[alpha]^ (Table 8). It is unlikely that these heifers would have conceived if bred at a TAI at 72 h. Second, heifers must be developed to an appropriate size (at least 65% of mature body size at breeding) and body condition. One method used to assess heifer development is reproductive tract scoring. The most commonly used method is one developed by Andersen et al. (1991) (Table 9). This table also demonstrates how heifer development significantly affects synchronized pregnancy rates. In fact, if heifers receive a tract score of 1 within a month of synchronization, they should be culled, as it is unlikely that they will conceive during a recommended short breeding season. In addition, if they do conceive, it will be late in the breeding season, and, thus, they will calve late, making it even less likely that they will conceive during the subsequent breeding season.

A third factor, one that goes along with the second factor, is puberty. When synchronizing and breeding heifers to calve at 2 yr of age, some will be prepubertal at the time the estrous synchronization program is initiated. One of the advantages of using a progestin-based estrous synchronization protocol (MGA(R) is a progestin) is that progestins have been demonstrated to hasten puberty. The literature regarding efficacy of various progestins in hastening puberty is summarized in Table 10. In general, all three progestins summarized in this table [MGA(R), norgestomet (the progestin in the Syncro-Mate B(R) protocol), and natural progesterone] hasten puberty. Other factors should be considered when interpreting this effect. In the study by Hall et al. (1997), norgestomet was demonstrated to hasten puberty in 12.5-mo-old beef heifers but was ineffective when heifers were only 9.5- to 11 mo of age. Additional factors that may influence onset of puberty include season, bull exposure, nutrition, age of weaning, and inclusion of ionophores in the diet.

Synchronization of Estrus with the CIDR(R)/PGF^sub 2[alpha]^ Protocol. A new reproductive tool, the CIDR[alpha] (Pharmacia and Upjohn)/PGF^sub 2[alpha]^ estrous synchronization protocol, was approved by the US PDA in 2002. The CIDR(R), an intravaginal progesterone insert, used in conjunction with PGF^sub 2[alpha]^ advances the first pubertal estrus in beef heifers, advances the first postpartum estrus in suckled beef cows, and synchronizes estrus in replacement beef and dairy heifers and suckled beef cows. The CIDR(R) was developed in New Zealand and has been used there and in many other countries for several years.

The CIDR(R) is a "T"-shaped insert. The wings of the insert collapse to form a rod that can be inserted into the vagina using an applicator. The CIDR(R) is left in the vagina for 7 d. An injection of PGF^sub [alpha]^ is administered on d 6, and the insert is removed on d 7. However, many academic researchers (G. C. Lamb, University of Minnesota, February 20, 2003 and D. J. Patterson, University of Missouri, February 20, 2003) are suggesting that the PGF^sub 2[alpha]^ be administered at the same time that the insert is removed. On the end of the insert opposite the wings, a tail is attached that facilitates removal at the end of the administration period. The backbone of the CIDR(R) is a nylon spine covered by a progesterone impregnated (1.38 g) silicone skin. During the 7 d in the vagina, the CIDR(R) releases a relatively constant dosage of progesterone. Upon removal of the insert, progesterone is quickly eliminated. Retention rate of the insert during the 7-d period is exceptionally high (>95%).

During the 7 d of CIDR(R) treatment, progesterone diffusion from the CIDR(R) insert does not affect spontaneous luteolysis. Assuming all heifers have a 21-d estrous cycle, there will be two populations of females after 6 d of CIDR(R) treatment: females without corpora lutea and females with corpora lutea >6 d after ovulation. All females, therefore, either do not have corpora lutea or have corpora lutea that are potentially responsive to an injection of PGF^sub 2[alpha]^. Although most research suggest that about 90% of corpora lutea in heifers >6 d after ovulation regress promptly to an injection of PGF^sub 2[alpha]^, only about 60% of the females will have corpora lutea at the time of PGF^sub 2[alpha]^ treatment. Therefore, about 95% of the females treated with the CIDR(R)/PGF^sub 2[alpha]^ procedure are synchronized to exhibit estrus within a few days of CIDR(R) removal. More than 95% of the treated females will be synchronized to exhibit estrus if estrous behavior is monitored for 5 d after removal of the CIDR(R) insert. However, the interval to estrus appears to be altered when compared with heifers synchronized with the MGA(R)/PGF^sub 2[alpha]^ protocol (Table 8). Heifers synchronized with the CIDR(R)/PGF^sub 2[alpha]^ protocol were detected in estrus about 1 d earlier than heifers synchronized with the MGA(R)/PGF^sub 2[alpha]^ protocol.

Studies that support the approval of the CIDR(R) in the US have been published (Lucy et al., 2001). These studies included 724 beef heifers, 851 beef cows, and 260 dairy heifers from six, seven, and four sites, respectively, across the US; they demonstrated that the protocol is efficacious (Table11). There is limited data available generated in the US because the product was just approved by FDA. This is a progesterone-based estrous synchronization protocol, and similar to the synthetic progestins MGA(R) and norgestomet, it hastens puberty (Table 10).

The following are recommendations that should be followed when using the CIDR(R)/PGF^sub 2[alpha]^ estrous synchronization protocol.

* Individuals handling the CIDR(R) should wear latex or nitrile gloves to prevent exposure to progesterone on the surface of the insert and to prevent introduction of contaminants from the hands into the vagina of treated heifers.

* The inserts were developed for a one-time use; multiple-use, which is not approved by the FDA, may cause vaginal infections

* Wipe fecal matter off the vulva before inserting the CIDR(R). A small amount of lubricant, the same lubricant as used for AI, will facilitate CIDR(R) insertion.

Because the CIDR(R)/PGF^sub 2[alpha]^ procedure may not be optimized, several alterations of the procedure are being evaluated.

* Inclusion of the CIDR(R) in the CoSynch procedure.

* Inclusion of an estradiol at the time of CIDR(R) insertion or 24 h after CIDR(R) removal.

CoSynch is an estrous cynchronization procedure that involves the administration of gonadotropin releasing hormone (GnRH) followed by an injection of PGF^sub 2[alpha]^7 d later. Cows are inseminated and a second injection of GnRH is administered 48 h after the injection of PGF^sub 2[alpha]^. OvSynch differs from CoSynch in that insemination is done 16 h after the second injection of GnRH rather than at the same time. When the CIDR(R) is included with the CoSynch protocol, it is inserted at the first injection of GnRH and removed at the PGF^sub 2[alpha]^ injection.

Although OvSynch and CoSynch have been reported to be unsatisfactory in heifers (per insemination pregnancy rates of 75 and 35% for control and OvSynch-treated heifers, respectively) (Pursley et al., 1997), when the CIDR(R) insert was included in the CoSynch protocol, pregnancy rates were exceptional (68%, Martinez et al., 2002a; 60%, Estrada et al., 2002; and 65%, Martinez et al., 2002b). Conversely, when estradiol was administered at the time of CIDR insertion, pregnancy rates were reduced (Steckler et al., 2001). Although more heifers administered estradiol about 24 h after CIDR(R) insert removal express estrus during the synchronized insemination period than untreated heifers, fertility is similar, and additional animal handling was required to administer the estradiol (Martinez et al., 2002b). Furthermore, the only estradiol commercially available in the US is estradiol cypionate (ECP(R); Pharmacia and Upjohn Company). Most studies have been done with estradiol benzoate, which is a far shorter-acting ester than ECP(R). In addition, Rhinehart et al. (2002) reported that estradiol benzoate and ECP(R) are equally effective in stimulating follicular atresia; however, recruitment of a new follicle wave may be delayed in heifers administered ECP(R). Currently, it is premature to conclude that any estradiol should be used in any estrous synchronization procedure.

Breeding and Managing Estrous-Synchronized Heifers

Because not all heifers will conceive to the synchronized estrus, subsequent breeding will be required. This is generally accomplished by observing for estrus and breeding by AI or using natural service. After using an effective estrous synchronization protocol, such as the MGA(R)/PGF^sub 2[alpha]^ (19-d interval) protocol, the return to estrus will likely be in the range of 40 to 50%. It is recommended that economic consideration be included in the decision of how to breed the heifers that do not conceive to the synchronized AI. Also, it should be noted that fertility at the return estrus will be less than at the synchronized estrus, because heifers with the highest fertility conceived to the first insemination. The results from four herds that our laboratory monitored during the past 2 yr are presented in Table 12.

Those heifers were synchronized with the MGA(R)/PGF^sub 2[alpha]^ procedure (19-d interval). Mean pregnancy rate to AI during a 5-d synchronization period was 71%. Mean estrus response and conception rates were 91 and 77%, respectively. At the return estrus, pregnancy rates were 41% (30% less than at the synchronized estrus). The lower pregnancy rates were due to both poor estrous responses (or estrous detection rates) and conception rates. These heifers were in good condition, as demonstrated by the reproductive tract scores, and were managed on pastures free of fescue. Using an electronic estrous detection system, Richardson et al. (2002) demonstrated that the number of standing events and the total duration of standing events were reduced in return estrus. Although the 24-d pregnancy rate was high (83%), it may not be maximized.

One method that has been developed to manage breeding at the return estrus and potentially improve the return estrus pregnancy rate is resynchronization. Resynchronization involves treatment such that the pregnant heifers are unaffected, yet the non-pregnant heifers return to estrus at a synchronized time. This was first reported by Ghallab et al. (1984) and subsequently by others (Table 13). In the study reported in Table 13, although the return estrus pregnancy rate was not improved (70% vs 75%), the detection of estrus was facilitated. (Ninety-three percent of the return estrus events were in a 3-d period for resynchronized heifers vs 52% for control heifers.) In other studies, it has been demonstrated that the estrus detection rate is greater when resynchronization is used (Domatob et al., 1997; Kesler et al., 1997). In the study summarized in Table 13 (Favero et al., 1993), the first synchronization was done with Synchro-Mate B(R), and the results were

Several factors must be considered when breeding heifers, not only at the return estrus, but at the first estrus as well. A summary of recommendations on breeding heifers synchronized with the MGA(R)/PGF^sub 2[alpha]^ procedure, the CIDR(R)/PGF^sub 2[alpha]^ procedure, and at return estrus is given in Figure 1.

One item in Figure 1 (#8) relates to breeding once a day. Most studies conducted on synchronization protocols bred heifers using the A.M./P.M. rule (estrus in the A.M., breed in the P.M.; estrus in the P.M., breed in the A.M.). However, there are substantial data accumulating that suggest that once-a-day breeding (and twice-a-day estrus detection) is sufficient. Data published by Nebel et al. (1994) and Newman (1983) demonstrate that pregnancy rates were unaffected when once-a-day breeding was done, and it should be applicable to synchronized heifers. Recommendation #6 (Figure 1) is based on using visual estrous detection. When electronic estrous detection systems are used, the onset of estrus is detected earlier, and conception rates increase as the interval from the onset of estrus to AI increases (Richardson et al., 2002). Therefore, the interval from the onset of estrus to AI may be extended up to 18 h if electronic estrous detection systems are used.

Conclusions

Synchronization of estrus in heifers has changed considerably during the past 40 yr. Today, valuable, effective, consistent, and convenient (not requiring extensive time and/or labor) estrous synchronization protocols are available. However, education programs are needed to overcome the producer's history of poor results that occurred when some of the earlier estrous synchronization protocols were used. The CDIR(R)/PGF^sub 2[alpha]^ and MGA(R)/PGF^sub 2[alpha]^ estrous synchronization programs both have applicability, and the advantage will depend on the situation. When considering the reasons why producers do not use AI (Table 2), both estrous synchronization programs have utility (Table 14).

* Neither are difficult to use.

* Both have value.

* Neither requires extensive time and labor.

* Both are cost effective.

* Consistent high satisfaction will be obtained with either procedure.

* Limited estrus detection is required by both procedures.

Therefore, which procedure should one use? An advantage analysis of the two procedures is summarized in Table 15. The MGA(R)/PGF^sub 2[alpha]^ procedure is the most cost effective, has the fewest number of animal handlings, and is robust. The CIDR(R)/PGF^sub 2[alpha]^ procedure has the shortest interval (from initiation), allows flexibility in working with subgroups in a larger pen, and there is less chance that producers will make scheduling errors. Time has not allowed researchers to evaluate the robustness (effectiveness in all situations) of the CIDR(R)/PGF^sub 2[alpha]^ procedure; however, because of the use of progesterone in the procedure, it is expected to be robust.

Implications

Reliable and effective methods to synchronize estrus in heifers without compromising fertility now exist; however, one ideal protocol for all operations does not exist. Within the reproductive toolbox, there are multiple protocols, and managers will need to match protocols and operations. The MGA(R)/PGF^sub 2[alpha]^ protocol requires considerable lead time; however, it is economical, requires two animal handlings, and heifers may be bred at a predetermined time. The CIDR(R)/PGF^sub 2[alpha]^ protocol requires more animal handling; however, it is less prone to scheduling errors and has a short interval from initiation to AI. Either protocol will improve reproductive management and facilitate the use of AI.

1 Presented at the Managing Reproduction in Beef Cattle symposium as a part of the 2002 Midwest ASAS and ADSA Regional Meeting in Des Moines, IA in March 2002.

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D. J. KESLER2

Departments of Animal Sciences and Veterinary Clinical Medicine, University of Illinois, Urbana, IL 61801

2 To whom correspondence should be addressed: djkesler@uiuc.edu

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