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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
* Department of Animal Science,
and
Agricultural Experiment Station, University of Missouri, Columbia 65211
Abstract
Two progestin-based protocols for estrus synchronization in postpartum beef cows were compared following treatment administration on the basis of estrous response, interval to and synchrony of estrus, and pregnancy. Cows were assigned to one of the two treatment protocols by age, body condition score (BCS), and days postpartum (DPP). The MGA Select-treated cows (MGA Select; n = 109) were fed melengestrol acetate (MGA; 0.5mg•cow-1•d-1) for 14 d, fed carrier for 8 d, GnRH (100 µg of Cystorelin) was injected i.m. 12 d after MGA withdrawal, and PG (25 mg of Lutalyse) was administered i.m. 7 d after GnRH. Cows assigned to the 7-11 Synch protocol (7-11 Synch; n = 111) were fed carrier for 15 d, fed MGA for 7 d, injected with PG on d 22 (d 7 of MGA), injected with GnRH on d 26, and injected with PG on d 33. Mean BCS (4.8 ± 0.1, MGA Select; 4.7 ± 0.1, 7-11 Synch) and DPP (40 ± 1, MGA Select; 40 ± 1, 7-11 Synch) did not differ between treatments. Blood samples were collected 8 d and 1 d before feeding of MGA or carrier to determine the pretreatment estrous cyclicity (progesterone 
1 ng/mL; 10/109 [9%], MGA Select; 12/111 [11%], 7-11 Synch), and again at PG on d 33 to evaluate treatment response (81/109 [74%], MGA Select; 84/111 (76%), 7-11 Synch). Serum concentrations of progesterone at PG on d 33 differed (P < 0.01) between treatments (3.3 ± 0.3 ng/mL [MGA Select] vs. 1.7 ± 0.1 ng/mL [7-11 Synch]). HeatWatch was used for 6 d after PG on d 33 to detect estrus, and AI was performed 12 h after the onset of estrus. Estrous response did not differ between treatments (100/109 [92%], MGA Select; 101/111 [91%], 7-11 Synch). Mean interval to estrus (65 ± 2.7 h, MGA Select; 52 ± 1.8 h, 7-11 Synch) and synchrony of estrus differed (P < 0.01) between treatments. Synchronized conception and pregnancy rates (61/100 [61%], 61/109 [56%], MGA Select; 71/101 [70%], 71/111 [64%], 7-11 Synch), and final pregnancy rates (94/109 [86%], MGA Select; 99/110 [90%], 7-11 Synch) did not differ between treatments. In summary, estrous response and fertility did not differ among cows assigned to the MGA Select or 7-11 Synch protocols. Synchrony of estrus, defined as the variance in the interval to estrus from PG, however, was improved following treatment with the 7-11 Synch protocol.
Key Words: Artificial Insemination • Estrus Synchronization • Gonadotropin-Releasing Hormone • Progestin
Introduction
Efforts to develop more effective protocols for the synchronization of estrus focused recently on synchronizing follicular waves by injecting GnRH followed 7 d later by injection of PG (Ovsynch [Pursley et al., 1995
]; CO-Synch [Geary et al., 2001]; and Select Synch [Geary et al., 2000]). A factor contributing to reduced synchronized pregnancy rates in beef cows treated with the preceding protocols is that 5 to 15% of cycling cows show estrus on or before PG injection (Kojima et al., 2000).
We proposed the general hypothesis that progestin treatment preceding a GnRH-PG protocol would induce ovulation in anestrous beef cows; diminish the occurrence of the short luteal phase among anestrous cows induced to ovulate; and, in concert with GnRH and PG, improve estrous response, conception, and pregnancy rates during the synchronized period. The MGA Select and 7-11 Synch protocols utilize this sequential approach to elicit the results previously described. These protocols work effectively in mixed populations of estrous cycling and anestrous beef cows (Kojima et al., 2000; Patterson et al., 2002). A previous study showed differences in the interval to and synchrony of estrus following administration of these protocols (Stegner et al., 2004); however, there have been no direct comparisons of fertility following their administration. Our objectives in this experiment were to compare the MGA Select and 7-11 Synch protocols for the synchronization of estrus in postpartum beef cows on the basis of estrous response and distribution of estrus after PG, and synchronized conception and pregnancy rates.
Materials and Methods
Experimental Design.
Crossbred, lactating, beef cows were assigned within age group by calving date (days postpartum, DPP) and BCS (1-to-9 scale, 1 = emaciated and 9 = obese) to one of two treatments. The MGA Select-treated (Figure 1) cows (MGA Select; n = 109) were fed melengestrol acetate (MGA; 0.5mg•cow-1•d-1) for 14 d, fed carrier for 8 d, GnRH (100 µg of Cystorelin; Merial, Athens, GA) was injected i.m. 12 d after MGA withdrawal, and PG (25 mg of Lutalyse Sterile Suspension; Pfizer Animal Health, New York, NY) was administered i.m. 7 d after GnRH. Cows assigned to the 7-11 Synch protocol (7-11 Synch; n = 111) were fed carrier for 15 d, fed MGA for 7 d, injected with PG on d 22 (d 7 of MGA), injected with GnRH on d 26, and injected with PG on d 33 (Figure 1). Cows in each treatment were maintained as separate groups on spring pasture and offered free choice access to prairie hay. The young cows (<5 yr old) and mature cows (5 yr old) were managed as separate herds on the same farm. The young and mature cow herds did not differ significantly for days postpartum, BCS, and estrous cyclicity status. The data from the two herds were, therefore, combined within treatment group (Table 1). The exact time at which PG injections were administered was recorded for each cow. All cows were fitted with transmitters capable of detecting estrus and monitored for estrous behavior continuously with the HeatWatch Estrus Detection System (DDx, Inc., Denver, CO) for 6 d beginning the day PG was administered. Estrus was defined by HeatWatch after cows experienced three or more mounts for a duration 2 s over a period of 4 h. Cows were inseminated approximately 12 h after onset of estrus. Cows were exposed to fertile bulls 14 d after the last AI for a 60-d natural service period. Cows were maintained on pasture and supplemented only during the time when carrier was provided.
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Blood samples were collected 8 d and 1 d before initiation of MGA or carrier feeding to determine pretreatment the cyclicity status of cows based on concentrations of progesterone in serum. Cows were considered to be cycling if concentrations of progesterone in serum from either one of two samples were elevated (1 ng/mL). Samples collected at PG on d 33 were used to determine induction of estrous cyclicity following treatment. Blood samples were allowed to clot and stored at 4°C for 24 h. Serum was then collected by centrifugation. Serum was stored at -20°C until hormone analyses were performed. Concentrations of progesterone in serum were determined by RIA using the Coat-A-Count Kit (Diagnostic Products Corporation, Los Angeles, CA; Kirby et al., 1997). Intra- and interassay coefficients of variation were 5.8 and 8.8%, respectively, with an assay sensitivity of 0.5 ng/mL.
Pregnancy Determination.
Pregnancy rate to AI was determined by transrectal ultrasonography (Aloka 500V equipped with a 5.0-MHz linear-array transducer; Aloka, Wallingford, CT) approximately 60 d after AI. Final pregnancy was determined by transrectal ultrasound 45 d after the end of the natural service breeding season.
Statistical Analyses.
Treatments were compared on the basis of days postpartum, BCS, age, and interval to estrus and were analyzed using GLM procedures of SAS (SAS Inst. Inc., Cary, NC). Logistic regression was used to develop a model to predict pregnancy rate for the following variables: treatment, BCS, DPP, age, pretreatment estrous cyclicity, serum concentrations of progesterone at PG on d 33, interval to estrus, estrous response, AI technician, AI sire, and all two-way interactions (Proc Log Reg of SAS). Pretreatment estrous cyclicity, serum concentrations of progesterone at PG on d 33, estrous response, synchronized conception rate, synchronized pregnancy rate, and final pregnancy were analyzed by 
2 analysis (Proc Freq of SAS). Synchrony of estrus was defined as the variance in the interval to estrus after PG on d 33. Differences in variances, associated with interval to estrus, were tested by dividing the greater variance by the lesser variance and performing a simple F-test (Snedecor and Cochran, 1989).
Results
Steroid Concentrations.
Mean serum concentrations of progesterone at PG on d 33 differed (P < 0.001) between treatments (MGA Select = 3.3 ± 0.3 ng/mL vs. 7-11 Synch = 1.7 ± 0.1 ng/mL of progesterone; Table 2); however, there were no differences in the percentage of cows in each treatment exhibiting elevated concentrations of progesterone (1 ng/mL; Table 2) at the time PG was administered.
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There were no differences in estrous response or mean interval to estrus between age groups within treatment. Estrous response did not differ significantly between treatments (100/109 [92%], MGA Select; 101/111[91%], 7-11 Synch); however, there were differences in mean interval to estrus when age groups were pooled (Figure 2; Table 2
). Mean interval to estrus was shorter (P < 0.01) for cows assigned to 7-11 Synch (52 h± 1.8) than for cows assigned to MGA Select (65 h ± 2.7). The interval to estrus ranged from 20 to 141 h and 27 to 86 h for cows assigned to MGA Select and 7-11 Synch, respectively. Variances associated with the synchrony of estrus were compared by performing an F-test and differed (P < 0.01) between treatments (MGA Select, variance estimate = 784, df = 108; 7-11 Synch, variance estimate = 324, df = 110).
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There were no differences between treatments (Table 3) in synchronized conception (61/100 [61%], MGA Select; 71/101 [70%], 7-11 Synch; P = 0.22) or synchronized pregnancy rates (61/109 [56%], MGA Select; 71/111 [64%], 7-11 Synch; P = 0.28). Pregnancy rate at the end of the breeding season also did not differ between treatments (94/109 [86%], MGA Select; 99/110 [90%], 7-11 Synch; Table 3). Treatment, DPP, BCS, age, interval to estrus, estrous response, pretreatment estrous cyclicity, serum concentrations of progesterone at PG on d 33, AI technician, and AI sire were not significant sources of variation for predicting pregnancy rates, as determined by logistic regression.
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The calving distribution is illustrated in Figure 3 for cows in this study that were assigned to the MGA Select and 7-11 Synch protocols. A high proportion of calves were delivered within the first 15 and cumulative 30 d of the calving season for each protocol, with no differences between treatments. The cumulative number of cows that calved within the first 30 d of the calving period was 93% (87 of 94 cows) and 89% (88 of 99 cows) for the MGA Select and 7-11 Synch groups, respectively.
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Artificial insemination allows producers to make more-rapid genetic improvement in their herds by facilitating the use of genetically superior bulls with high accuracy, and without having a bull physically present on the farm or ranch (Trenkle and Willham, 1977). 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 the detection of estrus. More-economical and effective protocols for synchronizing estrus must be developed to increase producer acceptance and use of these practices (Dziuk and Bellows, 1983). Research has focused on the development of strategies to effectively synchronize estrus in beef cows to reduce the period of time over which heat detection is required, or ultimately facilitate the use of fixed-time AI without detecting estrus.
The MGA Select and 7-11 Synch protocols have been shown to reliably induce estrous cyclicity, improve estrous response, and increase pregnancy rates during the synchronized period in herds of mixed populations of anestrous and estrous cycling cows (Kojima et al., 2000; Patterson et al., 2002). These two protocols provide a consistently high estrous response and predictable synchrony of estrus. Treatment differences in serum concentrations of progesterone at PG on d 33 in this experiment are likely a function of how the two protocols differ in synchronizing estrus among cows assigned to each treatment. Cows assigned to the MGA Select protocol are generally considered to be in the mid-luteal phase of the estrous cycle vs. 7-11 Synch-treated cows that are generally in the early-luteal phase of the cycle at the time PG is administered (Stegner et al., 2004). Despite low estrous cyclicity rates of cows before treatment initiation, estrous response during the synchronized period was high for both the MGA Select and 7-11 Synch treatment groups. It is important to point out, however, that the design of this study precluded an estimate of the effect that the treatments had on initiation of estrous cyclicity.
The higher degree of estrus synchrony exhibited by the 7-11 Synch–treated cows may have resulted from follicles that were more physiologically mature at the time PG was administered (Stegner et al., 2004). Differences in length of interval to estrus between MGA Select and 7-11 Synch cows were explained in a previous study (Stegner et al., 2004) by differences in follicular development and steroid hormone concentrations of cows assigned to the two treatments. Stegner et al. (2004) reported that the longer interval to estrus for MGA Select–treated cows may be caused by higher progesterone concentrations during the growth phase of the follicle and correspondingly lower E2 concentrations, a hormonal environment similar to the mid-luteal phase of the estrous cycle. The shorter interval to estrus exhibited by 7-11 Synch-treated cows may result from lower progesterone concentrations during the growth phase of the follicle and resulting higher estradiol concentrations, similar to the early-luteal phase of the estrous cycle. These results support this concept given the fact progesterone concentrations were lower on the day PG was administered to 7-11 Synch-treated cows.
Producers that consider using either of these protocols should know that 1) MGA is economical to use (approximately $0.02 per animal daily to feed); 2) MGA was recently cleared for use in reproductive classes of beef and dairy cattle (Federal Register, 1997); 3) methodology and understanding of the use of MGA is documented in the literature dating back as early as the 1960s (Zimbelman, 1963; Zimbelman et al., 1970; Patterson et al., 1989); and 4) MGA is easily administered in feed and does not require that animals be handled or restrained during administration.
The practical application of these protocols requires consideration of the relative advantages and disadvantages associated with their administration. The advantages shown here and reported in other studies (Kojima et al., 2000; Patterson et al., 2002), include the following: 1) each protocol works effectively in mixed populations of beef cows that were estrous cycling or were anestrous at the time treatments were imposed; 2) the timing of estrus following administration of these protocols is consistent; 3) the expression of estrus is closely synchronized; and 4) synchronized pregnancy rates of cows that were synchronized with these protocols have consistently ranged from 56 to 70%. The feasibility of feeding MGA to cattle on pasture is limiting in some production systems and is viewed as a disadvantage. The MGA Select protocol requires feeding and management of cows for 33 d, whereas the 7-11 Synch protocol involves an 18-d period. Conversely, the 7-11 Synch protocol requires that animals be handled four times, including AI, compared to the MGA Select protocol, which requires three handlings.
The high estrous response and synchrony of estrus exhibited by cows assigned to the protocols in this experiment and reported separately in others (Kojima et al., 2000; Patterson et al., 2002) suggest the potential for use of these protocols to synchronize estrous cycles of cows before fixed-time AI. These data support the design of future studies to directly compare the efficacy of the MGA Select and 7-11 Synch protocols for synchronizing estrous cycles of postpartum beef cows before insemination at a predetermined fixed time.
Lastly, and perhaps most importantly from the standpoint of reproductive management of beef cows, the calving distribution of cows assigned to each of these protocols must be carefully considered. One of the obvious benefits of the synchronization of estrus is a shortened calving season, which results in more uniform calves at weaning (Dziuk and Bellows, 1983). Reduced length of the calving season translates into a greater number of days for postpartum recovery of the cow to occur before the subsequent breeding season. Herd owners must be aware of the risks associated with a concentrated calving period, including inclement weather or disease outbreaks, which separately or together may result in a decrease in the number of calves weaned.
These data, however, support the use of the synchronization of estrus not only as a means of facilitating the more-rapid genetic improvement of beef herds, but perhaps, more importantly, as a powerful reproductive management tool. Profitability may be increased by decreasing the extent to which labor is required during the calving period, and increasing the total weight of calves weaned, which results from a more-concentrated calving distribution and a resulting increase in the age of calves at weaning.
Implications
The MGA Select and 7-11 Synch protocols produced similar results (i.e., estrous response, synchronized conception, and synchronized pregnancy rate) when administered to postpartum beef cows. The synchrony of estrus, however, following administration of these treatments can be expected to be improved among cows treated with 7-11 Synch vs. MGA Select.
Footnotes
1 Contribution from the Missouri Agric. Exp. Stn. This research was supported by National Research Initiative Competitive Grant 00-35203-9175 from the USDA Cooperative State Research, Education, and Extension Service, and Select Sires, Inc., Plain City, OH. The authors gratefully acknowledge Pfizer Animal Health, New York, NY; Merial, Athens, GA; and D. S. McAtee and J. J. D. Schreffler for their dedicated support of this research at the University of Missouri Thompson Farm, Spickard, MO. 
2 Correspondence—phone: 573-882-7519; fax: 573-882-4798; e-mail: pattersond{at}missouri.edu.
Received for publication July 30, 2003. Accepted for publication December 5, 2003.
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