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A comparison of progestin-based protocols to synchronize ovulation and facilitate fixed-time artificial insemination in postpartum beef cows¹

J. F. Bader^*, F. N. Kojima^*, D. J. Schafer^*, J. E. Stegner^*, M. R. Ellersieck, M. F. Smith^* and D. J. Patterson^*,2

^* Department of Animal Science and and Agricultural Experiment Station, University of Missouri, Columbia 65211

	Abstract

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The experimental objective was to compare pregnancy rates after fixed-time AI in postpartum suckled beef cows following administration of two progestin-based protocols to synchronize ovulation. Cows (n = 424) at three locations (n = 208, 122, and 92 per location) were stratified by age, BCS, and days postpartum (DPP) and assigned randomly to one of the two treatment protocols. The MGA Select-treated cows (MGA Select; n = 213) were fed melengestrol acetate (MGA, 0.5 mg•cow^–1•d^–1) for 14 d and carrier for 8 d, and then GnRH (100 µg i.m. Cystorelin; d 26) was injected 12 d after MGA withdrawal, and PG (25 mg i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol (7-11 Synch; n = 209) were fed carrier for 15 d and MGA for 7 d, and then injected with PG on d 22 (d 7 of MGA), GnRH on d 26, and PG again on d 33. Artificial insemination was performed at fixed times for cows in both treatments at 60 or 72 h after d 33 PG for 7-11 Synch and MGA Select groups, respectively. All cows were injected with GnRH (100 µg of i.m. Cystorelin) at AI. There was no treatment x location interaction for age (P = 0.90), BCS (P = 0.64), or DPP (P = 0.93), and the results were therefore pooled for the respective treatments (age [7-11 Synch, 5.5 ± 0.2; MGA Select, 5.5 ± 0.2], BCS [7-11 Synch, 5.7 ± 0.1; MGA Select, 5.6 ± 0.1], and DPP [7-11 Synch, 41.1 ± 1.1; MGA Select, 42.1 ± 1.1]). Blood samples were collected 8 and 1 d before MGA or carrier to determine pretreatment estrous cyclicity (progesterone 1 ng/mL; 7-11 Synch, 59/209 [28%]; MGA Select, 54/213 [25%]; P = 0.50) and again on d 33 PG to evaluate treatment response as a percentage of cows with progesterone concentrations in serum 1ng/mL (7-11 Synch, 184/209 [88%]; MGA Select, 177/213 [83%]; P = 0.15). Pregnancy rates resulting from fixed-time AI did not differ (P = 0.25) between treatments (7-11 Synch, 128/209 [61%]; MGA Select, 142/213 [67%]), nor did pregnancy rates (P = 0.77) at the end of the breeding season (7-11 Synch, 198/208 [95%]; MGA Select, 204/213 [96%]). These data indicate that pregnancy rates were comparable after fixed-time AI, following administration of the 7-11 Synch and MGA Select protocols. Both protocols provide opportunities for beef producers to use AI and eliminate the need to detect estrus.

Key Words: Beef Cow • Estrus Synchronization • Fixed-Time Artificial Insemination

	Introduction

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Artificial insemination allows beef producers to make genetic improvement in seedstock and market animals by using genetically proven sires with high accuracy; however, only a small percentage (fewer than 5%; NAHMS 1994, 1998) of beef producers utilize this management practice. The development of convenient and economical protocols to synchronize estrus and ovulation that facilitate use of fixed-time AI with resulting high fertility should result in increased adoption of these management practices (Patterson et al., 2003). Current research has focused on the development of methods that effectively synchronize estrus in postpartum beef cows by decreasing the period of time over which estrous detection is required and to facilitate the use of fixed timed AI.

Previous research from our laboratory led to the development of the MGA Select and 7-11 Synch protocols (MGA Select, Wood et al., 2001; 7-11 Synch, Kojima et al., 2000). Both protocols effectively synchronize estrus in mixed populations of estrous cycling and anestrous postpartum beef cows. The two protocols differ in length of treatment (MGA Select = 33 d; 7-11 Synch = 18 d) as well as in the length of the interval to estrus and resulting synchrony of estrus; however, there were no differences reported in pregnancy rates between these protocols among cows inseminated on the basis of observed estrus (Kojima et al., 2000; Patterson et al., 2001; Stegner et al., 2004b). The optimal and/or appropriate time to perform AI at fixed times following administration of these two protocols was reported (Perry et al., 2002; Kojima et al., 2003a; Stegner et al., 2004a); however, a direct comparison of the protocols to facilitate fixed-time AI has not been made. Therefore, the objective of this study was to compare pregnancy rates resulting from fixed-time AI among cows assigned to the MGA Select and 7-11 Synch protocols.

	Materials and Methods

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Animals

Crossbred lactating beef cows (n = 424) at three locations (n = 208, 122, and 92) were assigned by age, calving date (days postpartum, DPP), and BCS (1 to 9 scale; 1 = emaciated, and 9 = obese; Richards et al., 1986) to one of two treatment protocols (Table 1). Cows assigned to the MGA Select protocol (n = 213) were fed melengestrol acetate (MGA; Pfizer Animal Health, New York, NY; 0.5 mg•cow^–1•d^–1) for 14 d and carrier for 8 d; GnRH (100 µg i.m. of Cystorelin, Merial, Athens, GA) was injected 12 d after MGA withdrawal and PG (25 mg i.m. of Lutalyse sterile suspension, Pfizer Animal Health) was administered 7 d after GnRH. Cows assigned to the MGA Select protocol were inseminated 72 h after PG and GnRH (100 µg of i.m. Cystorelin) was administered at AI. Cows assigned to 7-11 Synch (n = 209) were fed carrier for 15 d and MGA for 7 d, were injected with PG on d 22 (d 7 of MGA); GnRH was injected on d 26, and PG was injected on d 33 (d 33 PG; Figure 1). Cows assigned to 7-11 Synch were inseminated 60 h after the last injection of PG and GnRH was administered at AI. Cows were supplemented with either MGA or carrier during the synchronization period to ensure equal supplementation between treatments (Figure 1). Cows in each treatment and at each location were maintained as separate groups on spring pasture and offered free-choice access to prairie hay. Inseminations were performed by the same two experienced technicians at all locations. Two AI sires were used at Locations 1 and 2, and one sire was used at Location 3. One of the AI sires used at Location 1 was the same sire that was used at Location 3. The AI sires and technicians were assigned equally to cows in each treatment by cow age and calving date. Cows were exposed to fertile bulls 14 d after AI for a 60-d natural service period.

View larger version (14K): [in this window] [in a new window]   Figure 1. Treatment schedule for cows assigned to the MGA Select and 7-11 Synch protocols. Cows assigned to the MGA Select protocol were fed melengestrol acetate (MGA; 0.5 mg•cow–1•d–1) for 14 d, GnRH was administered (100 µg i.m. Cystorelin) 12 d after MGA withdrawal, and PGF2 (PG; 25 mg i.m. Lutalyse) was administered 7 d after GnRH. Cows were inseminated 72 h after d 33 PG with an injection of GnRH at AI. Cows assigned to the 7-11 Synch protocol were fed carrier for 15 d and MGA for 7 d, and then were injected with PG on d 22 (d 7 of MGA) and GnRH on d 26, and again with PG on d 33. Cows were inseminated 60 h after d 33 PG with an injection of GnRH at AI.

Blood samples were collected 8 and 1 d before MGA or carrier to determine pretreatment estrous cyclicity status of cows based on concentrations of progesterone in serum. Cows were considered to be estrous-cycling if concentrations of progesterone in serum were 1 ng/mL at either one or both pretreatment sampling times. Presence of luteal tissue on d 33 PG was determined if concentrations of progesterone in serum were 1 ng/mL. Blood samples were allowed to clot and then stored at 4°C for 24 h. Serum was collected after centrifugation and stored at –20°C until hormone analyses were performed. Concentrations of progesterone in serum were determined by RIA using a Coat-A-Count Kit (Diagnostic Products Corp., Los Angeles, CA; Kirby et al., 1997). Intra- and interassay CV were 6.4 and 15.5%, respectively, with an assay sensitivity of 0.1 ng/mL.

Pregnancy Diagnosis

Pregnancy rates resulting from fixed-time AI were determined by ultrasonography (Aloka 500V equipped with 5.0-MHz linear-array transducer, Aloka, Wallingford, CT) 40 to 50 d after AI. Final pregnancy rate at the end of the breeding period was determined by ultrasonography 45 d after the breeding season ended.

Calving Distribution

Calf birth dates for cows that conceived to fixed-time AI were recorded at each location to confirm the initial pregnancy diagnosis determined from ultrasound. Pregnancy rate to fixed-time AI includes those cows that conceived to AI at a fixed-time on the same day and were diagnosed pregnant 40 to 50 d after AI. Pregnancy rate at the end of the breeding season includes all cows that were diagnosed pregnant (AI and natural service bred) 45 d after the breeding season ended.

Statistical Analyses

The random variables that included age, days postpartum, BCS, and progesterone concentrations at d 33 PG, were analyzed by ANOVA using the linear statistical model of location, treatment, and the interaction of location x treatment (GLM procedures of SAS; SAS Inst., Inc., Cary, NC). Logistic regression was used to develop a model to predict pregnancy rate based on the following variables: treatment, BCS, DPP, age, pre-treatment estrous cyclicity, presence of luteal tissue and progesterone concentrations at d 33 PG, AI technician, AI sire, and all appropriate two-way interactions (Proc Log Reg of SAS). Stepwise analyses were performed with stay and entry levels for variables of P = 0.30. Pretreatment estrous cyclicity, pregnancy rate resulting from AI, and final pregnancy at the end of the breeding period were analyzed by ² analysis (Proc Freq of SAS). Differences in shape of calving distribution among individual AI sires were analyzed with the Kolmogorov-Smirnov two-sample test (NPAR1WAY procedures of SAS; Conover, 1999), which compares differences in variance, skewness, and kurtosis between distributions. Gestation length for progeny from the respective sires was analyzed using a one-way ANOVA (GLM procedure of SAS).

	Results

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The number of cows at each location, days postpartum, BCS, and estrous cycling status of cows before the initiation of treatments are shown in Table 1. There were no differences between treatments at the respective locations for age, days postpartum, BCS, or estrous cyclicity status at the initiation of treatment; however, there were differences among locations (Table 1).

The number of cows with concentrations of progesterone in serum 1 ng/mL on d 33 PG and the mean progesterone concentrations on d 33 PG for cows assigned to the respective treatments at each location are shown in Table 2. There were no differences between treatments in the proportion of cows with progesterone concentrations 1 ng/mL, except at Location 2 (P < 0.05). There were differences (P < 0.05), however, between treatments at each location in the mean concentration of progesterone on d 33 PG (Table 2).

View this table: [in this window] [in a new window]   Table 2. Number of cows with progesterone concentrations in serum 1 ng/mL (mean ± SE) and serum progesterone concentrations at the time of prostaglandin injection on d 33

View larger version (30K): [in this window] [in a new window]   Figure 2. Calving distribution for cows that conceived to fixed-time AI at each location. Calving periods among cows that conceived on the same day to the respective sires (A, B, C, D, and B) varied by 21, 16, 16, 20, and 18 d, respectively. Sire B at Locations 1 and 3 was the same sire. Calving distributions were different (P < 0.05) for Sires A vs. B (Location 1), B (Location 1) vs. D, and D vs. B (Location 3). The shaded bar in each graph represents an anticipated 285-d gestation due date.

	Discussion

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The MGA Select protocol provides an established synchrony of estrus and improves total herd estrous response, particularly among herds with high rates of anestrus (Patterson et al., 2002). Peak estrous response among cows assigned to the MGA Select protocol typically occurs 72 h after d 33 PG (Patterson et al., 2001, 2002; Stegner et al., 2004b). Pregnancy rates were optimized for cows assigned to the MGA Select protocol when fixed-time AI was performed 72 h after PG injection (Perry et al., 2002; Stegner et al., 2004a), but were decreased when AI was compared 48 or 80 h after d 33 PG (Stevenson et al., 2003; Stegner et al., 2004a).

The 7-11 Synch protocol (Kojima et al., 2000) improves synchrony of estrus over other protocols (Select-Synch, MGA Select), and peak estrous response typically occurs 56 h after d 33 PG (Kojima et al., 2000; Patterson et al., 2002; Stegner et al., 2004b). Pregnancy rates resulting from fixed-time AI after administration of the 7-11 Synch protocol were improved when AI was performed 60 h after d 33 PG (Kojima et al., 2003a). Collectively, these studies led to the comparison of the two estrus synchronization protocols used in conjunction with the fixed-timed AI reported here.

Pregnancy rates resulting from fixed-time AI utilizing the two protocols involved in this study are consistent with previously published reports for the MGA Select (Perry et al., 2002; Stegner et al., 2004a) and 7-11 Synch protocols (Kojima et al., 2002, 2003a,Kojima et al., b). Furthermore, pregnancy rates resulting from fixed-time AI in this study compare favorably with pregnancy rates after cows were inseminated on the basis of detected estrus using the same protocols to synchronize estrus (Kojima et al., 2000; Patterson et al., 2002; Stegner et al., 2004b).

Perry (2003) reported differences in late embryonic/fetal mortality following fixed-time AI among cows assigned to a CO-Synch protocol. Late embryonic/fetal mortality occurred at higher rates among cows that were induced to ovulate follicles 11 mm in diameter. Follicles induced to ovulate in this smaller range (11 mm) are characterized as being less physiologically mature at the time of ovulation, which may subsequently result in decreased oocyte and/or luteal competence. When cows were detected in standing estrus, however, follicle size did not affect pregnancy rates or late embryonic mortality (Perry, 2003). The author suggested that oocyte and luteal competence might be more dependent on the steroidogenic capacity of the follicles from which they were ovulated than follicle size (Perry 2003). A key observation from the preceding study suggests that follicular competence is important for both the establishment and maintenance of pregnancy. Vasconcelos et al. (2001) observed decreased peak concentrations of circulating estradiol, resulting size of the corpus luteum, circulating concentrations of progesterone, and pregnancy rate to AI when dairy cows were induced to ovulate smaller follicles (14 mm).

Premature ovulation of a dominant follicle results in decreased ovulatory size, decreased luteal function, and compromised pregnancy rates compared with animals induced to ovulate larger, more mature dominant follicles (Mussard et al., 2003). One potential advantage in using either of these protocols (MGA Select, 7-11 Synch; Stegner et al., 2004c) to synchronize ovulation for fixed-time AI is that mean follicle diameter at the time ovulation was induced (Kojima et al., 2002, 2003a,Kojima et al., b; Perry et al., 2002) exceeds the range described by Perry (2003) and potentially minimizes the problems with late embryonic/fetal mortality described by Perry (2003) and Mussard et al. (2003).

The results from the step-wise logistic regression showed that the primary variable influencing pregnancy rate to fixed-time AI was the presence of luteal tissue on d 33 PG. The percentage of cows with functional CL on d 33 PG differed (P < 0.05) among locations. The difference (P < 0.05) in the percentage of cows with functional CL on d 33 between treatments at Location 2 (7-11 Synch = 90%; MGA Select = 74%) was not reflective of the subsequent pregnancy rates observed at the other two locations (7-11 Synch = 57%; MGA Select = 69%). This discrepancy cannot be explained from these data.

Although the presence of luteal tissue on d 33 PG affected subsequent pregnancy rate to fixed-time AI, the actual concentration of progesterone on d 33 PG was not important in determining subsequent pregnancy. The difference in serum concentrations of progesterone on d 33 PG between treatments stems from differences in hormonal environments under which the dominant follicle develops (Stegner et al., 2004c). The MGA Select-treated cows have higher concentrations of serum progesterone and lower estradiol-17ß during the growth phase of the dominant follicle, than cows treated with 7-11 Synch (Stegner et al., 2004c). This hormonal milieu is similar to the mid-luteal phase of the estrous cycle, whereas 7-11 Synch cows develop a dominant follicle under higher estradiol-17ß and lower progesterone concentrations similar to the early luteal phase. Pregnancy rates based on pretreatment estrous cyclicity status (estrous cycling vs. anestrus) did not differ between treatments or among locations, which points to the efficacy of both protocols in successfully synchronizing estrus before fixed-time AI in mixed populations of estrous cycling and anestrous cows.

Stegner et al. (2004b) discussed the advantages and disadvantages related to the practical application and successful administration of the MGA Select and 7-11 Synch protocols. The advantages shown here and reported in other studies (Patterson et al., 1989, 2003) include the following: 1) MGA is economical to use (approximately $0.02 per animal, per day, to feed, excluding cost of carrier); 2) each protocol works effectively in mixed populations of beef cows that were estrous cycling or anestrus at the time treatments are imposed; and 3) pregnancy rates resulting from insemination performed on the basis of detected estrus or at predetermined fixed times are comparable.

Stegner et al. (2004b) noted, however, that the feasibility of feeding MGA to cattle on pasture is limiting in some production systems and is viewed as a disadvantage. Furthermore, 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 with the MGA Select protocol, which requires the cows be handled three times.

Calving dates for cows that conceived on the same day to fixed-time AI were recorded to address concerns that pertain to the subsequent calving period. Cows in this experiment that conceived on the same day gave birth to calves over a 16- to 21-d period, with no more than 12 to 22% of calves being born on a single day, depending on the respective sire. Effect of sire on gestation length has been documented in previous research (Boyd and Hafs, 1965; Everett and Magee, 1965; Crockett and Kidder, 1967). The calving periods reported here and their associated distributions suggest that the successful use of fixed-time AI will not result in an overwhelming number of cows calving on the same day. This further suggests that current management practices will not need to be greatly altered to accommodate the early portion of the calving season. Conversely, these data suggest that successful application of estrus synchronization protocols that facilitate fixed-time AI support improvements in whole-herd reproductive management and expanded use of improved genetics.

	Implications

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These results indicate that estrus synchronization with the 7-11 Synch and MGA Select protocols, followed by fixed-timed artificial insemination at 60 or 72 h after administration of prostaglandin on d 33, respectively, results in comparable pregnancy rates in postpartum suckled beef cows. Producers may be able to use these protocols to facilitate artificial insemination and eliminate the need to detect estrus.

	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) for providing the Lutalyse sterile suspension; Merial (Athens, GA) for providing the Cystorelin; and D. S. McAtee and J. J. D. Schreffler for their dedicated support of this research at the Univ. of Missouri Thompson Farm, Spickard.

² Correspondence—phone: 573-882-7519; fax: 573-884-4798; e-mail: pattersond{at}missouri.edu.

Received for publication June 30, 2003. Accepted for publication September 29, 2004.

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				G. C. Lamb, J. E. Larson, T. W. Geary, J. S. Stevenson, S. K. Johnson, M. L. Day, R. P. Ansotegui, D. J. Kesler, J. M. DeJarnette, and D. G. Landblom Synchronization of estrus and artificial insemination in replacement beef heifers using gonadotropin-releasing hormone, prostaglandin F2{alpha}, and progesterone J Anim Sci, November 1, 2006; 84(11): 3000 - 3009. [Abstract] [Full Text] [PDF]

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