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ANIMAL PRODUCTION |
protocol for synchronization of estrus in beef heifers1
* Department of Animal Sciences and Industry, Kansas State University, Northwest Research and Extension Center, Colby 67701; and
and
Department of Animal Sciences, The Ohio State University, Columbus 43210
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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(PGF) protocol for synchronization of estrus in heifers. In each experiment, all heifers received 0.5 mg of MGA•animal–1•d–1 for 14 d (d –32 to –19) and PGF (25 mg, i.m.; d 0, 0 h) 19 d after the last feeding of MGA (MGA-PGF protocol). In Exp. 1, heifers (n = 709) were assigned to each of the following protocols: 1) the MGA-PGF protocol with AI 6 to 12 h after detection of estrus (estrus AI; MGA-PGF); 2) MGA-PGF plus 100 µg, i.m. of GnRH on d –7 (1xGnRH) and estrus AI; or 3) MGA-PGF, GnRH on d –7, and GnRH (100 µg, i.m.) at 48 h after PGF, coincident with insemination (2xGnRH-TB48). In Exp. 2, heifers (n = 559) received the MGA-PGF protocol and were inseminated by either estrus AI or fixed-time AI (TAI) at 60 h, coincident with an injection of GnRH (GnRH-TB60). In Exp. 3, all heifers (n = 460) received the MGA-PGF protocol and were inseminated by estrus AI when detected up to 73 h. Heifers not observed in estrus by 73 h received TAI between 76 and 80 h. Half the heifers inseminated by TAI received no further treatment (TB80), and the remaining half was injected with GnRH at insemination (GnRH-TB80). Variance associated with the interval to estrus and the proportion in estrus from d 0 to 5 was similar for 1xGnRH and MGA-PGF treatments in Exp. 1. Pregnancy rate (d 0 to 5) did not differ for the MGA-PGF and 1xGnRH treatments (62.5 and 60.4%, respectively), and both were greater (P <0.05) than TAI pregnancy rate in the 2xGnRH-TB48 treatment (42.3%). In Exp. 2, the peak estrous response occurred 60 h after PGF. Pregnancy rate during the synchrony period was greater (P <0.05) for the MGA-PGF (255/401; 63.6%) than the GnRH-TB60 (74/158; 46.6%) treatment. In Exp. 3, 75.7% of heifers (348/460) were detected in estrus by 73 h and were inseminated, with a conception rate of 74.4%. Pregnancy rates after TAI did not differ between TB80 and GnRH-TB80 (14/56 = 25% and 19/56 = 33.9%, respectively). Total pregnancy rate was 63.5% for heifers inseminated after detected estrus and by TAI. Collectively, these data indicate that the exclusive use of TAI for heifers treated with the MGA-PGF protocol resulted in lower pregnancy rates than when AI was performed after detection of estrus. However, estrus AI for 3 d and TAI at the end of d 3 could result in pregnancy rates similar to those achieved after a 5-d period of detecting estrus.
Key Words: Fixed-Time Artificial Insemination • Gonadotropin-Releasing Hormone • Heifers • Melengestrol Acetate • Pregnancy Rates • Synchronization
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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(PGF) system for synchronization of estrus in beef heifers (Brown et al., 1988
) has been widely adopted because it is simple, inexpensive, and yields consistent responses. Lack of time, labor, or both was the most common reason cited for not using AI or synchronizing estrus (NAHMS, 1997). Development of effective fixed-time AI systems for heifers could simplify the application of AI, decrease labor, and increase the use of AI. The original MGA-PGF protocol was modified to increase the interval between the last feeding of MGA and the injection of PGF from 17 to 19 d (Deutscher 2000; Lamb et al., 2000). The resulting improvement in estrus synchrony with this modification has encouraged the investigation of fixed-time AI protocols with the MGA-PGF system.
Incorporation of GnRH into a variety of synchronization protocols facilitates a new wave of follicular growth and controls timing of ovulation (Twagiramungu et al., 1995). Treatment with GnRH induced ovulation in 54% and initiated a new wave of follicular growth in 75% of randomly cycling heifers (Pursley et al., 1995). Synchrony of follicular growth in heifers was improved with an injection of GnRH 7 d before PGF in an MGA-PGF system (Wood et al., 2001). Ovulation has been synchronized with GnRH when administered at the time of, or 16 to 24 h before, insemination in fixed-time AI protocols (Burke et al., 1996; Pursley et al., 1997; Geary and Whittier, 1998).
We hypothesized that the improved MGA-PGF protocol (Lamb et al., 2000) provided sufficient regulation of ovarian cycles to permit fixed-time AI and that addition of GnRH for control of follicle waves, synchronization of ovulation, or both, would enhance the efficacy of this general approach for fixed-time AI. The aim of these experiments was to explore methods of follicular and ovulation control that would result in pregnancy rates after fixed-time AI comparable to those for heifers inseminated after detection of estrus.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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). Heifers were assigned to three treatments within herd. Heifers in treatments that included insemination 6 to 18 h after detected estrus (estrus AI) received either no further treatment (n = 253; MGA-PGF) or were administered i.m. 100 µg of GnRH (Cystorelin, Merial, Inselin, NJ) on d –7 (1xGnRH, n = 260). The third treatment included GnRH on d –7 and 48 h after PGF (2xGnRH-TB48, n = 196), and heifers were inseminated based on a set time at 48 h (fixed-time AI). Heifers in this treatment that were detected in estrus before 26 h (n = 18) were inseminated 6 to 18 h after detected estrus. Of this group, six heifers were in estrus at the time of PGF or up to 48 h before PGF, and three heifers were in estrus at 24 h after PGF. For determination of fixed- time AI pregnancy rate, these heifers were classified as nonpregnant. Day of insemination was recorded for all herds, whereas time of estrus (before or after noon within day) was only recorded for Herds A and B.
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). Within herd, heifers were assigned to receive either no further treatment and be inseminated after detected estrus (MGA-PGF; n = 401) or to receive an injection of 100 µg of GnRH (Factrel, Fort Dodge Animal Health, Fort Dodge, IA) at timed insemination at 60h after PGF (GnRH-TB60; n = 158). The disproportionate numbers of heifers assigned to treatments resulted from restrictions imposed by the cooperating producer. Heifers in the MGA-PGF treatment that were observed in estrus between dawn (0630) and 0800, 0800 and 1200, and 1200 until dark (1700) were inseminated beginning at 1330, 1800, and 0800 the following morning, respectively, from d 0 to 5 after PGF. Heifers assigned to the GnRH-TB60 treatment that were observed in estrus before 37 h were inseminated after detected estrus and were not reinseminated at 60 h (n = 7). For determination of pregnancy rate to fixed-time AI, these seven heifers were classified as nonpregnant.
Experiment 3
Angus and Angus crossbred heifers at a single location (n = 460) received the MGA-PGF protocol described in Exp. 1. Heifers detected in estrus up to, and including, 72 h were inseminated 6 to 18 h after observed estrus. After estrus detection was performed at 72 h, all heifers not observed in estrus by this time received fixed-time AI at 76 to 80 h. Half of these heifers received no further treatment (TB80), whereas the remaining heifers were treated with 100 µg of GnRH at the time of insemination (GnRH-TB80).
Data Collection and Analyses
Heifers used for these studies were handled in a manner consistent with normal production practices, and all inseminations were performed by experienced technicians. The synchrony period was defined as the 5-d period following injection of PGF (d 0 to 5). To determine differences in distribution of estrus, heifers were categorized into groups based on detection before or after noon and were classified into a time group that reflected an approximate median time after injection of PGF. Following the synchrony period, detection of estrus and AI continued, or at least 10 d passed, before heifers were exposed to natural service sires. Conception rates after AI were determined via transrectal ultrasonography between 30 and 55 d after the initial insemination. Final pregnancy rates were confirmed by either palpation per rectum, transrectal ultrasonography, or both at least 40 d after the end of the breeding season. Conception rate was calculated as the number of pregnant heifers as a percentage of those inseminated. Synchronized pregnancy rate was defined as the number of heifers becoming pregnant during the synchrony period, after estrus AI, fixed-time AI, or a combination of estrus AI and fixed-time AI as a percentage of those receiving the respective treatment. Final pregnancy rate was calculated as the total number of heifers pregnant at the end of a 30- to 60-d breeding season as a percentage of the number treated.
Experiments 1 and 2 were designed as randomized complete block experiments and analyzed with the MIXED procedure of SAS version 8.1 (SAS Inst., Inc., Cary, NC). Time of estrus, sire, and technician were recorded for heifers in Herds A and B, but not C and D, of Exp. 1. Thus, any sire or technician effects were confounded with herd variation and were excluded from the statistical model. The model included fixed effects of treatment and random effects of herd and herd x treatment, with location included in the repeated statement for the variable day of insemination. Levene’s test for heterogeneity of variance (Milliken and Johnson, 1984) was used to compare the treatment variances for the interval in hours from PGF to estrus. The percentages of heifers that were in estrus after PGF were tested by Fisher’s exact test (Steele and Torrie, 1980). In Exp. 2, treatment was a fixed effect, and random effects were herd, sire, and technician nested within herd and their interactions with treatment. In Exp. 3, the model included fixed effects of treatment and random effects of sire and technician.
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Results |
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), although this difference was not significant for the 1xGnRH treatment. In the MGA-PGF treatment, 82.2% of heifers were detected in estrus during the synchrony period, similar to the response in 1xGnRH heifers (85.4%). In the 2xGnRH-TB48 treatment, 18/196 (9.2%) heifers were detected in estrus before 26 h and were inseminated 6 to18 h after detected estrus. Removal of these 18 heifers from the fixed-time AI group resulted in 178 heifers being time inseminated at 48 h. In Herds A, C, and D, heifers in the 2xGnRH-TB48 treatment were observed for estrus after fixed-time AI. In Herd A, 23/107 (21.5%) heifers exhibited estrus between d 5 and 8. These heifers were reinseminated 6 to 18 h after detected estrus. For statistical analyses, it was assumed that they had not conceived to the fixed-time AI. In Herds C and D, no heifers were observed in estrus during this time period.
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Experiment 2
The greatest proportion (P <0.05) of heifers in the MGA-PGF treatment was in estrus 60 h after PGF (38.9%; Figure 2). The mean time of estrus was 65 ± 1.5 h, and the cumulative proportions of heifers in estrus by h 72, 84, and 96 were 72.1, 77.8, and 83.3%, respectively. Seven heifers in the GnRH-TB60 treatment exhibited estrus between h 0 and 37 and were inseminated 6 to 12 h later. These heifers were not inseminated at 60 h. Actual time of AI and GnRH injection for the GnRH-TB60 heifers ranged from 63.5 to 68.5 h in both herds.
Conception rate was greater (P < 0.05) for the MGA-PGF (255/364 = 70%) than GnRH-TB60 (74/151 = 49%) treatment. Pregnancy rate during the synchrony period was greater (P <0.05) for the MGA-PGF treatment (255/401 = 63.6%) than after fixed-time AI for the GnRH-TB60 (74/158 = 46.6%) treatment. Of the seven heifers in estrus before 37 h, only two became pregnant. Inclusion of the two early pregnancies for the GnRH-TB60 treatment resulted in a pregnancy rate over the 60 h after PGF of 48.1%. Final pregnancy rate was 90.7% and did not differ between treatments.
Experiment 3
Through h 72, 75.7% (348/460) of heifers were detected in estrus (Figure 2). Of the heifers that received fixed-time AI (n = 56 for TB80; n = 56 for GnRH-TB80) 17 and 6 heifers, respectively, were detected in estrus during the 5 d following fixed-time AI (d 4 to 9). These heifers were reinseminated, but none were inseminated less than 1.5 d after fixed-time AI. Conception rate for heifers detected in estrus was 74.4% (259/348). Conception rate after fixed-time AI at h 76 to 80 did not differ between the TB80 (14/56 = 25%) and GnRH-TB80 (19/56 = 34%) treatments (P >0.30). After combining pregnancies from those inseminated after detected estrus and fixed-time AI, synchronized pregnancy rate was 63.5%. Heifers that received a second AI and became pregnant in the TB80 (13/17) and GnRH-TB80 (4/6) treatments were presumed to have become pregnant to the second insemination rather than to the fixed-time AI.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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) did not translate into improved breeding performance in heifers. Failure of GnRH to improve the precision of estrus in this study, or in heifers in general, might result from the failure of GnRH to cause ovulation of dominant follicles or turnover of the dominant follicle to initiate a new follicular wave. Although Wood et al. (2001) reported that 100% of heifers ovulated in response to GnRH when treated on the 12th day after a 14-d period of MGA feeding, only approximately half the heifers ovulated in response to GnRH when treated at random stages of the estrous cycle (Silcox et al., 1993; Pursley et al., 1995; Martinez et al., 2000). In most cases, heifers have failed to achieve the pregnancy rates in a GnRH plus PGF synchronization system realized with MGA-PGF (Lamb, 2002). One possible explanation for this difference is an inconsistent response to GnRH in heifers. Even though blood samples to determine the proportion of pubertal heifers were not collected, the high estrous responses and high AI pregnancy rates indicate that a majority of heifers were cycling and/or responded to the MGA-PGF protocol. Perhaps the synchronized pregnancy rate achieved with MGA-PGF protocol is already near maximum and any improvement in synchrony of follicular growth by GnRH could not be measured in pregnancy rate.
In all three experiments, the peak estrous response in the MGA-PGF protocol was at 60 h after PGF. The mean interval to AI based on detection of estrus was 56.2 h for heifers that received PGF 19 d after the last feeding of MGA (Lamb et al., 2000). Thus, the 48-h intervals in Exp. 1 and 60-h intervals in Exp. 2 for the single fixed-time AI were logical choices. However, pregnancy rates to fixed-time AI at either 48 or 60 h after PGF (plus the concurrent injection of GnRH) were significantly lower than those following AI based on detected estrus. Including the results of the present studies, and several recent reports, pregnancy rates after the MGA-PGF protocol that included a fixed-time AI plus injection of GnRH, ranged from 38 to 54% (weighted average of 48%; L. H. Anderson, University of Kentucky, Lexington, unpublished data; Sasongko et al., 2000; Kojima et al., 2001; Baker et al., 2001). In this collection of reports, insemination times ranged from 48 to 72 h after PGF, and collectively these data indicated that conception rates increased as the interval to AI approached 60 to 72 h. Using the MGA-PGF protocol without GnRH at insemination, Patterson et al. (1999), reported that pregnancy rates to a single timed insemination at 72 h after PGF (53%) or a double insemination at 65 and 85 h after PGF (49%) did not differ. It seems that a relatively wide window exists in which fixed-time AI can be performed with the MGA-PGF protocol, but the expectation is a lower pregnancy rate than can be achieved after AI based on detected estrus. Estrous response and pregnancy rates to fixed-time AI are likely less consistent with the 17-d interval between MGA and PGF (King et al., 1994; Larson et al., 1996). When estrous response was low, little difference existed between pregnancy rates after detected estrus or fixed-time AI. However, when estrous response was higher, the difference between the two was similar to the present studies.
A common approach currently practiced with the MGA-PGF protocol, known as "clean-up AI," includes AI after detected estrus through 72 h after PGF, and then a fixed-time AI concurrent with administration of GnRH in all remaining heifers not yet observed in estrus. In Exp. 3, approximately 25% of heifers (112/460) were inseminated after fixed-time AI at 76 to 80 h, and conception rate was 34% for those receiving GnRH at AI. This group of heifers included those expected to be in estrus after 72 h (14 to 18% of those treated in Exp. 1, and 2; Figure 2), as well as those that would not exhibit estrus (10 to 20% of females treated based on appropriate controls in Exp.1 and 2). Thus, when estrus detection was extended for two more days in Exp. 1 and 2, approximately 12% more pregnancies were gained during this time. If 25% of heifers are bred by fixed-time AI and 34% become pregnant, 8.5% of the total group would become pregnant. Thus, it seems that both approaches should give approximately equal pregnancy rates. The comparison is the cost of an additional 2 d of detecting estrus, balanced against the cost of semen and GnRH for all females not detected in estrus by 73 h.
Administering GnRH at the time of insemination was hypothesized to synchronize ovulation in heifers that were off pace in follicular development. However, addition of GnRH did not statistically improve pregnancy rates after the fixed-time AI, with the limited numbers represented in the fixed-time AI group in Exp. 3. Administering GnRH at fixed-time AI ensures that ovulation should occur in a relatively narrow window: 24 to 32 h after injection (Pursley et al., 1995). For fixed-time AI, the inclusion of GnRH increases the confidence that ovulation is occurring at an appropriate time relative to AI. However, if one chose to terminate detection of estrus and use a clean-up AI without the GnRH injection, the consequences might be tolerable. In fact, if no GnRH was administered and time for further detection of estrus was not limited, more heifers might be detected in estrus than if they received GnRH at the timed insemination, as demonstrated in Exp. 3.
In conclusion, the addition of a single injection of GnRH 7 d before the PGF injection of the MGA-PGF protocol did not improve pregnancy rates of heifers in the present studies. Likewise, compared with an estrus AI program, fixed-time AI programs with GnRH at either 48 or 60 h after PGF decreased pregnancy rates. Use of fixed-time AI at 80 h after PGF, in animals not detected in estrus by 73 h, resulted in pregnancies in approximately 30% of females inseminated. It is unclear whether GnRH is necessary in conjunction with fixed-time AI when this approach is taken.
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Footnotes |
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2 Correspondence: P.O. Box 786 (phone: 785-462-6281; fax: 785-462-2315; e-mail: sandyj{at}ksu.edu).
Received for publication December 1, 2004. Accepted for publication June 18, 2004.
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Literature Cited |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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to Syncro-Mate B for estrous synchronization in beef heifers. Theriogenology 30:1–12.
interval from 17 to 19 days in an estrus synchronization system for heifers. Theriogenology 53:691–698.[Medline]
system: Efficiency of timed insemination 72 hours after prostaglandin treatment. Theriogenology 45:851–863.[Medline]
J. Anim. Sci. 77(Suppl 1):237. (Abstr.)
protocol. J. Anim. Sci. 78(Suppl 2):119. (Abstr.)
synchronization treatment in beef heifers. J. Anim. Sci. 79:2210–2216.This article has been cited by other articles:
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  J. A. Atkins, D. C. Busch, J. F. Bader, D. H. Keisler, D. J. Patterson, M. C. Lucy, and M. F. Smith Gonadotropin-releasing hormone-induced ovulation and luteinizing hormone release in beef heifers: Effect of day of the cycle J Anim Sci, January 1, 2008; 86(1): 83 - 93. [Abstract] [Full Text] [PDF]
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