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Department of Dairy Science, University of Wisconsin, Madison 53706
Corresponding author: P. M. Fricke; e-mail: pmfricke{at}wisc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGEMENTSREFERENCES |
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; d 8: 100 µg of GnRH + TAI) either without (GPG; n = 95), or with inclusion of a CIDR insert (CIDR; n = 94) from d 0 to 6. No CIDR heifers received AI before d 8 compared with 24% of GPG heifers, and pregnancy rate per AI (PR/AI) at 30 d after TAI did not differ between treatments. To synchronize return to estrus for heifers failing to conceive after TAI, heifers (n = 166) receiving TAI to first service were randomly assigned to receive no further treatment (control; n = 85) or receive a new CIDR insert between 14 and 20 d after TAI (Resynch; n = 81). No Resynch heifers received AI during CIDR treatment compared with 35% of control heifers, and the proportion of heifers receiving AI within 72 h after the day of CIDR removal was 78 vs. 50% for Resynch vs. control heifers, respectively. No treatment x inseminator interaction was detected at first or second AI; however, overall PR/AI was modest for heifers throughout the experiment due to poor performance of 2 of the 3 herd inseminators (14, 6, and 58% PR/AI, respectively). Inclusion of CIDR inserts suppressed estrus during the TAI protocol with no reduction in PR/ AI. Resynchronization of estrus using CIDR inserts resulted in tighter synchrony of estrus among nonpregnant heifers compared with untreated controls.
Key Words: controlled internal drug-releasing insert • dairy heifer • timed artificial insemination • resynchronization
Abbreviation key: CIDR = controlled internal drug-releasing insert, GPG = synchronization regimen using sequential injections of GnRH and PGF2 to control luteal regression and ovulation for timed insemination, PR/AI = pregnancy rate per artificial insemination, P4 = progesterone, TAI = timed artificial insemination.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGEMENTSREFERENCES |
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Ovsynch was the first protocol developed to successfully synchronize ovulation thereby allowing for a TAI in lactating dairy cows (Pursley et al., 1995). Unfortunately, dairy heifers respond poorly to Ovsynch and TAI, exhibiting a pregnancy rate per AI (PR/AI) 20 to 40% lower than heifers receiving AI to a standing estrus (Schmitt et al., 1996; Pursley et al., 1997). The greater incidence of estrus before TAI in dairy heifers (18%; Rivera et al., 2004) compared with lactating cows (6 to 9%; Roy and Twagiramungu, 1996) likely accounts for the low PR/AI to TAI for dairy heifers. Reduced PR/AI likely occurs when heifers express premature estrus that is not detected, but receive a TAI that is too late relative to estrus for conception to occur. Thus, suppression of premature estrus expression during a GnRH-PGF2-based protocol for synchronization of ovulation may improve the practical implementation of these protocols as a reproductive management tool in dairy heifers. Furthermore, because some heifers submitted to TAI at first breeding will fail to conceive, a strategy to resynchronize estrus for second breeding may be useful for submitting heifers for second AI and further minimize time devoted to detection of estrus.
The objectives of the present study were to: 1) evaluate the effect of a controlled internal drug-releasing (CIDR) insert as a tool to increase the proportion of heifers submitted to TAI after a hormonal protocol to synchronize ovulation, and 2) determine the effect of CIDR inserts to resynchronize return to estrus for dairy heifers failing to conceive to first TAI. Our hypothesis was that inclusion of CIDR inserts between the first 2 injections of a GnRH-PGF2-based synchronization protocol would suppress premature expression of estrus before PGF2 treatment, and that resynchronization of estrus in nonpregnant heifers using CIDR inserts might result in tighter synchrony of estrus among heifers failing to conceive to first TAI.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGEMENTSREFERENCES |
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Synchronization of Ovulation for First AI Service
At onset of the AI breeding period (d 0), Holstein heifers were assigned randomly to receive GnRH (Cystorelin; Merial Ltd; Duluth, GA) and PGF2 (Lutalyse; Pharmacia Animal Health, Kalamazoo, MI) as follows: 100 µg of GnRH on d 0; 25 mg of PGF2 on d 6; 100 µg of GnRH + TAI on d 8, either without (GPG; n = 95) or with insertion of a new CIDR insert (Eazi-breed CIDR, Pharmacia Animal Health) containing 1.38 g of progesterone (P4) from d 0 to 6 (CIDR; n = 94). Heifers in both treatments received TAI within 30 min after the second GnRH injection. Any heifer in which tail chalk removal was detected before scheduled TAI on d 8 was inseminated and the subsequent synchronization protocol components were discontinued. Artificial insemination throughout the experiment was performed by the herd manager and 2 full-time employees. A maximum of 92 TAI were performed per replicate on d 8 of the experiment.
Synchronization of Estrus for Second AI Service
On d 14 after TAI, heifers receiving TAI were assigned randomly within treatment (GPG and CIDR) to receive no further treatment (control; n = 85) or to receive a new CIDR insert from d 14 to 20 after TAI (Resynch; n = 81). All heifers were inseminated after rubbed tail chalk that was evaluated once daily at the morning feeding period when heifers were restrained in self-locking head gates.
Blood Sampling and Radioimmunoassay
Blood samples were collected from GPG and CIDR heifers via venipuncture of the median caudal vein or artery immediately before administration of the second GnRH injection of each protocol. Blood samples were allowed to clot for 24 h at 4°C, and centrifuged (1500 x g for 15 min). Blood serum was harvested and stored at –20°C until assayed for P4 concentration by radioimmunoassay (Coat-a-Count Progesterone, Diagnostic Products Corporation, Los Angeles, CA). All samples were run in a single assay, and the intraassay coefficient of variation was 6.8%. Serum samples were classified as either low (
1.0 ng/mL) or high (>1.0 ng/mL), to assess the presence (high P4) or absence (low P4) of a functional corpus luteum at the second GnRH injection. Blood sampling and hormone injections for heifers inseminated after removed tail chalk during the protocol were discontinued after AI.
Transrectal Ultrasonography
Ovarian structures were monitored using an ultrasound machine equipped with a transrectal 7.5-MHz linear-array transducer (Aloka 500V; Corometrics Medical Systems, Inc., Wallingford, CT). During each ultrasound examination conducted on d 8 and 10, a sketch of the location and diameter of all ovarian structures 
8 mm in diameter was recorded for all heifers receiving TAI. For each ovarian structure, 2 diameter measurements, taken at right angles, were recorded on a single frozen image of the presumed maximal diameter of the structure using the digital calipers of the ultrasound machine. Diameters were calculated as the mean of these 2 measurements. Ovulatory response after the second GnRH injection was determined by the presence of 1 or more dominant follicles at the time of the second GnRH injection and the absence of 1 (single ovulation) or 2 (double ovulation) of those follicles at an ultrasound examination conducted 48 h later (Fricke et al., 1998). Ultrasound examinations were conducted to determine diameter of the largest follicle at the time of CIDR removal in Resynch heifers (d 20 postTAI). Rate of double ovulation was calculated as the number of heifers that ovulated 2 follicles within 48 h of the second GnRH injection, and was expressed as a percentage of heifers that ovulated after the second GnRH injection. Synchronization rate was defined as the proportion of heifers with low P4 at the second GnRH injection, and in which ovulation occurred within 48 h after the second GnRH injection, expressed as percentage of the total number of heifers treated with the protocol.
Pregnancy Rate per AI
Pregnancy status was determined 30 ± 0.1 and 65 ± 0.1 d after TAI using transrectal ultrasonography. Visualization of a fluid-filled uterine horn and presence of a conceptus were used as positive indicators of pregnancy (Fricke et al., 1998). Pregnancy rate per artificial insemination to TAI assessed at 30 and 65 d after AI and pregnancy loss between these 2 stages of gestation were calculated for heifers in both treatments. Fetal sex was determined for all heifers confirmed pregnant at d 65 of gestation. A fetus was recorded as a male when the genital tubercle was located in the area caudal to the umbilicus and as a female when the same structure was located beneath the tail (Curran and Ginther, 1991).
Body Size Traits
Body condition scores were assigned to each heifer before first AI by the same evaluator throughout the experiment using a quarter-point scale from 1.0 to 5.0, where 1.0 = emaciated and 5.0 = obese (Ferguson et al., 1994). Live BW was estimated for each heifer before first AI using a commercial dairy tape (Weight-By-Breed Dairy Cow Tape; Nasco, Fort Atkinson, WI), and hip height and wither height were measured for each heifer at first AI on a level concrete surface with the heifer in a normal standing position. Wither height index was calculated as BW divided by wither height as described by Looper and Bethard (2000), but using metric rather than imperial units of measure.
Statistical Analyses
Dichotomous data (ovulatory response after GnRH and PR/AI) were analyzed by multifactorial logistic regression using the LOGISTIC procedure of SAS (SAS Institute, 1989). The model used to analyze PR/AI to first breeding included treatment, inseminator, and treatment x inseminator interaction, with BCS, wither height, hip height, and BW at first AI as regression variables. Because all heifers per replicate were 13 mo of age within a 2-wk period, age was not included in the model.
To assess potential carryover effects of CIDR inserts used for first TAI on PR/AI to second AI, treatments from TAI (CIDR vs. GPG) and treatments from second AI (Resynch vs. control) were combined to yield 4 treatment groups for first/second AI (CIDR/CIDR, CIDR/ noCIDR, noCIDR/CIDR, noCIDR/noCIDR). Thus, the new model to analyze PR/AI to second AI included treatment, inseminator, and treatment x inseminator interaction. No difference in PR/AI was detected among these 4 treatments, possibly due to the small number of heifers in each of the 4 treatments. Because no statistical difference was detectable, however, PR/AI to second AI was analyzed using a model that included treatment (control vs. Resynch), inseminator, replicate, and the interaction of treatment x inseminator.
Continuous data (days to first AI, days to conception, and follicular diameter) were analyzed using the GLM procedure of SAS. Effect of treatment on interval to first AI and to pregnancy was evaluated by survival analysis using the Log-Rank method of the LIFETEST procedure of SAS including all heifers assigned to each treatment. The Log-Rank method also was used in survival analysis to evaluate interval to pregnancy including all heifers in the model. Heifers not inseminated during the breeding period were censored. Fetal sex ratio was analyzed by 
2 for specified proportions using the FREQ procedure of SAS. Because no published reports of the sex ratio of calves born to nulliparous Holstein heifers in the literature were found, the specified proportions (expected values) used in this analysis were based on the observed sex ratio of newborn calves reported by Ryan and Boland (1991) for primiparous Holstein dairy cows.
For all analyses, a significant difference was stated when P 0.05, whereas a statistical tendency was considered to occur when P = 0.05 and 0.10.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGEMENTSREFERENCES |
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). A similar effect of inseminator was reported in a field trial conducted on this farm (Rivera et al., 2004). Because groups of heifers were enrolled into the present experiment (August 27 to September 10, 2002) immediately after completion of the enrolment phase of the preceding experiment (May 21 to August 13, 2002), we were unable to identify and mitigate the poor performance of the 2 herd inseminators before initiating the present experiment. Because treatment x inseminator interactions were not detected for pregnancy outcomes in the present experiment, main effects of treatment on experimental endpoints are valid. Moreover, the unacceptably low PR/AI in this field trial conducted on the largest commercial custom heifer growing operation in Wisconsin serves as a teaching opportunity for dairy farmers and the industry consultants who advise them.
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). No treatment x inseminator interaction was detected indicating that treatment effects on PR/AI in the present study were not confounded by the inseminator effect. Conception rates of dairy heifers inseminated after standing estrus have been reported to be 47% (Donovan et al., 2003), 59% (Gwazdauskas et al., 1981), 55 to 66% (Butler and Smith, 1989), and 74% (Pursley et al., 1997). A recent study reported a 60% PR/AI for Jersey and Holstein heifers submitted to a TAI protocol using estradiol cypionate, PGF2, GnRH, and CIDR inserts (Peeler et al., 2004).
Few herd-level or animal-level factors affect fertility in dairy heifers (Donovan et al., 2003), and most herd-level variation in conception rate in heifers is due to variation among inseminators and service sire (Ron et al., 1984). Barth (1993) reported that timing of AI, semen quality, semen handling, and inseminator expertise influenced pregnancy outcomes after AI in cattle. Improper semen placement in the reproductive tract also affects outcomes to AI with more than half of inseminators depositing semen into the cervix rather than correctly into the uterine body (Peters et al., 1984). A common mistake was for inseminators to deposit semen into the cervix while withdrawing the pipette or straw during AI (Zavy and Geisert, 1994). A similar effect of AI technician on conception rate was reported for beef heifers synchronized using melengestrol acetate and PGF2 (Salverson et al., 2002). Had the herd manager inseminated all heifers in the present experiment, PR/AI would have been comparable with previous reports in dairy heifers (Gwazdauskas et al., 1981; Butler and Smith, 1989).
Effect of Treatment on Synchronization and Pregnancy Outcomes to First AI Service
A primary objective of the present experiment was to determine if inclusion of CIDR inserts from d 0 to 6 (i.e., insertion at the first GnRH injection and removal at the PGF2 injection) would suppress estrus during the protocol without negatively affecting fertility. As expected, none of the CIDR heifers displayed estrus during the protocol before scheduled TAI, whereas 24% of GPG heifers expressed estrus 4.5 ± 0.4 d after the first GnRH injection (Figure 1, upper panel; Table 2). Similar observations were reported for dairy (Rivera et al., 2004) and beef (Roy and Twagiramungu, 1996) heifers subjected to protocols for synchronization of estrus when the GnRH injection was initiated at a random stage of the cycle followed 6 d later by PGF2. No treatment difference was detected in ovulatory response to the second GnRH injection (Table 2). In the present study, 2% (3/146) of heifers that ovulated after the second GnRH injection ovulated 2 follicles, whereas the proportion of heifers carrying twin fetuses at 65 d of gestation was 2% (1/57). These results are consistent with previous reports of 1% double ovulation rates and <1% twinning rates in dairy heifers (Ryan and Boland, 1991; Rivera et al., 2004). Synchronization rate (proportion of heifers on d 8 having low P4 and subsequently ovulating a follicle after the second GnRH injection) was greater (P < 0.01) for CIDR heifers due to suppression of estrus during the protocol, whereas no treatment differences were detected for PR/AI at 30 or 65 d after AI or for pregnancy loss from 30 to 65 d after TAI (Table 2). Although fertility in the present experiment is less than that reported previously for this custom heifer growing operation (Rivera et al., 2004), the rate of pregnancy loss is similar between experiments, but greater than the 2% reported by Peeler et al. (2004) for Jersey and Holstein heifers during a similar stage of gestation.
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treatment on d 6 compared with AI after synchronized estrus with PGF2. Similarly, Martinez et al. (2002) reported improved pregnancy rates in beef heifers after addition of a progestin to a Cosynch or Ovsynch protocol. Diameter of the largest follicle on d 8 affected (P < 0.01) ovulatory response (11.9 ± 0.1 and 9.8 ± 0.4 mm for ovulatory and nonovulatory follicles, respectively), which is consistent with a previous report (Rivera et al., 2004). Growing ovarian follicles in lactating dairy cows generally must reach a minimum diameter of 10 mm before they acquire the capacity to ovulate to an injection of GnRH (Sartori et al., 2001). Interestingly, follicular diameter for heifers that ovulated after the second GnRH injection was greater (P < 0.01) for heifers diagnosed pregnant 30 d after TAI compared with non-pregnant heifers (12.3 ± 0.3 and 11.3 ± 0.2 mm, respectively). Vasconcelos et al. (2001) reduced the size of the ovulatory follicle in lactating dairy cows submitted to Ovsynch by follicular aspiration and reported less serum concentrations of estradiol at the time of the second GnRH injection, subsequent diameter and volume of the GnRH-induced corpus luteum, serum P4 concentrations, and fertility.
Effect of Treatment on Second AI Service
To determine the effect of using CIDR inserts to synchronize return to estrus for heifers failing to conceive after first service, heifers receiving TAI at first service were randomized within their original treatments and assigned to resynchronization treatments (85 and 81 for control and Resynch, respectively) for second AI (Table 3). Because 25 control heifers and 23 Resynch heifers were diagnosed pregnant 30 d after TAI, 60 and 58 nonpregnant heifers were eligible to receive second AI service in the control and Resynch treatments, respectively (Table 3). Although service rate for heifers eligible to receive a second AI was similar between treatments (Figure 2, upper panel; Table 3), the proportion of heifers receiving AI after rubbed tail chalk by 72 h after CIDR insert removal was greater (P < 0.01) for Resynch than for control heifers (Table 3). Furthermore, 35% (21/60) of nonpregnant control heifers were inseminated after rubbed tail chalk during the CIDR resynchronization period (Figure 2, upper panel), whereas none of the nonpregnant Resynch heifers were inseminated during the CIDR resynchronization period. Fewer noninseminated eligible heifers at the end of the experiment for control vs. Resynch heifers (illustrated in Figure 3) is an artifact caused by a greater proportion of control than Resynch heifers receiving more than 2 inseminations.
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). Exposure to a CIDR insert for first service did not affect PR/AI to second service. Although a treatment x inseminator interaction was not detected for second AI, inseminator 3 had a greater PR/AI for Resynch than for control heifers (Table 1). The possibility of Type I or Type II errors (declaring a difference when a difference among groups does not exist or declaring no difference when a difference exists), should be considered when interpreting these data. The likelihood of detecting inseminator effects on fertility was limited in the present study because less than 50 breedings per treatment were included for inseminator 3. Further research is needed to determine the effect of this resynchronization strategy on fertility of dairy heifers. Contrary to the effect of follicular size on PR/AI to TAI detected in this study, follicular size at the time of CIDR removal (assessed only in Resynch heifers) did not differ between Resynch heifers diagnosed pregnant vs. not pregnant 30 d after second AI (10.2 ± 0.9 and 10.9 ± 0.4 mm, respectively). Assessment of follicular size in heifers submitted to TAI (i.e., synchronized ovulation) was performed at a known stage relative to the expected time of ovulation after GnRH treatment (28 to 30 h). In contrast, Resynch heifers inseminated after rubbed tail chalk probably had larger follicles (greater than 10.2 to 10.9 mm) at the time of AI compared with the time of CIDR removal, given that these follicles could have grown larger later on. Therefore, the smaller follicular size detected in heifers inseminated in the Resynch group compared with heifers receiving TAI, as well as the similar follicular size of pregnant vs. not pregnant Resynch heifers could be explained by the uncertain time from their CIDR insert removal to the endogenous GnRH/LH surge, which might affect the final stages of follicular growth.
Heifers failing to conceive to TAI returned to second service beginning as early as 4 d after TAI; however, P4 concentration was not evaluated at second AI. A total of 16 nonpregnant heifers received a second service by 14 d after TAI, and only 1 of these heifers conceived (Figure 3). Because both luteal regression and ovulation in this heifer was synchronized by the protocol, but returned to service 9 d after TAI, it was assumed to have had a short luteal phase. From CIDR reinsertion (d 14) until 19 d after TAI, 14 nonpregnant control heifers were re-inseminated, compared with none of the Resynch heifers. Only 1 of these heifers became pregnant (d 19). Because PR/AI to second AI by 19 d after TAI was only 7% (2/30), most of these services likely represent timing of AI errors due to inaccurate detection of estrus based on rubbed tail chalk (Figure 3). Comparison of the percentage distribution of pregnancies after the second AI service (Figure 3, lower panel) to the percentage distribution of second AI services illustrates these trends. Although patterns of estrus behavior were not examined in the present study, Richardson et al. (2002) reported a decreased number and total duration of standing events for a second AI service when heifers were submitted to a synchronization protocol before first AI.
A surprisingly large proportion of pregnant heifers (17%; 10/58) were re-inseminated after the TAI, but before pregnancy diagnosis at 30 d. Prevalence of this error was reported to be 8% for dairy heifers diagnosed pregnant approximately 30 d after AI (Rivera et al., 2004). Although the effect of this error on PR/AI was not evaluated, some heifers diagnosed nonpregnant 30 d after AI conceived to first AI and subsequently lost these pregnancies due to insemination after secondary signs of estrus. In fact, 1 of the heifers (25%, 1/4) having pregnancy loss (diagnosed pregnant at 30 d and open at 65 d after TAI) received AI on d 22 of gestation. Unnecessary re-insemination during diestrus reduced the PR/AI to a previous insemination at the correct time by over 30% (Macmillan et al., 1977). Weaver et al. (1989) reported reduced PR/AI for cows re-inseminated in the uterine body between 12 and 24 d after previous inseminations at estrus (4 vs. 41% for re-inseminated and control, respectively). Artificial insemination during pregnancy may induce loss, through mechanical trauma to fetal membranes or from the introduction of infection (Noakes et al., 2001).
Effect of Body Size Parameters
No effect of body size traits on pregnancy status 30 d after first AI was detected in the present study. This supports the observation that animal-level factors are not strongly associated with fertility in dairy heifers (Donovan et al., 2003). Overall, BW (377.5 ± 2.1 kg), hip height (131 ± 0.7 cm), and wither height (122 ± 0.3) were within body size recommendations by age reported for growing dairy heifers (Hoffman, 1997; Looper and Bethard, 2000). Heifers were evaluated using both wither height index (3.1 ± 0.0) and BCS (3.0 ± 0.0) to avoid a possible bias due to visual errors in BCS (Looper and Bethard, 2000).
Fetal Sex Ratio
Observed sex ratio at 65 d of gestation based on ultrasonographic location of the genital tubercle tended (P = 0.06) toward more female fetuses compared with the expected sex ratio reported for primiparous dairy cows inseminated after a standing estrus (49% female; n = 7852; Ryan and Boland, 1991). In the present study, the proportion of female fetuses resulting from TAI (diagnosed at 65 d of gestation) was 61 and 65% for CIDR and GPG heifers, respectively. In the present analysis, sex ratio was assumed to be an effect of timing of AI in relation to GnRH treatment (i.e., ovulation), and it was not expected to differ between treatments. Therefore, sex ratio data from both treatments was combined (Table 4). Data from the present study were combined with sex ratio data from a previous study (Rivera et al., 2004) that used the same breeding protocol for TAI. Overall, there was a tendency (P = 0.06) for heifers receiving TAI to have more female fetuses compared with the expected sex ratio reported for primiparous cows (Ryan and Boland, 1991). Increasing the interval from TAI to ovulation by synchronizing ovulation has been reported to skew the sex ratio to increase the number of female calves born in dairy cattle (Pursley et al., 1998; Xu and Burton, 1999). Further research is needed to determine the effect of time of AI relative to ovulation on sex ratio in dairy heifers.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGEMENTSREFERENCES |
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Received for publication July 19, 2004. Accepted for publication November 18, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONACKNOWLEDGEMENTSREFERENCES |
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for synchronizing estrus and shortening the interval to pregnancy in postpartum beef cows, peripubertal beef heifers, and dairy heifers. J. Anim. Sci. 79:982–995. and GnRH. Theriogenology 44:915–923.
, and progesterone. J. Anim. Sci. 80:2792–2800.-GnRH method for beef females. J. Anim. Sci. 74(Suppl. 1):462. (Abstr.)
: An efficacy comparison of cloprostenol and dinoprost tromethamine. Theriogenology 57:853–858.[Medline]
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