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* Department of Dairy Science, University of Wisconsin, Madison 53706
Miltrim Farms, Inc., Athens, Wisconsin 54411
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 DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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at 32 ± 3 d and 46 ± 3 d (Presynch); 100 µg GnRH at 60 ± 3 d; 25 mg PGF2 at 67 ± 3 d; and 100 µg GnRH + TAI at 69 ± 3 d (Ovsynch). At first TAI, cows were randomly assigned to initiate the first GnRH injection of a hormonal protocol for resynchronization of ovulation (Resynch; 100 µg GnRH, d 0, 25 mg PGF2, d 7, 100 µg GnRH + TAI, d 9) at 19 (D19), 26 (D26), or 33 d (D33) after first TAI to set up a second TAI service for cows failing to conceive to Ovsynch. Overall pregnancy rate per artificial insemination (PR/AI) to Ovsynch assessed 68 d after TAI was 31% and did not differ among treatment groups. For Resynch, PR/AI was assessed 26 d after TAI for D19 and D26 cows and 33 d after TAI for D33 cows. Overall PR/AI to Resynch was 32%. However, the PR/AI for D26 (34%) and D33 (38%) cows to Resynch was greater than for D19 cows (23%). Cows with a CL at the PGF2 injection (D19 cows) or at the first GnRH injection (D26 + D33 cows) of Resynch exhibited greater PR/AI to Resynch compared with cows without a CL. Survival analysis (failure time) of cows in the D26 and D33 treatment groups across the first three TAI services did not differ statistically. Although administration of GnRH to pregnant cows 19 d after first TAI service did not appear to induce iatrogenic embryonic loss, initiation of Resynch 19 d after first TAI service resulted in a lower PR/AI compared with initiation of Resynch 26 or 33 d after first TAI service.
Key Words: synchronization • resynchronization • dairy cow • timed artificial insemination
Abbreviation key: Cosynch = timed insemination coincides with last injection of GnRH in Ovsynch, Ovsynch = synchronization regimen using sequential injections of GnRH and PGF2 to control ovulation for timed insemination, Presynch = postpartum regimen using 2 injections of PGF2 to prepare cows for Ovsynch, PR/AI = pregnancy rate per artificial insemination, Resynch = synchronization regimen using GnRH and PGF2 to resynchronize ovulation in cows after first insemination, TAI = timed AI
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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(Ovsynch; Pursley et al., 1995; Burke et al., 1996; Pursley et al., 1997a, 1997b) and is an effective method for hormonally programming cows to receive TAI. A presynchronization strategy in which cows receive two injections of PGF2 administered 14 d apart beginning 26 to 28 d before initiation of Ovsynch (Presynch) has been reported to increase conception rate to TAI in lactating dairy cows compared with Ovsynch alone (Moreira et al., 2001; El-Zarkouny et al., 2002; Navanukraw et al., 2002). Many dairy farms in Wisconsin have incorporated synchronization protocols as the primary strategy for submitting cows for first TAI service. Although reliance on synchronization of ovulation and TAI for improving service rate to first AI reduces the impact of poor estrus detection efficiency, the high AI submission rate to first TAI often is followed by a time lag that can exceed 60 d before cows failing to conceive are reinseminated. Because AI conception rates of high producing lactating dairy cows are reported to be 40% or less (Pursley et al., 1997a, 1997b; Fricke et al., 1998; Jobst et al., 2000), 60% or more of the cows that receive TAI will fail to conceive and, therefore, require a resynchronization strategy for aggressively initiating subsequent AI services. Coupling a nonpregnancy diagnosis with a management decision to rapidly reinitiate AI service improves reproductive efficiency and pregnancy rate by decreasing the interval between AI services, thereby increasing AI service rate (Fricke, 2002).
Fricke (2002) proposed an aggressive resynchronization protocol in which groups of cows past the voluntary waiting period receive their first postpartum TAI after synchronization of ovulation. On d 18 after TAI, all cows would receive an injection of GnRH regardless of their pregnancy status. Nonpregnant cows would receive a PGF2 injection on d 25 after TAI based on a nonpregnancy diagnosis using ultrasound and would continue resynchronization with Ovsynch, whereas pregnant cows would discontinue the resynchronization protocol. A field trial conducted in Florida in which cows received GnRH on d 20 after TAI followed by transrectal ultrasound and PGF2 administration to nonpregnant cows on d 27 was reported by Moreira et al. (2000b). These authors reported an interaction for cows resynchronized with GnRH on d 20 after TAI in which embryonic loss from d 20 to 27 after TAI was increased for bST-treated pregnant cows receiving GnRH but not for non-bST-treated pregnant cows. Because of this observation, this resynchronization strategy was discontinued (Moreira et al., 2000b). Hormonal resynchronization systems that program nonpregnant cows to receive subsequent AI services need to be developed and further assessed so that systematic reproductive management programs can be implemented to aggressively manage reproduction in lactating dairy cows on commercial dairies (Fricke, 2002).
The objective of this study was to evaluate pregnancy rate per AI (PR/AI), and early embryonic loss after initiation of a resynchronization of ovulation protocol initiated at three intervals after first postpartum TAI on a commercial dairy farm in Wisconsin.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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(25 mg of Lutalyse; Pharmacia Animal Health, Kalamazoo, MI). Cow lists for injection schedules, pregnancy examinations, and reproductive events for individual cows were generated, tracked, and recorded using a commercial on-farm computer software program (Dairy Comp 305, Valley Agricultural Software, Tulare, CA). Cows assigned to the study were coded by treatment, and the "cowfile" was archived and saved every 3 to 5 wk throughout the study to capture individual cow data throughout the study period. Data from cowfile archives were transferred into a computer spreadsheet program (Microsoft Excel 2002, Microsoft Corporation, Redmond, WA) for organization and manipulation of data before statistical analysis using SAS (SAS Inst. Inc., Cary, NC).
Submission of Cows for First Postpartum TAI Service: Presynch + Ovsynch
Lactating cows (n = 711) were allocated weekly to breeding groups, each of which included cows that had calved within a given calendar week (12.7 ± 0.7 cows/group; range = 3 to 23 cows/group). In this way, cows were managed in groups to receive hormone injections and first postpartum TAI on 2 preselected days of the week (Tuesdays and Thursdays). All cows received a hormonal synchronization protocol (Presynch + Ovsynch) to receive first postpartum TAI as follows: 25 mg of PGF2 (d 32 ± 3 and 46 ± 3; Presynch), 100 µg GnRH (d 60 ± 3), 25 mg PGF2 (d 67 ± 3), and 100 µg GnRH (d 69 ± 3) postpartum (Ovsynch). All cows received TAI immediately after administration of the second GnRH injection of the Ovsynch protocol (d 0). Thus, the average DIM at first postpartum TAI for all cows enrolled in this experiment was 69 ± 3 d.
Submission of Cows for Second Postpartum TAI Service: Resynch
At first TAI service, cows were randomly assigned to each of three treatment groups for resynchronization of ovulation [100 µg of GnRH (d -9); 25 mg of PGF2 (d -2) and 100 µg of GnRH + TAI (d -0)] to induce a second TAI for cows failing to conceive to Ovsynch (i.e., Resynch). Within a breeding group, cows were blocked by parity (primiparous vs. multiparous) as part of the randomization procedure to minimize confounding of this variable among treatment groups. All cows (n = 235) in the first group (D19) received a GnRH injection (100 µg) 19 d after Ovsynch TAI, a PGF2 injection (25 mg) at a nonpregnancy diagnosis using transrectal ultrasound 26 d after Ovsynch TAI, and a second GnRH injection (100 µg) and Resynch TAI 28 d after Ovsynch TAI. Cows (n = 240) in the second group (D26) received a GnRH injection (100 µg) at a nonpregnancy diagnosis using transrectal ultrasound 26 d after Ovsynch TAI, a PGF2 injection (25 mg) 33 d after Ovsynch TAI, and a second GnRH injection (100 µg) and Resynch TAI 35 d after Ovsynch TAI. Cows (n = 236) in the third group (D33) received a GnRH injection (100 µg) at a nonpregnancy diagnosis using transrectal ultrasound 33 d after Ovsynch TAI, a PGF2 injection (25 mg) 40 d after Ovsynch TAI, and a second GnRH injection (100 µg) and Resynch TAI 42 d after Ovsynch TAI.
Submission of Cows for Third and Greater Postpartum TAI Service
Throughout the study, cows that failed to conceive to a given TAI service were resynchronized until they became pregnant or were culled from the herd. Because of the potential detrimental effects of GnRH administered to pregnant cows receiving bST reported by Moreira et al. (2000b), cows in the D19 group that failed to conceive to second TAI service (e.g., Resynch) were resynchronized using the D26 Resynch schedule. Thus, D19 cows did not receive a GnRH injection 19 d after the first Resynch TAI, but received an injection of GnRH after a nonpregnancy diagnosis 26 d after the first Resynch TAI. For the third TAI service, cows in the D26 and D33 groups were resynchronized according to the resynchronization schedule for each treatment group. After third TAI service nonpregnant cows in all 3 treatment groups were resynchronized for subsequent TAI services similar to the D26 treatment group. As time progressed, treatment intervals after the first Resynch TAI service were unintentionally extended due to missed injections and failure to locate specific cows on injection days, or intentionally due to injury or illness.
Transrectal Ultrasonography
All ultrasound examinations and hormone injections were conducted by the herd veterinarian, who is also an ownership partner of the dairy, and who had managed reproduction in dairy cattle using transrectal ultrasonography for the previous 7 yr. Pregnancy examinations and hormone injections were conducted immediately after milking by restraining cows in a palpation rail located in the return alley exiting the milking parlor. Visualization of a fluid-filled uterine horn and the presence of a conceptus were used as positive indicators of pregnancy 26 d after TAI using an ultrasound machine equipped with a transrectal 7.5 MHz linear-array transducer (Aloka 500V; Corometrics Medical Systems, Inc., Wallingford, CT; Fricke et al., 1998). The number of cows diagnosed pregnant to TAI expressed as a percentage of cows within that treatment group receiving TAI was defined as the PR/AI. In addition, both ovaries of each cow were visualized at each pregnancy exam using ultrasound and the presence or absence of a CL was recorded. A cow was recorded as having a CL when the CL diameter was estimated to be 
10 mm. This definition of a CL was adopted because it allowed for rapid and accurate evaluation of CL size using the 10-mm hash marks on the ultrasound screen without repeated freezing of the ultrasound image during weekly herd checks.
Pregnancy loss was assessed for cows that conceived to first TAI service during the data collection period. Cows diagnosed pregnant were scheduled for a pregnancy recheck using transrectal ultrasound on d 68 of gestation. A cow that was pregnant at the first pregnancy exam but in which the fetus was dead (assessed by lack of a fetal heartbeat) was considered pregnant and the loss was recorded as occurring on that day. Cows with questionable fetal viability were rechecked 1 wk later. Pregnancy loss for cows diagnosed pregnant to Ovsynch was calculated as the number of pregnant cows that lost a pregnancy by d 68 of gestation expressed as a percentage of cows diagnosed pregnant at 26 (D19 and D26 cows) or 33 d (D33 cows) after Ovsynch TAI.
Temperature Data
Official temperature data (Midwestern Climate Center, Champaign, IL) reported at a research station located within 20 km of the farm (Medford, WI; Station ID: 475255; latitude: 45° 07', longitude: 90° 20') were collected retrospectively. Temperature data were collected for days that TAI was performed for each breeding group throughout the experiment. The high and low temperatures on the day of TAI were recorded, and the high temperature was used to evaluate the effect of temperature at TAI on conception rate for cows within a weekly breeding group.
Statistical Analyses
Procedure LOGISTIC of SAS (SAS Inst. Inc.) was used to analyze the effect of treatment on PR/AI. The model included treatment, parity, and the two-way interaction, with high temperature at TAI as a regression variable. The effect of AI sire was not included in this model, but sires were distributed evenly among treatments. Within each treatment group, the effect of the presence or absence of a CL on PR/AI was tested by chi-square analysis using the Cochran-Mantel-Haenszel statistic of SAS.
Survival analysis (i.e., failure time analysis), of cows in the D26 and D33 groups was compared across the first three TAI services using a Weibull proportional hazards model. Number of days to pregnancy across the first three TAI services was calculated using the hazard function. This function reflects the instantaneous probability (risk) of an individual cow becoming pregnant at time t and was modeled as:
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where
| hijk(t) | = | hazard function of a given cow at time t;
| h0(t,  ,  ) | = | Weibull baseline hazard function with scale parameter and shape parameter (these parameters were estimated from the data);
| Ti | = | time-independent effect of treatment;
| Lj | = | time-independent effect of parity;
| Sk(t) | = | time-dependent effect of season at TAI, assumed to be piecewise constant with change points at each TAI for a given cow.
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Four seasons per year were defined, and their change points occurred on March 21, June 21, September 21, and December 21 of each calendar year during the experiment. Observations from cows that became pregnant during the study period were considered as uncensored. Observations from cows that were sold before becoming pregnant, cows that failed to become pregnant by the end of the study period, and cows that experienced embryonic loss after a positive pregnancy diagnosis were considered as censored. For cows that experienced embryonic loss, the date of the last TAI service was used as the time of censoring. The Survival Kit Version 3.12, a set of Fortran programs written by Ducrocq and Sölkner (1998), was used for the survival analysis. Details regarding the algorithms for estimation are given by Ducrocq (1994), and theoretical aspects are discussed by Ducrocq and Casella (1996).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Initiation times for Resynch for each of the 3 treatment groups in this study were chosen to occur on Tuesdays so that injection schedules would remain consistent for all cows assigned to weekly breeding groups at any given time. To adhere to the Tuesday/Thursday schedule, all pregnancy examinations were conducted on Tuesdays. To fit the reproductive management system, the first pregnancy examination using transrectal ultrasound was conducted 26 d after TAI for the D19 and D26 cows, and 33 d after TAI for the D33 cows. Under most on-farm conditions, pregnancy diagnosis can be rapidly and accurately diagnosed using transrectal ultrasound as early as 26 d post AI (Kastelic et al., 1991; Filteau and DesCôteaux, 1998). Sensitivity and specificity of pregnancy diagnosis using ultrasound was 45 and 82%, respectively, when conducted between 21 and 25 d post AI but increased to 98 and 88%, respectively, when conducted between 26 and 33 d post AI (Pieterse et al., 1990a). Thus, the reproductive management systems assessed in this trial allow for administration of all hormone injections, Ovsynch and Resynch TAI services, and pregnancy examinations to be restricted regularly to either Tuesdays or Thursdays.
Pregnancy Rate per Artificial Insemination After First TAI Service: Ovsynch
Inherent to the experimental design, first assessment of pregnancy status was not conducted at the same interval after the Ovsynch TAI among the three treatment groups. Pregnancy status after the Ovsynch TAI was first assessed 26 d after TAI for cows in the D19 and D26 groups, whereas pregnancy status was assessed 33 d post Ovsynch TAI for cows in the D33 group (Table 1
). Overall PR/AI to Ovsynch was 40% (286/711) and was greater for D19 and D26 cows than for D33 cows (Table 1). This difference is likely due to a greater period in which embryonic loss can occur in the D33 cows due to the increased interval from TAI to pregnancy diagnosis (26 vs. 33 d). When pregnancy status was reassessed for all treatment groups at 68 d after Ovsynch TAI, overall PR/AI to Ovsynch was 31% (219/711) and did not differ among treatments (Table 1). Thus, differences in PR/AI at the first pregnancy exam and pregnancy losses between the first and second pregnancy exams among treatment groups likely represent an artifact of time of assessment of pregnancy status after TAI inherent to the experimental design rather than to treatment differences.
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injection of Presynch and the first GnRH injection of the Ovsynch protocol. Although this study was not designed to evaluate the effect of presynchronization on PR/AI, the 8-percentage-unit difference in overall PR/AI to the Presynch/Ovsynch versus first Resynch TAI (Table 1
vs. Table 3) in the present study may reflect the increased PR/AI to TAI due to presynchronization reported previously (Moreira et al., 2001; El-Zarkouny et al., 2002; Navanukraw et al., 2002).
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). Although not a direct comparison, the numbers of pregnancies lost in each treatment group almost fully accounted for differences in PR/AI to the first pregnancy exam after the Ovsynch TAI among treatment groups (Table 1). Because cows diagnosed pregnant at an early ultrasound exam have a greater risk of early embryonic loss, these cows must undergo subsequent pregnancy examinations to identify and rebreed cows that experience such loss (Fricke, 2002). If left unidentified, cows experiencing embryonic loss after an early pregnancy diagnosis would actually reduce reproductive efficiency by extending intervals from calving to conception.
Of cows diagnosed pregnant at 28 d post TAI, 10 to 16% experience embryonic loss by 56 d after TAI (Mee et al., 1994; Vasconcelos et al., 1997; Fricke et al., 1998). Although the magnitude of embryonic loss in this study is greater that that reported in previous studies, the period over which loss was assessed beginning earlier in gestation (26 to 33 d). In a previous study (Vasconcelos et al., 1997), pregnancy loss in lactating dairy cows was 11% from 28 to 42 d, 6% from 42 to 56 d, and 2% from 56 to 98 d post AI, suggesting that losses are highest early and subsequently decrease as gestation ensues. In the present study, at least 46% of cows in the D19 group were pregnant at the time of GnRH administration (Table 1), whereas GnRH was administered only to cows after a nonpregnancy diagnosis in the D26 group. Both PR/AI and embryonic losses were similar between these treatment groups (Table 1), suggesting that no iatrogenic embryonic loss occurred in pregnant cows receiving GnRH in the D19 group from d 26 to 68 of gestation. Although the present study does not include the appropriate control groups to substantiate or refute the observation of Moreiera et al. (2000b), a study reported during the course of this experiment in which GnRH was administered to cows 21 d after AI before a pregnancy examination 28 d after AI showed no negative effect of GnRH on conception rate or embryonic losses compared to untreated cows (Chebel et al., 2002). Collectively, these data support that GnRH does not induce iatrogenic embryonic loss when administered to pregnant cows.
Pregnancy Rate per Artificial Insemination After Second TAI Service: Resynch
A total of 41 cows diagnosed nonpregnant to Ovsynch were not enrolled for Resynch for second TAI service for various reasons (Table 2). Only 5.6% of cows failing to conceive to Ovsynch were visually detected in estrus and inseminated (Table 2), underscoring the reliance of this farm on TAI for inseminating cows. Overall PR/AI to Resynch was 32% and was greater for D26 and D33 cows than for D19 cows (Table 3). Although PR/AI to TAI protocols can vary widely among farms, PR/AI after Resynch for the D26 and D33 cows in the present study is similar to conception rate after Ovsynch reported previously (Pursley et al., 1995; Fricke et al., 1998; Jobst et al., 2000).
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). This occurred in spite of the longer interval from Resynch TAI to pregnancy diagnosis in the D33 group (33.7 d) compared to the D26 group (26.6 d) and in addition to the expected increased incidence of embryonic loss among treatment groups similar to that observed for PR/AI to Ovsynch.
Progesterone status or the presence or absence of a CL at Resynch initiation, both of which are related to stage of the estrous cycle also can affect PR/AI. Although progesterone status during the Resynch protocol was not assessed in the present study, presence or absence of a CL at pregnancy examinations was recorded. Although these data are limited in the present study because prostaglandin responsiveness and presence or absence of ovarian follicles was not known, these data are pertinent because this information can be collected via transrectal ultrasonography or rectal palpation by a bovine practitioner in a commercial setting. In the present study, PR/AI was nearly triple (P < 0.05) for D19 cows with a CL (28%) than for cows without a CL (10%) at the PGF2 injection of Resynch (Table 4). Nearly one-fourth of D19 cows lacked a CL at PGF2 administration suggesting that induction of ovulation by the first GnRH injection of Resynch may have been poor, because all of those cows received GnRH 7 d before the pregnancy diagnosis. Similarly, PR/AI to Resynch tended to be greater (P = 0.09) for D26 + D33 cows with a CL (39%) than for cows without a CL (29%) at the first GnRH injection of Resynch (Table 4). Based on previous reports (Vasconcelos et al., 1999; Moreira et al., 2000a), these cows may have been at an unfavorable stage of the estrous cycle when Resynch was initiated. Interestingly, presence or absence of a CL at Resynch initiation did not affect PR/AI in the D26 group. Perhaps ovulatory response to the first GnRH injection was high in this group because they were in the early stage of the estrous cycle when a dominant follicle with ovulatory capacity is present (Vasconcelos et al., 1999). Further experiments are required to define the physiology that determines PR/AI after initiation of Resynch at various intervals post TAI.
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), this difference was not statistically significant. Thus, the 7-d advantage of the D26 group over the D33 group was negated by the increased rate of embryonic loss observed in the D26 group due to the earlier pregnancy exam and the longer period in which embryonic loss could occur.
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). In addition, retrospective analysis of official temperature data showed a relationship between monthly PR/AI to Ovsynch and Resynch TAI versus mean high daily temperature at TAI for the breeding groups (Table 6 and Figure 1). For Ovsynch, a 1°C increase in mean daily high temperature at TAI resulted in a 2-percentage-unit decrease in PR/AI, whereas for the first Resynch, a 1°C increase in mean daily high temperature at TAI resulted in a 3-percentage-unit decrease in PR/AI to TAI (Table 6). Although heat stress has been well documented in Southern US states (Hansen et al., 1992), these observations highlight the impact of heat stress on PR/AI in lactating cows in Northern US states during seasons and months when ambient temperatures are high.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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at a nonpregnancy diagnosis via rectal palpation conducted 1 wk later. Alternatively, veterinarians who have incorporated transrectal ultrasound for reproductive management could adopt the D26 Resynch strategy as reported herein, or alternatively, could administer GnRH to all cows regardless of their pregnancy status 26 d post TAI. Cows would then receive PGF2 at a nonpregnancy diagnosis via transrectal ultrasound conducted 1 wk later. Adoption of systematic Ovsynch/Resynch strategies by large dairy operations may provide a research model to investigate important aspects of reproductive biology in lactating dairy cows. For example, systematic resynchrony effectively eliminates the vagaries of low estrous detection rates when trying to identify factors associated with differences between cows that influence success or failure to initiate pregnancy. Using records from large herds adopting systematic resynchrony systems, individual cows that lose or retain pregnancies could be coupled with health, production, and environmental information obtained for the same cows at resynchrony preceding subsequent inseminations. Future studies may be developed to answer these as well as other questions regarding reproductive events in dairy cattle in herds using systematic synchronization and resynchronization systems.
Although use of ultrasound is not required to implement either the D26 or D33 Resynch protocols, use of ultrasound for reproductive management of lactating dairy cows is not without benefit (Fricke, 2002). The observation that cows without a CL at initiation of Resynch had poor conception rates provides a management opportunity to detect such cows and employ an alternative treatment strategy. Because specificity and sensitivity of detection of ovarian structures is greater for ultrasound than for rectal palpation (Pieterse et al., 1990b), use of ultrasound may be beneficial if such a differential management strategy is developed. Finally, it is possible that the optimal resynchronization strategy for this herd may not perform optimally in other herds. A difference in populations with varying proportions of cows exhibiting two or three follicular waves per cycle has been suggested to impact conception rate to Ovsynch (Cordoba and Fricke, 2002). Further research is needed to further develop successful resynchronization strategies for managing reproduction in lactating dairy cows.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication May 30, 2003. Accepted for publication August 18, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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, gonadotropin-releasing hormone, and timed artificial insemination. J. Dairy Sci. 83:2366–2372.[Abstract]
and GnRH. Theriogenology 44:915–923.
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