New Strategies to Increase Pregnancy Rates

 

W.W. Thatcher, F. Moreira and C.A. Risco

Department of Animal Sciences and Large Animal Clinical Sciences

University of Florida, Gainesville

 

Introduction

The goal of maintaining herd pregnancy rates in current production systems is a challenge due to large herds, confinement systems that are not necessarily conducive to heat detection, cow identification, and the challenge of implementing nutritional management systems that meet individual cow requirements during both the transition and lactation periods that ultimately impact reproductive performance.  Reproductive management is further compromised by the seasonal period of heat stress that reduces both heat detection rates and pregnancy rates to inseminations at detected heats.  Under our current production systems, the heat detection rate in high-producing cows is 50% at best.  Programs in the Department of Animal Sciences have strived to optimize the efficiency of dairy production systems.  Objectives of this presentation are to update dairy producers with several new strategies to increase herd reproductive performance, and to emphasize that use of these new practices has to be founded on an understanding of the technology and its integration with good nutritional management.

Principles and Limitations of Implementing a Timed Insemination Program

Herd pregnancy rate is the product of heat detection rates and pregnancy rates to inseminations at detected heats (conception rate).   Intensive research investigations have focused on trying to optimize these two biological components.  One strategy to control heat detection rates is to control precisely the time of ovulation so that all cows can be inseminated at a fixed time, which is equivalent to a 100% heat detection rate.  If our ability to precisely control ovulation time can be achieved in all cows and pregnancy rate to a timed insemination is normal, then a major advancement in reproductive management will have occurred.  At the present time, there are only two drugs available to dairy producers for use in lactating dairy cows.  Prostaglandin F_2a (PGF_2a) drugs (e.g., Lutalyse, Pharmacia Upjohn) can be used effectively to regress the corpus luteum (CL) but are ineffective on CL that are developing on days 1 to 5 of the estrous cycle.  The other major drug is the Gonadotrophin Releasing Hormone (GnRH) like drugs (e.g., Cystorelin, Merial Co.) that release both LH and FSH from the pituitary of the cow.  GnRH has the ability to ovulate a mature follicle to form a CL, and induce recruitment of a new follicle.  Research at the University of Florida demonstrated that injection of GnRH will recruit development of a new dominant follicle which will induce the cow to express estrus when PGF_2a is injected 7 days later (Macmillan and Thatcher, 1991).  Investigators at the University of Wisconsin demonstrated that an additional treatment with GnRH after injection of PGF_2a would induce a timed ovulation (Pursley et al., 1995).  This procedure is known as the Ovsynch Program because it synchronizes ovulation and permits a timed insemination (Pursley et al., 1997).  Pioneering studies at the University of Wisconsin and University of Florida demonstrated that in lactating dairy cows pregnancy rates are normal following a timed insemination (see review by Risco et al., 1999).  However, the challenge is to further optimize this system based on an understanding of how the system works and recognizing the physiological constraints that limit performance of the system.

 

The Ovsynch/TAI protocol is idealized in Figure 1.  In this example, concentrations of progesterone are monitored to document presence of a CL (the CL produces progesterone), and an idealized description of follicle development is presented.  In this example, the cow is injected with GnRH (Monday, 5:00 PM) on day 5 of the estrous cycle.  At this time, the cow has a dominant and healthy follicle that ovulates in response to the GnRH-induced release of LH; furthermore, the increase in FSH induced by the GnRH injection induces recruitment of a new pool of follicles in approximately 2 days (day 7) and one of the follicles is selected to become the dominant follicle (Moreira et al., 2000b).  On day 12 of the cycle (7 days after the injection of GnRH), PGF_2a is injected (e.g., Monday, 5:00 PM) to regress both the original CL present at day 5 of the cycle and a newly formed CL that was induced by the injection of GnRH on day 5 of the cycle.  The decrease in progesterone associated with regression of the CL accelerates growth of the newly recruited dominant follicle and a second injection of GnRH is made 2 days after the injection of PGF_2a (e.g., Wednesday, 5:00 PM).  The second injection of GnRH induces ovulation 24 to 32 hours later (Pursley et al., 1995).  Knowing that ovulation will occur at this time, the timed insemination is given at approximately 16 hours after the injection of GnRH  (e.g., 9:00 AM  on Thursday).  This permits sufficient time for sperm to develop the capacity to fertilize the egg so that when it ovulates, a fertile population of sperm is present to carry out fertilization or initiate a pregnancy.  This is an idealized scenario and the timing of injections is considered critical to the success of the program.  If an interval of less then 7 days is used between GnRH and PGF_2a injection, the ability to effectively regress a newly developed CL is reduced.  If the second injection of GnRH is delayed to longer than 48 hours or 2 days, then more cows are detected in heat prior to injection of GnRH, cows become asynchronized, and the timing of insemination is not correct.  With our present availability of drugs for lactating dairy cows, it is essential that producers not alter the protocol.  One commonly asked question is, can cows be inseminated at the time of the GnRH injection or at 24 h after the injection of GnRH to make the insemination process more convenient?  Pregnancy rates will be lowered at the 24 h insemination, and optimal insemination times appear to be between 12-18 hours after the GnRH injection (Pursley et al., 1998). 

Success of the program is dependent on whether lactating dairy cows are cycling as well as stage of the estrous cycle at the time the Ovsynch/TAI protocol is initiated in cycling cows. Clearly, if cows in the group are not cycling then it is a given that pregnancy rates of the group will be somewhat less even though the Ovsynch/TAI protocol itself may induce some cows to begin to cycle and perhaps conceive.  This will be addressed later in the paper. Figures 2 and 3 provide examples as to specific stages of the estrous cycle that lead to lower pregnancy rates when the Ovsynch/TAI protocol is initiated.  In Figure 2, a

 

cow initiates the program at day 15 of the estrous cycle when a normal second wave follicle is under development.  This follicle may or may not ovulate depending upon how mature it is.  In many instances the second wave follicle is too small to ovulate and a new CL does not develop.  At 2 to 4 days after the injection of GnRH, the cow spontaneously induces regression of the CL by releasing PGF_2a from the uterus.  Thus, at the time of the PGF_2a injection given 7 days after the GnRH injection, the cow has already regressed the CL and may even be in heat (Moreira et al., 2000b).  Such cows will be asynchronized in that they will ovulate prematurely and, if we follow the protocol, the insemination is made too late and the cow does not conceive.   A second problematic stage of the cycle is in the early phases of the estrous cycle (e.g., day 2) as demonstrated in Figure 3.  In this

 

scenario, the cow already has been in heat, ovulated and is recruiting a new dominant follicle.  This is a small follicle and the injection of GnRH on day 2 does not alter development of the dominant follicle and recruit a new dominant follicle (Moreira et al., 2000b).  As a consequence, at the time of the second GnRH injection, the dominant follicle is considered aged and has expressed dominance for 5 days or longer.  Follicles that have periods of dominance longer than 5 days (Austin et al., 1999) or cows that initiate the Ovsynch/TAI program in early stages of the estrous cycle are less fertile (Moreira et al., 2000b).  Follicles may ovulate but oocytes are less fertile, or some cows may fail to ovulate their follicle in response to the second injection of GnRH. We can project what the success rate of the Ovsynch/TAI program will be in an idealized situation in which all cows are cycling and are at random stages of the estrous cycle when the program is initiated (Table 1.).  Assuming a 20-day estrous cycle, we would expect 5% of the cows to be at each day of the estrous cycle. Thus, for a group of 100 cows, the percent of cows in early stages of the cycle (problematic), early diestrus, late diestrus (problematic) and proestrus are depicted in Table 1 with expected pregnancy rates at each stage for the reasons described above.  An expected overall pregnancy rate of 36% is anticipated.   However, it is possible to manipulate the estrous cycle of cows such that they are in the ideal stage of the estrous cycle when the Ovsynch/TAI program is initiated.  One idealized scenario is to inject all cows twice with PGF_2a at an interval of 14 days and to initiate the first GnRH injection of the Ovsynch/TAI program on day 12 after the second injection of PGF_2a.  Such an idealized manipulation is demonstrated in Table 2.  If all cows were cycling, we would expect 90% of the cows to be in the ideal stage of the estrous cycle, between 5 to 10 days, when the Ovsynch/TAI program is initiated 12 days after the second injection of PGF_2a.  With this scenario, an expected

pregnancy rate to the Ovsynch/TAI program is 48%.  Such a proposed treatment program prior to implementation of the Ovsynch/TAI program is called pre-synchronization with a standard protocol that is practiced in the industry.   Effectiveness of such a program will be described later.  However, it is imperative that the producer and veterinarian have a thorough understanding of the principles of ovarian manipulation in order to understand how the system functions when they make herd management decisions as to how to implement the program.

Impact of Body Condition Score On Pregnancy Rates To The Ovsynch/TAI Program

There is the perception that pregnancy rates are lower in lactating dairy cows with poor body condition.  Retrospective analyses of our field experiments indicate that as body condition score increases pregnancy rate increases to the Ovsynch/TAI program (Burke et al., 1996).  We recently completed an experiment that examined pregnancy rates of the Ovsynch/TAI program in cows that had Body Condition Scores (BCS) of <2.5 versus >2.5 (Moreira et al., 2000d).  Pregnancy rates at days 27 and 45 after insemination were 18.1% and 11.1% for cows with a low BCS (81 cows) versus higher rates of 33.8% and 25.6% for cows with BCS >2.5.  The proportion of cows conceiving to the first synchronized service was lower for the cows in low body condition, and this was a temporary decrease since rates of cumulative pregnancies during the ensuing 120 days

 

postpartum were similar (Figure 4).  This demonstrates the importance of optimizing fertility to the first service.  Utilizing the differences in pregnancy rates in cows with body condition scores <2.5 versus >2.5, dynamic modeling was used to estimate net revenue per cow per year when considering what percentage of the herd had a low BCS of <2.5 (Figure 5).  The difference in net revenue was $10.33 per cow per year as to whether 10% versus 30% of the herd had low body condition scores at the time the Ovsynch/TAI program was initiated (Moreira et al., 2000d).  Thus, it is essential that producers try to nutritionally manage the dynamics of body condition postpartum to optimize fertility rates.   Why does a low body condition score result in a lower pregnancy rate to the Ovsynch/TAI program?  Is the rate of anestrus (non-cycling cows) responsible for the poor pregnancy rates or are pregnancy rates in cows cycling reduced due to defective eggs and/or the reproductive tract is unable to maintain a pregnancy?  These are researchable questions that warrant further investigation and some further insight on this issue is described later in the paper.

 

Optimization Of Reproductive Performance With Pre-Synchronization Implemented Prior To The Ovsynch/TAI Program and Use of Bovine Somatotropin

Of concern to dairy producers is whether Bovine Somatotropin (bST; Posilac, Monsanto, St. Louis, MO; 500 mg) treatment can be initiated in the ninth week of lactation and be continued without compromising reproductive performance.  Our previous research findings (Moreira et al., 2000c) indicated that first service pregnancy rates to the Ovsynch/TAI protocol were increased when cows received bST treatment.  Furthermore, the same experiment provided no evidence that bST treatment had any detrimental effect on subsequent services and reproductive performance at 120 or 305 days postpartum. Treatment with bST was initiated at day 63 postpartum concurrently with first injection of GnRH given as part of the Ovsynch/TAI program.  An additional challenge is to document whether pregnancy rates to the Ovsynch/TAI program can be improved with prior implementation of a pre-synchronization program.  A field trial was conducted with the objectives of: determining whether pre-synchronization of lactating cows prior to the initiation of the Ovsynch/TAI program would improve pregnancy rates; to verify prior results indicating that bST increased pregnancy rates to the Ovsynch/TAI program; to determine whether the possible beneficial effect of bST on pregnancy rates occurred prior to or after timed artificial insemination.  Development and implementation of the Ovsynch/TAI program has provided investigators a means to methodically test various factors that may improve pregnancy rates.  Measuring the impact of any therapy or management system on pregnancy rates is a challenge because the experimental response is pregnancy rate in which a considerable number of cows are needed to detect potential differences, and the investigator has to cope with numerous management factors.  At least with the Ovsynch/TAI program, the management errors associated with heat detection are eliminated and the precise timing of insemination can be controlled tightly.

Experimental design and treatment of lactating dairy cows:

The experimental design for pre-synchronization is depicted in Figure 6.  A total of 543 cows were assigned randomly to the experiment in which half of the cows received the pre-synchronization program.  The pre-synchronization program was initiated on a

weekly basis such that cows 34 to 40 days postpartum (37+ 3 days) received an injection of PGF_2a (Lutalyse, Pharmacia-Upjohn Co.; 25 mg; i.m.) and this was followed 14 days later (51 +3 days) with a second PGF_2a injection.  In contrast, control cows (no pre-synchronization) did not receive the two injections of PGF_2a.  The rationale for the pre-synchronization program is described above and potential impact on pregnancy rates presented in Table 2.  On day 63 + 3 days, the first injection of GnRH of the Ovsynch/TAI program was initiated, and this was 12 days after the second injection of PGF_2a of the pre-synchronization program.  The pre-synchronization program will place cows between days 5 to 10 of the cycle at the time of the GnRH injection depending upon what day they expressed estrus after the second injection of PGF_2a (Figure 7).  Days 5 to 10 of the cycle are considered by us as an optimal time to begin the Ovsynch/TAI program as discussed above. Following the protocols depicted in Figure 6, all cows will be time inseminated at day 73 + 3 days postpartum.  The other factors tested in this experiment are the initiation of bST treatment at day 63 (time of the GnRH injection of the Ovsynch/TAI program), day 73 (time of artificial insemination as part of the Ovsynch/TAI program) or bST-control in which first injection of bST was not given until 147 days of lactation (well after first and second services).  With this experimental design, the array of six treatment groups and number of cows in each group are shown in Figure 8.  A series of blood samples were collected on days 51, 63, 70 72 and 79.  Relative comparisons of progesterone concentrations in plasma allow us to determine if

cows are cycling (samples on days 51 and 63), stage of the cycle at the beginning of the Ovsynch/TAI program (samples on days 63 and 70), whether CL regression was successfully induced (samples on days 70 and 72) and whether cows had a synchronized ovulation (samples on days 72 and 79).  All cows were examined by ultrasonography for pregnancy on day 32 after timed insemination and pregnant cows re-examined for pregnancy by rectal palpation on day 74 after timed insemination.  This allowed us to characterize fetal losses between 32 and 74 days of pregnancy.  All cows diagnosed open at day 32 after the first timed insemination were injected with GnRH and the Ovsynch/TAI program repeated with second insemination occurring at 115 days of lactation.

Impact of Anestrous Cows:

With our ability to measure plasma progesterone in two plasma samples collected 12 days apart (on days 51 and 63 postpartum), it is possible to identify exactly what cows are anestrus when the Ovsynch/TAI program is initiated.  If cows had progesterone <1ng/ml in both samples they were considered to be anestrus.  It was important for us to determine which cows are cycling since pre-synchronization treatments and potential effects of bST on pregnancy rates will not occur in cows that are not cycling.  Furthermore, this assessment of anestrous status will allow us to document the frequency of this condition and its impact on reproductive performance of the herd.

For the assessment of anestrous status, 499 cows had blood samples collected on both days 51 and 63 postpartum.  It is interesting that overall 23.4% of the cows were anestrus or had not started to cycle by 63 days postpartum (Table 3).  Not surprising is the observation that the frequency of anestrus was greater for primiparous or first-calf heifers than multiparous cows (two lactations or more).

The frequency of anestrus also was associated with body condition score as shown in Figure 9.  The occurrence of anestrus decreased as body condition scores recorded at initiation of the Ovsynch/TAI improved.  Therefore, body condition may be used to estimate the relative nutritional status of lactating dairy cows, and its impact on the frequency of anestrous cows at initiation of a reproductive management system. 

Figure 9.  Relationship between body condition scores and frequency of cows in anestrus at 63 days postpartum.

Body condition scores could only account for 7.8% of the variation in occurrence of anestrus.  Thus, body condition score is not an absolute predictor of what cows are cycling.  Some cows with body condition scores of 3.0 were anestrus. As anticipated, anestrous cows did not perform as well as cyclic cows in terms of pregnancy rates to the first-service Ovsynch/TAI protocol.  Pregnancy rate at 74 days after insemination was   only 22.4% for anestrous cows, which was lower than the 41.7% pregnancy rate at 74 days after insemination for cyclic cows.  Since cows in anestrus do not respond to injections of PGF_2a, pre-synchronization did not affect pregnancy rates of anestrous cows.  Also, pregnancy rates of anestrous cows following first-service Ovsynch/TAI were not affected by administration of bST.  Collectively, these results indicate that use of advanced technology to tightly control the reproductive cycle and stimulate embryonic development to increase pregnancy rates may not produce the expected results if there is a high frequency of anestrous cows at breeding.  Therefore, postpartum management of lactating dairy cows is of extreme importance and will greatly affect reproductive performance.  Efforts to maximize cow health, comfort, and nutritional status following parturition (e.g., enhance dry matter intake) will be reflected later in the lactation in terms of a higher incidence of cycling cows and improved reproductive performance.

Reproductive performance of  Cyclic Cows:

Since anestrus had such a highly significant affect on pregnancy rates, reproductive performance was examined only in cyclic cows. First-service pregnancy rates to the Ovsynch/TAI protocol were affected by both bST pre-synchronization and bST treatments (Figure 10).  Cows initiating bST treatment at 63 or at 73 days postpartum had increased pregnancy rates compared to controls among cows not pre-synchronized and also among cows pre-synchronized.  The fact that a similar stimulation in pregnancy rates was observed in cows treated with bST at 63 (day of first GnRH injection) and at 73 days postpartum (day of timed insemination) indicates that bST is probably enhancing embryonic development and survival following insemination.  Concentrations of bST are elevated throughout the 14-day period between injections such that bST injection at the time of the first GnRH injection will still elevate concentrations of bST 10 days later when the cow was inseminated.  Since pregnancy rate was elevated when the first injection of bST was delayed to the time of insemination, the effects of bST appear to be targeted on the reproductive tract or on the embryo directly to enhance embryo survival.

Moreover, increased pregnancy rates were detected when cows were pre-synchronized (Figure 10; gray bars; 52.3%) compared to cows not pre-synchronized (white bars; 31.1%).  An additional comparison is the effect of pre-synchronization in the two groups

Figure 10.  First-service pregnancy rates to the Ovsynch/TAI protocol for cyclic cows (n = 375; LSM + SE).

that did not receive bST in which pre-synchronized cows had a 42.6% pregnancy rate compared to 25.3% for the control group (Figure 10).  This difference associated with pre-synchronization (17.3%) approximates the predicted differences (12.0%) that were estimated in Tables 1 and 2. 

The reason for increased pregnancy rates to the Ovsynch/TAI protocol in cows pre-synchronized was related to the frequency of cows initiating the synchronization protocol at favorable stages of the estrous cycle.  As indicated above, it was hypothesized that pregnancy rates to the Ovsynch/TAI protocol would be increased if cows received the first GnRH injection between days 5 to 10 of the cycle, and that pre-synchronization would synchronize approximately 90% of the cycling cows such that these cows would be between days 5 and 10 of the cycle when the Ovsynch/TAI protocol was initiated.  By collecting blood samples at the first injection of GnRH (at day 63) and again when PGF_2a was injected (at day 73), we were able to indirectly identify cows that initiated the synchronization program at the early luteal phases of the estrous cycle (e.g., between days 5 to 10 of the cycle).  Cows with high plasma progesterone (> 1.0 ng/ml) at both day 63 and day 73 (i.e., HIGH-HIGH cows) probably initiated the Ovsynch/TAI protocol at the optimal stage of the estrous cycle.  Results from the frequency of cows classified as HIGH-HIGH indicated that approximately 87.4% of pre-synchronized cows were classified as HIGH-HIGH versus only 71.7% of cows not pre-synchronized were HIGH-HIGH cows (Figure 11). Therefore, we were successful in programming the cows to be

Figure 11.  Frequency of cyclic cows classified as HIGH-HIGH among treatment groups.  More cows pre-synchronized were classified as HIGH-HIGH than cows not pre-synchronized.

in the optimal stage of the cycle to begin the Ovsynch/TAI program. As a consequence, pre-synchronization increased first-service pregnancy rates by enhancing the rate of synchronized ovulations and this increased the percentage of inseminated cows to respond to bST treatments.

Results from this experiment indicate for the second time that bST increased first-service pregnancy rates to the Ovsynch/TAI protocol.  Such an observation impels us to review previous reports of decreased reproductive performance in cows receiving bST (Cole et al., 1992; Collier et al., 1997) and find explanations for such a discrepancy.  It has been reported that use of bST may reduce the rate of estrous detection (Kirby et al., 1997) that may reduce reproductive performance of lactating cows.  However, when estrous detection was eliminated though the use of the Ovsynch/TAI protocol, the inefficiency of heat detection possibly associated with bST may have been eliminated.  We surely found no evidence that bST had any detrimental effect on reproductive performance in our studies.  In contrast, pregnancy rates were increased at first service to timed inseminations as part of an Ovsynch/TAI program.

Since bST increased pregnancy rates to the timed insemination, several possible mechanisms may contribute to this effect and they appear to occur after timed insemination.  Thus, a model for the possible effects of bST is depicted in Figure 12.

 

Figure 12.  Model for the possible effects of bST on fertility of lactating cows.

The bST may be affecting final development of the ovulatory follicle after insemination (circle 1).  Previous research has indicated that maturation of the oocyte in vitro is enhanced by both bST and IGF-I (Izadyar et al., 1996; Izadyar et al., 1997).  Furthermore, bST also may be increasing plasma progesterone following insemination (circle 2) as demonstrated previously (Lucy et al., 1994).  Increased plasma progesterone after insemination has been associated with greater pregnancy rates (Thatcher et al., 1994; Butler et al., 1996). However, in the present study we failed to see any difference in plasma progesterone concentrations measured at 6 days after the timed insemination.  Studies in vitro indicated that embryonic development was enhanced by IGF-I (Palma et al., 1997), and embryonic development was accelerated in superovulated cows treated with bST (Moreira et al., 2000a).  Thus, a direct effect of bST or an indirect effect of bST via IGF-I may be stimulating embryonic development and survival (circle 3).  Also, treatment with bST may decrease the production of PGF_2a by the endometrium at the time the embryo decreases secretion of PGF_2a to maintain the CL (defined here as pregnancy recognition (circle 4; Badinga et al., 2000). This potential effect would increase the chances for embryo survival.  All those are possible mechanisms that need to be further investigated to clarify the physiological processes that are influenced by bST treatment to increase pregnancy rates at first service to the Ovsynch/TAI protocol.  Use of bST by dairy producers in coordination with the reproductive management system may enhance herd reproductive performance of lactating cows at first service and such a strategy provides the opportunity to further maximize the return on investment for the use of bST.

Implications for Dairy Producers

A vast array of options has been recently developed for the reproductive management of lactating dairy cows.  Such management systems have been fine-tuned to result in maximum pregnancy rates and increase the overall reproductive efficiency of lactating dairy herds.  As it can be observed in the experiment described above, pregnancy rates as great as 50% to a single service were achieved.  It is important to emphasize, that as reproductive systems become more efficient and incorporate several levels for the control of reproductive processes, a thorough understanding of the technology is needed. 

It is important for producers to realize that such reproductive management systems cannot solve all reproductive problems per se.  For instance, incidence of cows in anestrus greatly reduces the reproductive efficiency of dairy herds and such a problem may not be solved by synchronization systems.  Providing optimal nutritional management, maximizing cow comfort, and maintaining a good herd health program are all pre-requisites for the success of any reproductive program.

To date, management of dairy cows has been driven by the necessity to maximize milk production with a high overall success. With the advent of new technologies to precisely manipulate reproductive function in lactating dairy cows, dairy producers are presented with a new opportunity.  Coordination of management strategies to maximize both milk production and reproductive performance may optimize the economical return of dairy herds, and allow for the industry to take complete advantage of the genetic potential to improve milk production through artificial insemination. Further research is necessary to fine tune management systems that are both practical and able to fulfill this objective.  Producers can look forward to new strategies to reduce anestrus, synchronize return services and enhance embryo survival.

References

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Burke, J. M., R. L. de la Sota, C. A. Risco, C. R. Staples, E. J.-P. Schmitt, and W. W. Thatcher.  1996.  Evaluation of timed insemination using a gonadotropin-releasing hormone agonist in lactating dairy cows.  J. Dairy Sci. 79:1385-1393.

 

Butler, W. R., J. J. Calaman, and S. W. Beam.  1996.  Plasma and blood urea nitrogen in relation to pregnancy rate in lactating dairy cattle.  J, Anim. Sci. 74:858-865.

 

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Izadyar, F., B. Colenbrander, and M. M. Bevers.  1996.  In vitro maturation of bovine oocytes in the presence of growth hormone accelerates nuclear maturation and promotes subsequent embryonic development.  Mol. Reprod. Dev. 45:372-377.

 

Izadyar, F., H. T. Van-Tol, B Colenbrander, and M. M. Bevers.  1997.  Stimulatory effect of growth hormone on in vitro maturation of bovine oocytes is exerted through cumulus cells and not mediated by IGF-I.  Mol. Reprod. Dev. 47:175-180.

 

Kirby, C. J., S. J. Wilson, and M. C. Lucy.  1997.  Response of dairy cows treated with bovine Somatotropin to a luteolytic dose of prostaglandin F_2a.  J. Dairy Sci. 80:286-294.

 

Lucy, M. C., T. L. Curran, R. J. Collier, and W. J. Cole.  1994.  Extended function of the corpus luteum and earlier development of the second follicular wave in heifers treated with bovine Somatotropin.  Theriogenology 41:561-572.

 

Macmillan, K. L. and W. W. Thatcher.  1991.  Effects of an agonist of gonadotropin-releasing hormone on ovarian follicles in cattle.  Biol. Reprod. 45:883-889.

 

Moreira, F., L. Badinga, C. Burnley, and W. W. Thatcher.  2000a.  Superovulatory responses in cows receiving bovine Somatotropin.  J. Dairy Sci. 83 (Suppl. 1): (in press).

 

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Moreira, F., C. A Risco, M. F. A. Pires, J. D. Ambrose, M. Drost, and W. W. Thatcher.  2000c.  Use of bovine Somatotropin in lactating dairy cows receiving timed artificial insemination.  J. Dairy Sci. 83:1245-1255.

 

Moreira, F. C. Risco, M. F. A. Pires, J. D. Ambrose, M. Drost, M. DeLorenzo, and W. W. Thatcher.  2000d.  Effect of body condition on reproductive efficiency of lactating dairy cows receiving a timed insemination.  Theriogenology 53:1305-1319.

 

Palma, G. A., M. Muller, and G. Brem.  1997.  Effect of insulin-like growth factor I (IGF-I) at high concentrations on blastocyst development of bovine embryos produced in vitro.  J. Reprod. Fertil. 110(2):347-353.

 

Pursley, J. R., M. R. Kosorok, and M. C. Wiltbank.  1997.  Reproductive management of lactating dairy cows using synchronization of ovulation.  J. Dairy Sci.  80:301-306.

 

Pursley, J. R., M. O. Mee, and M. C. Wiltbank.  1995.  Synchronization of ovulation in dairy cows using PGF_2a and GnRH.  Theriogenology 44:915-923.

 

Pursley, J. R., R. W. Silcox, and M. C. Wiltbank.  1998.  Effect of time of artificial insemination on pregnancy rates, calving rates, pregnancy loss, and gender ratio after synchronization of ovulation in lactating dairy cows.  J. Dairy Sci. 81:2139-2144.

 

Risco, C.A., M. Drost, L. Archbald, F. Moreira, R. L. de la Sota, J. Burke and W. W. Thatcher. 1998. Timed artificial insemination in dairy cattle-Part I. Compend. Contin. Educ. Pract. Vet. 20:280-287.

 

Thatcher, W. W., C. R Staples, G. Danet-Desnoyers, B. Oldick, and E. J. –P. Schmitt.  1994.  Embryo health and mortality in sheep and cattle.  J. Anim. Sci. 72:16-30.

 

 

 

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