Effects of hormonal treatments on reproductive performance and embryo production

Posted on May 20th, 2007

EFFECTS OF HORMONAL TREATMENTS ON REPRODUCTIVE PERFORMANCE AND EMBRYO PRODUCTION

W.W. Thatcher1′4 ,F. Moreira1, J.E.P.Santos2, R.C. Mattos1, F.L. Lopes1 , S.M. Pancarcil and C.A. Risco3

Department of Animal Sciences, IFAS1 and Large Animal Clinical Sciences, College of Veterinary Medicine 3, University of Florida, GainesviUe, FL 32611-0920 and Veterinary Medicine Teaching and Research Center 2, University of California-Davis, Tulare,CA 93274

ABSTRACT

Developments in the use of drugs to improve reproduction and embryo production have focused on estrus and ovulation synchronization protocols and embryonic survival. Protocols for synchronization of ovulation eliminate the need for detection of estrus and allow timed insemination of all cows enrolled. Various estrogenic, progestational, GnRH and PGF2~-like drugs are used to synchronize follicle development, CL regression and induction of ovulation. Strategies are discussed to optimize such programs to maximize herd pregnancy rates. Use of bovine Somatotrophin (bST) in combination with the Ovsynch protocol resulted in increased pregnancy rates, indicating possible effects on oocyte and embryonic development= Treatment of embryo donor cows with bST reduced the proportion of unfertilized oocytes and increased the number of transferable embryos. Furthermore, bST increased pregnancy rate when given to the recipient. Sub-luteal plasma progesterone concentrations after insemination have been associated with lower pregnancy rates. Injection of hCG on day 5 post-insemination resulted in induction of an accessory CL, increased plasma progesterone concentrations and increased conception rates. Strategies involving the use of sustained GnRH agonists to enhance CL development and alter follicular development are considered for future programs to enhance pregnancy rates. © 2000 by Elsevier Science Inc.

Key words: timed-insemination, GnRH, somatotrophin, hCG, embryo transfer

INTRODUCTION

Integrated reproductive management in cattle is considered essential to optimizing reproductive performance in current production systems. The physiological and environmental stresses of high milk production, inadequate nutrient intake, low body condition, and intensive management systems impair aspects of reproductive performance (e.g., onset of ovarian cycles, intensity of estrus, pregnancy rates) in both dairy and beef cattle. New technology has advanced the development of various systems to improve herd pregnancy rates by synchronizing follicle development with occurrence of CL regression, precisely controlling the time of ovulation, application of a fixed timed insemination and improving embryo survival. These systems involve the use of various drugs available to producers, and their effective uses have to be founded on a thorough understanding of the reproductive events that are being managed. The objective of this presentation is to describe the use of various hormonal treatments for reproductive management with application to cattle systems with an emphasis in dairy cattle.

MANAGEMENT OF OVARIAN FOLLICLE AND CL FUNCTIONS TO SYNCHRONIZE OVULATION AND ENHANCE PREGNANCY RATES WITH A TIMED INSEMINATION

Synchronization of the estrous cycle using progesterone and/or luteolytic agents is based on controlling lifespan of the CL. However, estrus culminates from growth of an estrogenic dominant follicle in coordination with regression of the CL. It is now evident that variations in efficacy of synchronization protocols based on progesterone and/or PGF2a are related to the stage of follicular development when the synchronization treatment was initiated. Thus, synchronization of follicular growth and CL regression has the potential to increase precision of estrus synchronization and to optimize quality of the follicle to maximize pregnancy rates.

Follicular growth may be synchronized with different drugs such as progesterone, estradiol, a combination of progesterone and estradiol, gonadotropin releasing hormone (GnRH) and its agonists (GnRHa). The use of progesterone or progestins in high doses may suppress LH support of the dominant follicle and induce atresia and follicle turnover (1). Estradiol treatments, either given in conjunction with a progestin or alone can induce follicular turnover (2,6). However, steroid treatments generally are prohibited in lactating dairy cattle of the USA and are not condoned in countries of the Europeon Union. Therefore, GnRH and GnRHa are the drugs of choice to synchronize follicular growth in lactating dairy cattle.

The injection of GnRH or its agonists induces a surge of LH and FSH (8). The LH release induces ovulation or luteinization of large ovarian follicles present at the time of treatment (20). Ovulation of the dominant follicle would allow for an endogenous surge of FSH, which would complement the GnRH-induced surge of FSH to initiate recruitment of a new follicular wave. Martinez and co-workers (22) compared the ability of porcine LH and GnRH to induce wave emergence in beef heifers injected on days 3, 6 and 9 after ovulation (day 0) of the first follicular wave. New wave emergence was induced consistently in only those heifers that ovulated in response to treatment. However, 74% of heifers had a growing dominant follicle 7 days after treatment, and 85% of the dominant follicles were > 9 mm and capable of ovulating. The ability to induce wave emergence in cattle that ovulate, but not compromise ovarian follicle wave development in animals whose dominant follicles were not capable of ovulating, resulted in the presence of a functional dominant follicle at 7 days after GnRH treatment. The net effect is that follicular synchronization has occurred.

Estrus synchronization with administration of a GnRHa followed 7 days later by an injection of PGFzt~ was tested in cattle compared to a single injection of PGF2g (20). Results indicated an increase in estrous detection rates and also an enhanced precision of synchrony with estrus being detected 2 to 3 days after PGF2~ injection. Furthermore, there were no differences in conception rates between the two experimental groups. Use of GnRH followed 7 days later with PGF2~ is a satisfactory system for synchronization of estrus with good fertility (39).

Pursley et al., (29) demonstrated that an additional injection of GnRH given 48 h after PGF2~ would induce a timed ovulation at approximately 30 h. The sequence of injections with GnRH (day 0), PGF2a (day 7) and GnRH (day 9) is called the Ovsynch Program because it synchronizes ovulation and permits a timed insemination (30). This system of management for timed insemination results in normal pregnancy rates for lactating dairy cows (see review, 33) and offers the producer a powerful management tool to precisely control time of first insemination in the postpartum period. The ability to control ovarian follicular and CL development has allowed producers to time inseminate cattle without the need for detecting estrus. In essence, with a timed insemination program, heat detection rates are 100% and from an economical perspective the Ovsynch Program increases net revenue per cow (34).

Producers should follow the Ovsynch protocol carefully. If an interval of less then 7 days is used between GnRH and PGF~ 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, then more cows are detected in heat prior to injection of GnRH. Cows become asynchronized, and timing of insemination is not correct. The current challenge is to further optimize the Ovsynch Program. Success of the program is dependent on whether lactating dairy cows are cycling. When a portion of the cows are anestrus, pregnancy rates of the group will be somewhat less even though the Ovsynch protocol itself may induce some anestrous cows to cycle and perhaps conceive.

Initiation of the Ovsynch protocol at certain stages of the estrous cycle is associated with reduced fertility and is therefore a problem. For example, injection of the first GnRH on day 15 may not result in ovulation and formation of a CL. At this stage, COWS that have two-wave cycles commonly have a small potentially dominant follicle that is not responsive to GnRH. Consequently, a new CL does not develop. At 2 to 4 days after the injection of GnRH, the cow secretes PGF2~ spontaneously from the uterus that causes regression of the CL. Thus, at the time of the PGF2~ injection given 7 days after the GnRH injection, the cow has already regressed the CL and may even be in heat (25). Such cows ovulate prematurely relative to insemination and are not likely to conceive. A second problematic stage occurs early in the estrous cycle (e.g., day 2 or 3). At this stage, spontaneous ovulation has already occurred and the potentially new dominant follicle is too small (5. 9mm) to ovulate in response to GnRH injection (25). Consequently, the dominant follicle will be aged and have expressed dominance for 5 days or longer at the second GnRH injection. Follicles that have periods of dominance longer than 5 days (4) or cows that initiate the Ovsynch program in the early estrous cycle are less fertile (25,43). These aged follicles may not or may ovulate, but oocytes produced are likely to be less fertile.

Strategies can be developed to minimize the proportion of cows in these problematic stages of the cycle for initiation of the Ovsynch program. One approach is to inject cows twice with PGF2~ at an interval of 14 days and to give the first GnRH injection of the Ovsynch program 12 days after the second injection of PGF,,. If all cows were cycling, 90% of the cows are expected to be in the ideal stage of the estrous cycle, between 5 to 10 days, when the Ovsynch program is initiated. Under this management scenario, an expected pregnancy rate to the Ovsynch program would be 48%. Pre-synchronization with a standard PGF2a protocol is implemented prior to initiation of the Ovsynch program. In contrast, expected pregnancy rate if all cows are cycling and the Ovsynch program is inititated at random stages of the cycle is 36%. Of concern to dairy producers is whether the drug Bovine Somatotropin (bST; Posilac, Monsanto, St. Louis, MO; 500 mg) can be administered at 9 weeks of lactation and be continued without compromising reproductive performance. Moreira et al., (28) reported that first service pregnancy rates to the Ovsynch protocol actually were increased when cows received bST treatment.

Impact of Anestrus and Pre-synchronization Prior to Ovsynch on Pregnancy Rates

Measuring the impact of any drug or management system on pregnancy rates is a challenge because the experimental response is a 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 such as detection of estrus. At least with the Ovsynch program, the management errors associated with heat detection are eliminated and the precise timing of insemination can be controlled tightly. A field trial was conducted with the objectives of: determining whether pre-synchronization of lactating cows prior to the initiation of the Ovsynch program would improve pregnancy rates; to verify prior results indicating that bST increased pregnancy rates to the Ovsynch program; to determine whether the possible beneficial effect of bST on pregnancy rates occurred prior to or after timed artificial insemination (26).

A total of 543 cows was assigned randomly to a 2×3 factorial 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~, (Lutalyse, Pharmacia-Upjohn Co.; 25 mg; i.m.), followed 14 days later (51 +3 days) with a second PGF2~ injection. In contrast, control cows (no pre-synchronization) did not receive the two injections of PGF2,. On day 63 + 3 days, the first injection of GnRH of the Ovsynch program was initiated, and this was 12 days after the second injection of PGF2~ of the pre-synchronization program. The pre-synchronization program will induce cows to be 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~. All cows were timed-inseminated at day 73 + 3 days postpartum. Other factors tested in this experiment were the initiation of bST treatment at day 63 (time of the GnRH injection of the Ovsynch program), day 73 (time of artificial insemination as part of the Ovsynch program) or bST-control in which first injection of bST was not given until 147 days of lactation (well after first and second services). A series of blood samples was collected on days 51, 63, 70 72 and 79 postpartum. Paired comparisons of plasma progesterone concentrations were used to determine cycling status (samples on days 51 and 63), stage of the cycle at the beginning of the Ovsynch program (samples on days 63 and 70), occurrence of complete CL regression (samples on days 70 and 72), and occurrence of synchronized ovulation (samples on days 72 and 79). All cows were examined for pregnancy using ultrasonography on day 32 after timed insemination. Pregnant cows were re-examined for pregnancy by rectal palpation on day 74 after timed insemination. All cows diagnosed open at day 32 after the first timed insemination were injected with GnRH and the Ovsynch program repeated with second insemination occurring at 115 days of lactation.

Cows with progesterone concentrations of <1 ng/ml on days 51 and 63 postpartum were considered to be anestrus (quiescent ovaries or anovulatory follicles). Determination of cycling status is important because pre-synchronization and bST effects on fertility would be expected to occur only in cycling animals. Overall, 23.4% of the cows were anestrus or had not started to cycle by 63 days postpartum. Frequency of anestrus was greater for primiparous (35.6%) than multiparous cows (16.9%). The occurrence of anestrus decreased (P<0.01) as body condition scores (BCS) at initiation of the Ovsyneh program increased from 2.0 to 3.0. Since BCS only accounted for 7.8% of the variation in occurrence of anestrus, it is an imperfect predictor of which cows are cycling. Pregnancy rate at 74 days after first insemination was only 22.4% for anestrous cows, which was lower than the 41.7% pregnancy rate for cyclic cows (P<0.01).

However, anestrous cows that ovulated twice to the two successive injections of GnRH, given 9 days apart in the Ovsynch program, had normal pregnancy rates (39.1%). Therefore, postpartum management of lactating dairy cows to minimize stress and maximize cow health, comfort, and nutritional status (e.g., enhance dry matter intake) will be reflected later in lactation in terms of a higher incidence of cycling cows and improved reproductive performance. Moreira et al., (27) demonstrated that pregnancy rate to first service of an Ovsynch program was lower in cows with body condition scores <2.5 versus >2.5. Dynamic modeling was used to estimate the effect of BCS on net revenue per cow. Net revenue per cow increased $10.33 when percentage of cows with BCS <2.5 was reduced from 30% to 10%. Why does a low body condition score result in a lower pregnancy rate to the Ovsynch 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? In the present study, an interaction between cycling-anestrus statuses on pregnancy rates was detected (P<0.01) in which pre-synchronization and bST effects were detected in cycling but not anestrous cows.

Increased pregnancy rates were detected (P<0.01) when cycling cows were presynchronized (52.3%) compared to cows not pre-synchronized (31.1%). An additional comparison is the effect of pre-synchronization in the two groups that did not receive bST in which pre-synchronized cows had a 42.6% pregnancy rate compared to 25.3% for the control group. This latter difference associated with pre-synchronization (17.3%) approximates the predicted difference of 12.0% that was estimated for pregnancy rates of cows initiating the Ovsynch program at random stages of the estrous cycle (36%) versus those pre-synchronized with two injections of PGF2a and the Ovsynch program beginning at 12 days after the second injection of PGF2a (48%).

The reason for increased pregnancy rates with the Ovsynch protocol in cyclic cows presynchronized was related to the frequency of cows initiating the Ovsynch protocol at favorable stages of the estrous cycle (i.e., days 5 to 10 of the cycle). By collecting blood samples at the first injection of GnRH (at day 63) and again when PGF2a was injected (at day 73), it was possible to 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 (HIGH-HIGH) probably initiated the Ovsynch protocol at the optimal stage of the estrous cycle. Approximately 87.4% of pre-synchronized cows were classified as HIGH-HIGH versus only 71.7% of cows not pre-synchronized(P<0.01). Therefore, pre-synchronization successfully programed cows to be in a favorable stage of the cycle to begin the Ovsynch program and likely increased first-service pregnancy rates to the Ovsynch protocol by enhancing the rate of synchronized ovulations.

bST Effects on Pregnancy Rate

Cycling cows (n=375) initiating bST treatment at 63 or at 73 days postpartum had increased pregnancy rates at day 74 of gestation compared to controls among cows not pre-synchronized (Control=25.3%, bST-day 63 = 34.2% and bST-day 73=33.7%) and also among cows pre-synchronized (Control=42.6%, bST-day 63 = 58.1% and bST-day 73=56.1%). 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 CmRH injection increased concentrations of bST 10 days later when the cow was inseminated. Since pregnancy rate also increased when the first injection of bST was given at the time of insemination, bST appears to affect the maturing occyte, reproductive tract or the subsequent developing embryo and to enhance embryonic survival. Injections of bST were repeated every 14 days. The bST-induced increase in pregnancy rate was only evident at the first timed insemination and not at the second timed insemination. The potential management problem of estrous detection in bST treated cows was eliminated with the use of the Ovsynch program. Furthermore, responsiveness to bST is enhanced if cows are presynchronized to enhance responsiveness to the Ovsynch program. There is no evidence that the use of bST at the 9th week of lactation is detrimental to fertility when used with a timed breeding protocol such as Ovsynch.

EFFECT OF BST ON DEVELOPMENT OF EMBRYOS FROM SUPEROVULATED DONORS AND ON PREGNANCY RATES IN RECIPIENT COWS.

An experiment was designed to examine potential bST effects on embryo and/or transfer recipient utilizing an embryo transfer model (23,24). The objective was to determine if injection of bST into superovulated donor cows at the time of insemination would affect yield and development of embryos recovered 7 d later, and whether subsequent transfer of frozen/thawed embryos from bST or control donor cows had effects on pregnancy rates of recipient cows that received either bST or no bST treatments.

Lactating (n = 8) and non-lactating (n = 4) cows were superovulated as follows: on day 0,cows were injected with estradiol-17[~ (2.5 mg; i.m.) and progesterone (50 mg; i.m.) to induce follicular turnover and two norgestomet implants (6.0 mg; s.c.) inserted to sustain an elevated progestin environment; in the afternoon of d 4 and until the morning of d 8, cows were injected with eight decreasing doses of FSH (Folltropin; 20 mg/ml; i.m.) given at a decreasing rate(d 4 [PM, 4 ml], d 5 [PM, 3 ml], d 6 [PM, 2 ml], and d 7 [PM, 1 ml]); in the PM of d 6 and AM of d 7, cows were injected with (POF2=; Lutalyse; 25 mg; i.m.); on PM of d 7, norgestomet implants were removed, and cows observed for estrus and inseminated at estrus and every 12 h until cessation of estrous behavior. At the time of the first insemination, cows were assigned randomly to receive a single dose of bST (Posilac; 500 mg; s.c.) or no bST. At 7 d after insemination, cows were non-surgically flushed and embryos were collected. Flushed ova and embryos were classified for stage of development and quality, and transferable embryos (Code 1-excellent and Code 1-good) were frozen in ethylene glycol for subsequent direct transfer. Donor cows were superovulated repeatedly at a minimum interval of >30 days with treatments sequentially alternated within each cow. Such a design was adopted in an attempt to control cow variability in evaluating treatment effects. Two sets of 26 flushes were derived from bST-treated cows and control cows, respectively. No differences in the total number of ova/embryos per flush were detected between bST and control cows (9.3 _+ 1.5 and 9.4 _+ 1.5 embryos per flush, respectively). However, the number of unfertilized oocytes recovered per flush was reduced (P < 0.05) in bST treated donors compared to control cows (1.0 ± .9 < 3.7 ± .9). No differences due to treatment were observed when the number of degenerate embryos per flush was analyzed (bST = 1.2 _+ .3 and control = 0.4 ± .3). Treatment with bST increased (P < 0.01) the percentage of embryos classified as transferable relative to the total number of ova/embryos flushed (77.2% > 56.4%). Number of embryos determined to be in the blastocyst stage of development per flush was greater for bST than for control cows (2.4 ± 0.7 > 0.4 ± 0.7; P < 0.04). In addition, distribution among stages of development differed between bST and control groups, respectively: morula, 18.4 and 28.0%; early blastocyst, 41.3 and 53.4%; blastocyst, 32.5 and 9.3%; expanded blastoeyst, 7.8 and 9.3% (P < 0.001).

Treatment of donor cows with bST increased the capacity of the oocyte to become fertilized following insemination since the number of unfertilized oocytes was reduced for bSTtreated cows compared to control-treated cows. In addition, percentage of embryos classified as transferable was increased in bST-treated donor cows. Since the total number of ova/embryos recovered per flush was similar between bST and control treatments, the difference in the percentage of embryos classified as transferable was probably due to an increased fertilization rate among bST-treated donors. Such an observation agrees with prior results indicating that treatment of bovine oocytes with growth hormone during maturation enhances the process of in vitro maturation and results in an increased development of embryos to the blastocyst stage (14,15,16). In the present experiment, 80% of transferable embryos were classified in the earlier stages of development (i.e., morula and early blastocyst) and only 20% were classified in the later stages of embryonic development 0alastocyst and expanded blastocyst) among control donors. In contrast, approximately 60% of transferable embryos from bST-treated donor cows were classified in the early stages of development and the remaining 40% were classified in more advanced stages of development. Collectively, these embryonic responses indicate that increased pregnancy rates due to bST given as part of the Ovsynch protocol may be due partially to bST stimulation on oocyte maturation and early embryonic development.

In the second experimental phase (24), transfer of bST or control-treated embryos to bST treated and control recipients were conducted to evaluate subsequent pregnancy rates. Embryos classified as transferable and frozen in ethylene glycol for subsequent direct transfer were obtained from superovulated donor Holstein cows that received bST treatment at insemination(bST embryos; n = 101) or no bST treatment at insemination (control embryos; n = 80), as described above. Lactating Holstein cows designated as recipients received either bST treatment 1 d after estrus was observed (Posilac, 500 rag, s.c.; bST recipients n = 97) or served as untreated controls (control recipients; n = 84). After initial bST treatment, bST-treated recipients were reinjected with bST every 14 d until 30 d prior to cessation of lactation. Either a bST or a controlfrozen embryo was thawed and transferred directly to a recipient cow (bST or control) at 7 d after detection of estrus. The experiment was analyzed as a 2×2 factorial of four treatment groups: control embryos transferred to control recipients (n = 43), bST embryos transferred to control recipients (n = 41), control embryos transferred to bST recipients (n = 37), and bST embryos transferred to bST recipients (n = 60). Pregnancy was determined by palpation 40 to 45 d after embryo transfer. Both a main effect ofbST treatment on superovulated donors (P < 0.02)and an interaction (P < 0.04) between bST treatments on suuperovulated donors and recipients were detected on pregnancy rates (Figure 1).

Figure 1. Pregnancy rates of recipient cows treated with bST or control and receiving frozenthawed embryos collected from superovulated donor cows treated with bST or control at the time of insemination (see text for experimental details).

Among control recipients, transfer of bST embryos increased pregnancy rates compared to transfer of control embryos (P < 0.01), but no differences were observed among bST recipients (P > 0.1). Further analyses indicated that pregnancy rates of bST recipient cows were 17.7% units greater (P < 0.05) than pregnancy rates for control recipient cows receiving a control embryo. No difference (P > 0.1) was detected between pregnancy rates of the two groups of bST recipient cows compared to the control group of recipient cows receiving a bST embryo. Transfer of Codel-excellent quality embryos resulted in greater (P < 0.01) pregnancy rates than transfer of Code 1- good quality embryos (47.4% > 11.1%).

The interaction and subsequent analyses indicate that transfer of a bST-treated embryo increased pregnancy rates following embryo transfer among control recipient cows. Such an effect of embryo treatment was not observed when recipient cows were treated with bST. The absence of an effect of embryo treatment on pregnancy rates among recipient cows treated with bST may be due to the fact that bST also increased pregnancy rates of recipient cows after embryo transfer compared to control recipient cows that received a control-treated embryo. Collectively results from the experiment indicate that bST may be increasing pregnancy rates of lactating dairy cows via: enhancing oocyte maturation and increasing fertilization rates; accelerating early embryonic development, and affecting maternal components following fertilization (e.g., hystotrophe production, differential modulation of growth factors and other proteins secreted from the oviduct and uterus, and a diminished luteolytic signal at the time of pregnancy recognition [5]). Further investigation is warranted to identify which of those factors may be influenced by bST treatment to significantly impact fertility of lactating cows.

PROGESTERONE SUPPLEMENTATION TO INCREASE PREGNANCY RATE

Several investigators have demonstrated lower progesterone concentrations in plasma of cows that failed to conceive and this was evident as early as day 6 after insemination (7,19,21,42). Development of the embryo is related to concentrations of progesterone and ability of the conceptus to secrete the antiluteolytic hormone, interferon-x (21). In fact, exogenous progesterone stimulated embryo development (see review, 42). Studies to supplement progesterone during the luteal phase after insemination (i.e., afterh day 5) with insertion of intravaginal progesterone releasing devices for 6 to 12 days have had inconsistent effects on pregnancy rates. An alternative strategy to increase progesterone concentrations is to induce an accessory CL by ovulating the dominant follicle at day 5 of the estrous cycle with an injection of hCG (e.g., 3,300 IU, i.m.). The hCG induced CL elevates progesterone concentrations to a greater degree than observed with the use of GnRH (38).

A study was designed to determine the effects ofhCG (3,300 IU i.m.) administered on d 5 after AI on accessory CL formation, plasma progesterone concentration, conception rate, and pregnancy loss in high producing Holstein dairy cows (37). Following synchronization of estrus (GnRH followed 7 d later by PGF2,~) and AI at detected estrus, 406 cows were injected with either hCG or saline on d 5 after AI (203/treatment). Blood sampling and ultrasonography of ovaries were conducted once between days 11 and 16 after AI. Pregnancy diagnosis was performed by ultrasonography on d 28 and by rectal palpation on days 45 and 90 after AI. Treatment with hCG on d 5 induced formation of one or more accessory CL in 86.2% of the hCG-treated cows compared with 23.2% in the controls. Differences in progesterone concentrations between hCG and control cows were +6.3 ng/ml for primiparous cows and +3.1 ng/ml for multiparous cows (treatment by parity; P<0.02). Accessory CL increased progesterone concentration in hCG-treated cows but not in controls. Treatment with hCG increased (P < 0.01) conception rates on days 28 (45.8>38.7%), 45 (40.4>36.3%), and 90 (38.4>31.9%) after AI. Pregnancy losses between days 28 and 45, 45 and 90, and 28 and 90 were similar between the two groups. Progesterone concentration and number of CL after AI affected conception rate (P < 0.01) such that pregnant cows had higher progesterone concentrations and a greater frequency of accessory CL. Body condition score at AI and milk yield affected conception rate, but no interaction between these variables and treatment were observed. However, hCG improved conception rate in cows loosing BCS between AI and d 28 after AI (P < 0.01). Treatment with 3,300 IU ofhCG on d 5 after AI induces the formation of one or more accessory CL, increases plasma progesterone concentrations, and improves conception rate of high producing dairy cows.

Multiple mechanisms may contribute to the increase in conception rate in response to hCG. Diaz et al. (13) increased progesterone concentrations and induced a three wave follicular cycle when hCG was injected at day 5 after estrus. In all hCG-treated heifers, the dominant follicle of the third follicular wave did not reach 9 to 10 mm in size until approximately d 20. Thus the potential estrogenic follicle for hCG treated heifers would not occur until d 20 versus d 14 in control heifers with a two-wave follicular cycle (13). Therefore, hCG treatment would decrease the estrogenic environment during the period of pregnancy recognition. Injection ofhCG on day 7 has increased conception rates in lactating dairy cows (40) with a slight increase in plasma progesterone concentrations.

It is likely that systems of progesterone delivery are needed that increase substantially both the rate of progesterone rise and absolute luteal phase concentrations of progesterone after insemination. Such systems need to deliver progesterone in an amount equivalent to what the normal CL can produce at various physiological stages. Further efforts to influence CL differentiation and subsequent function are a fruitful area of investigation. Insertion of an implant containing 700 gg or 2100 Bg Deslorelin (GnRH agonist) on day 5 of the estrous cycle induced formation of an accessory CL and a hyper-progestational response as detected with hCG (32). Insertion of a 700 I.tg Deslorelin implant to induce ovulation of the dominant follicle within an Ovsynch protocol induced a greater rise in progesterone following ovulation (3). In both experimental models, subsequent follicular and CL dynamics were altered in a manner that suggested a transitory period of sufficient FSH and LH secretion to induce follicular wave emergence and turnover as well as normal CL development followed by a desensitization of gonadotrophs to Deslorelin that led to a suppression in follicle development but did not alter CL maintenance (3,32). Several studies documented a stimulation in CL function following induced ovulations with Deslorelin implants (3,9,32).

In principle, incorporation of a Deslorelin implant to induce ovulation within an Ovsynch program for timed insemination could induce optimal CL development and first wave follicle emergence to support development of the embryo and then attenuate follicle development to support maintenance of pregnancy during the antiluteolytic period. Inhibition of episodic LH pulses from 2 days before the preovulatory surge of LH until day 7 of the estrous cycle had an inhibitory effect on development of the CL (31). However, suppression of LH secretion after day 12 had no effect on CL function. Recently, Davis et al., (11) reported that chronic administration of a GnRH agonist enhanced luteal function due to an increase in basal LH when infused between days 2 - 21 of the estrous cycle that was greater than infusions between days 12-21. Collectively these experimental results indicate that the transitory effects of an appropriate dose of a Deslorelin implant may have regulatory effects on ovarian function that could benefit reproductive management systems in dairy cattle. Various strategies for management of beef cattle with GnRH agonist implants have been described (10).

ALTERNATIVE SYNCHRONIZATION SYSTEMS

Dairy systems throughout the world vary as to whether production systems are intensive or extensive with the latter being dependent upon grazing of grasses (e.g., New Zealand and Ireland). In the latter system, efficiency of production is dependent upon synchronizing the calving pattern with the onset of the season for growing grass. This can be successfully achieved by precisely synchronizing first artificial inseminations so that the target population of cows has a highly synchronized estrous response and high fertility to the synchronized estrus. Ryan et al., (36) developed an estrous synchronization program that incorporated injection of a GnRH agonist at the time of inserting an intravaginal progesterone-releasing insert (IPI) for an 8 day period, and PGF2a treatment 24 h prior to withdrawal of the device. This system provided an 88.5% estrous detection rate and a 58% conception rate. The system was further refined (35) with an injection of estradiol benzoate (1 nag) after withdrawal of the intravaginal progesterone device which resulted in excellent synchronized estrus detection (95.7%), conception (60.4%) and pregnancy (57.8%) rates.

Patterson and coworkers (17,44) approached the concept of pre-synchronizaton to optimize subsequent synchronization and conception rates by feeding melengestrol acetate (MGA) for 14 days and injecting PGF2a at 19 days after MGA feeding or an injection of CmRH 7 days prior to PGF2a (44). An alternative program was to feed MGA for 7 days coupled with a PGF2~ injection at MGA withdrawal and subsequent injections of (3nRH and PGF2a at 4 and 11 days after MGA feeding, respectively (17). The latter program reduces the time required for presynchronization (7 days of MGA+PGF2a) and insures presence of progesterone during the final period of synchronization, which also synchronizes follicle recruitment with CL regression GnRH+PGF2a). These pre-synchronization systems with MGA resulted in excellent pregnancy rates to artificial insemination at detected estruses. The feeding of MGA in these integrated systems of synchronization appear to stimulate estrous cycles in a proportion of non-cycling prepubertal or lactating beef animals.

Exogenous estradiol has the ability to induce preovulatory surges of LH and FSH during late dlestrus and proestrns (low progesterone environment) by stimulating hypothalamic secretion of GnRH. Bo et al., (6) reported that 5 rag of estradiol administered i.m. in combination with a progestin implant at various stages of dominant follicle development would synchronize emergence of a new follicular wave in 4.3 days. In a study completed in New Zealand (12), cows received an IPI for a 7-day period and were injected twice with estradiol benzoate at IPI insertion (2mg, i.m.), to induce synchronized follicular wave emergence, and at 48 h after IPI removal (ling, i.m.) to induce estrus and ovulation. Cloprostenol sodium (500 Ixg; a synthetic-PGF2a) was injected at IPI removal. Greater than 90% of cows were in estrus within a 5 day period beginning 1 day after IPI removal. Conception rate to ftrst service (61.7%) was similar to an unsynchronized Control group (57.0%) but occurred 10.1 days earlier in the synchronized group. Precision of estrus was sufficient in the New Zealand (12) and Ireland (35) studies to possibly justify a fixed timed insemination.

Recently, a series of experiments was conducted to evaluate whether estradiol cypionate could be used as part of a timed insemination program in dairy animals to replace the second GnRH injection of an Ovsynch program to induce ovulation (18). All experimental animals were synchronized with injection of GnRH (100 ktg) on day 0, fed MGA on days 1-6 (0.5rag/d) and injected with PGF~ on day 7. Estradiol cypionate (ECP) injected i.m. on day 8 in 14 dairy heifers (0.5 mg) and 8 multiparous non-lacatating dairy cows (I rag). Time of ovulation was determined by ultrasound at 6 h intervals beginning at 48 h after ECP injection. Ovulation occurred at 62 _+ 2h and 60 + 2h after ECP for heifers and cows, respectively. A field trial was conducted to examine pregnancy rates to a Timed Insemination following injection of ECP. Heifers (n=158) were assigned to control or ECP timed insemination groups during the summer heat stress period in South Florida.

Following synchronization (GnRH, MGA, PGF~ as described above), control heifers were inseminated at estrus, and ECP treated heifers were injected with 0.5mg ECP on day 8 and timed inseminated 48 h later on day 10. Pregnancy rates did not differ between control (39.2%) and estradiol cypionate treated (39.2%) heifers. However a decrease in pregnancy rates was observed for the heifers that initiated the synchronization treatment at late diestrus and were timed inseminated. These two experiments indicated that estradiol cypionate (0.5 rag) was able to induce a synchronized ovulation with normal pregnancy rates in dairy heifers. A second field trial (41) was conducted in March, 2000 in South Florida prior to the heat stress season to test whether pregnancy rates to the ECP-TAI system could be increased by extending the MGA feeding until day 7 (day of PGFec, injection). All experimental heifers (n=160) were synchronized with GnRH (100 ~tg) on day 0, 25 mg of PGF2~ on day 7, injected with ECP (0.5 mg) on day 8 to induce ovulation, and insemination was performed at a fixed time at 48 h after ECP injection on day 10. Heifers were divided randomly into two treatment groups: MGA feeding from d 1 to d 6 (n=79) or from d 1 to d 7 (n=81). Blood samples were collected on day 0 and day 7 to characterize cyclicity and/or stage of the cycle of the heifers at initiation of treatment. Pregnancy diagnosis was performed by rectal palpation at 55 d after insemination. No difference in pregnancy rate was observed among the groups that received MGA from days 1 to d 6 (46.8%) or from days 1 to d 7 (48.1%). Furthermore, the decrease in pregnancy observed previously in the heifers that initiated the treatment in late diestrus also was observed in the present experiment (P<0.02), and was not avoided by extending MGA feeding by one day. This decrease in pregnancy rate could have resulted from a possible reduced fertility of an aged follicle/oocyte that was induced to ovulate by the ECP injection. ECP can be used effectively as part of a TAI system, in dairy heifers, resulting in good pregnancy rates. Future investigations are needed to evaluate whether estradiol cypionate can be used to: induce follicular turnover in the beginning of a Timed Insemination protocol and be used as part of a re-synchronization program for second service in cows that did not conceive to the first service.

SUMMARY

A vast array of options is available for the reproductive management of cattle. Such management systems have been fine-tuned to result in maximum pregnancy rates and can increase the overall reproductive efficiency of both dairy and beef cattle. Pregnancy rates as great as 50% to a single synchronized service or timed insemination are achieve able under commercial conditions. It is important to emphasize, that as reproductive systems become more efficient and incorporate several levels of control (e.g., ovarian and embryonic) the producer, veterinarian and consultants need to have a thorough understanding of the technology to insure effective implementation. Second generation strategies to improve CL function and embryo development will further improve reproductive efficiency. Incidence of anestrous cows greatly reduces the reproductive efficiency of the herd group, and current synchronization systems cannot fully overcome this physiological state. Providing optimal nutritional management, minimizing stress, maximizing cow comfort, and maintaining a good herd health program are all pre-requisites for the success of any reproductive program. Coordination of management strategies to maximize production and reproductive performances will optimize the economical return of the production unit, and allow for the industry to take complete advantage of the genetic potential to improve production through artificial insemination and embryo technology. Producers can look forward to the implementation of new strategies to reduce anestrus, synchronize return services and enhance embryo survival.

REFERENCES

  1. Adams GP, Matteri RL, Ginther OJ. Effect of progesterone on ovarian follicles, emergence of follicular waves and circulating follicle-stimulating hormone in heifers. J Reprod Fertil 1992; 95:627-640.
  2. Adams GP. Control of ovarian follicular wave dynamics in cattle: implications for synchronization and superstimulation. Theriogenology 1994;41:19-24.Theriogenology 87
  3. Ambrose JD, Pires MFA, Moreira F, Diaz T, Binelli M, Thatcher WW. Influence of Deslorelin (GnRH-agonist) implant on plasma progesterone, first wave dominant follicle and pregnancy in dairy cattle. Theriogenology 1998; 50:1157-1170.
  4. Austin EJ, Mihm M, Ryan MP, Williams DH, Roche IF. Effect of duration of dominance of the ovulatory follicle on onset of estrus and fertility in heifers. J Anim Sci 1999; 77:2219-2226.
  5. Badinga L, Guzeloglu A, Binelli M, Thatcher WW. Bovine somatotropin attenuates phorbol ester-induced PGF2ot release in bovine endometrial cells. Biol Reprod 2000; 62 (Suppl. 1): 151 abstr.
  6. Bo GA, Adams GP, Pierson RA, Mapletoft RJ. Exogenous control of follicular wave emergence in cattle. Theriogenology 1995; 43:31-40.
  7. Bulman DC, Lamming GE. Milk progesterone levels in relation to conception, repeat breeding and factors influencing acyclicity in dairy cows. J Reprod Fertil 1978;54:447-458.
  8. Chenault JR, Kratzer DD, Rzepkowski RA, Goodwin MC. LH and FSH response of Holstein heifers to fertirelin acetate, gonadorelin and buserelin. Theriogenology 1990;34: 81-98.
  9. D’Occhio MJ, Aspden WJ, Whyte TR. Controlled, reversible suppression of estrous cycles in beef heifers and cows using agonists of gonadotropin-releasing hormone. J Anim Sci 1996; 74:218-225.
  10. D’Occhio MJ, Fordyce G, Whyte TR, Aspden WJ, Trigg TE. Reproductive responses of cattle to GnRH agonists. Anim Reprod Sci 2000; 60-61;433-442.
  11. Davis T, Mussard M, Jimenez-Severiano H, Enright W, Kinder J. Enhanced luteal function in cattle chronically administered gonadotropin-releasing hormone (GnRH) agonist Azagly-Nafarelin. Biol Reprod 2000; 62(Suppl 1):211 abstr.
  12. Day ML, Burke CR, Taufa, VK, Day AM, Macmillan KL. The strategic use of estradiol to enhance fertility and submission rates of progestin-based estrus synchronization programs in dairy herds. J Anim Sci 2000; 78:523-529.
  13. Diaz T, Schmitt EJ-P, Thatcher M-J, Thatcher WW. Human chorionic gonadotropininduced alterations in ovarian follicular dynamics during the estrous cycle of heifers. J. Anim Sci 1998;76:1929-1936.
  14. Izadyar F, Colenbrander B, Bevers MM. In vitro maturation of bovine oocytes in the presence of growth hormone accelerates nuclear maturation and promotes subsequent embryonic development. Mol Reprod Dev 1996;45:372-377.
  15. Izadyar F, Hage WJ, Colenbrander B, and Bevers MM. The promotory effect of growth hormone on the developmental competence of in vitro matured bovine oocytes is due to improved cytoplasmic maturation. Mol Reprod Dev 1998;49:~4a.~453.
  16. Izadyar F, Van-Tol HT, Colenbrander B, Bevers MM. 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 1997;47:175-180.
  17. Kojima FN, Salfen BE, Bader IF, Ricke WA, Lucy MC, Smith MF, Patterson DJ. Development of an estrous synchronization protocol for beef cattle with short-term feeding of melengestrol acetate: 7-11 Synch. J Anim Sci (In Press).
  18. Lopes FL, Arnold DR, Williams J, Pancarci SM, Thatcher M-J, Drost M, Thatcher WW. Use of estradiol cypionate for timed insemination. J Anim Sci 2000;78:Supplement 1:216 abstr.
  19. Lukaszewska J, Hansel W. Corpus luteum maintenance during early pregnancy in the cow. J Reprod Fertil 1980;59:485-493.
  20. Macmillan KL, Thatcher WW. Effects of an agonist of gonadotropin-releasing hormone on ovarian follicles in cattle. Biol Reprod 1991;45 883-889.
  21. Mann GE, Lamming GE, Robinson RS, Wathes DC. The regulation of interferon-z production and uterine hormone receptors during early pregnancy. J Reprod Fertil 1999; 54(Suppl):317-328.
  22. Martinez MF, Adams GP, Bergfelt DR, Kastelic JP, Mapletoft RJ. Effect of LH or GnRH on dominant follicle of the first follicular wave in beef heifers. Anim Reprod Sci 1999;57:23-33.
  23. Moreira F, Badinga L, Burnley C, Thatcher WW. Superovulatory responses in cows receiving bovine somatotropin. J Anita Sci 2000; 78 (Suppl, I):214 abstr.
  24. Moreira F, Badinga L. Bumley C, Thatcher WW. Effects of bovine somatotropin on embryo transfer in lactating dairy cows. Theriogenology 2001; 55:367, abstr.
  25. Moreira F, de la Sota RL, Diaz T, Thatcher WW. Effect of day of the estrous cycle at the initiation of a timed artificial insemination protocol on reproductive responses of dairy heifers. J Anita Sci 2000;78:1568-1576.
  26. Moreira F, Orlandi C, Risco C, Lopes F, Mattos R, Thatacher WW. Pregnancy rates to a timed insemination in lactating dairy cows pre-synchronized and treataed with bovine somatotropin: cyclic versus anestrus cows. J Anita Sci 2000;78 Suppl 1 : 134 abstr / J Dairy Sci 2000;83 Suppll: 134 abstr.
  27. Moreira F, Risco C, Pires MFA, Ambrose JD, Drost M, DeLorenzo M, Thatcher WW. Effect of body condition on reproductive efficiency of lactating dairy cows receiving a timed insemination. Theriogenology 2000;53:1305-1319.
  28. Moreira F, Risco C, Pires MFA, Ambrose JD, Drost M, Thatcher WW. Use of bovine Somatotropin in lactating dairy cows receiving timed artificial insemination. J Dairy Sci 2000; 83:1245-1255.
  29. Pursley JR, Mee MO, Wiltbank MC. Synchronization of ovulation in dairy cows using PGF2(x and GnRH. Theriogenology 1995 ;44:915-923.
  30. Pursley JR, Wiltbank MC, Stevenson JS, Ottobre JS, Garverick HA, Anderson LL. Pregnancy rates per artificial insemination for cows and heifers inseminated at a synchronized ovulation or synchronized estrus. J Dairy Sci 1997;80:295-300.
  31. Quintal-Franco JA, Kojima FN, Melvin EJ, Lindsey BR, Zanella E, Fike KE, Wehrman ME, Clopton DT, Kinder JE. Corpus luteum development and function in cattle with episodic release of luteinizing hormone pulses inhibited in the follicular and early luteal phases of the estrus cycle. J Anim Sci 1999;61:921-926.
  32. Rajamahendran R, Ambrose JD, Schmitt EJ-P, Thatcher M-J, Thatcher WW. Effects of Buserelin injection and Deslorelin (GnRH-agonist) implants on plasma progesterone, LH, accessory CL formation, follicle and corpus luteum dynamics in Holstein cows. Theriogenology 1998;50:1141-1155.
  33. Risco CA, Drost M, Archbald L, Moreira F, de ia Sota RL, Burke J, Thatcher WW. Timed artificial insemination in dairy cattle-Part I. Compend Contin Educ Pract Vet 1998;20:280- 287.
  34. Risco CA, Moreira F, DeLorenzo M, Thatcher WW. Timed artificial insemination in dairy cattle-Part II. Compend Contin Educ Pratt Vet 1998;20:1284-1289.
  35. Ryan DP, Galvin JA, O’Farell KJ. Comparison of oestrous synchronization regimens for lactating dairy cows. Anita Reprod Sci 1999;56:153-168.
  36. Ryan DP, Snijders S, Yaakub H, O’Farrell KJ. An evaluation of estrus synchronization programs in reproductive management of dairy herds. J Anim Sci 1995;73:3687-3695.
  37. Santos, JEP, Thatcher WW, Pool L, Overton MW, Reynolds JP. Human chorionic gonadotropin influences the number of corpora iutea, plasma progesterone, and conception rates of dairy cows. 14th International Congress on Anita Reprod. Stockholm, Sweden, July 2-6, 2000;1:144 abstr.
  38. Schmitt EJ-P, Diaz T, Barros CM, de la Sota RL, Drost M, Fredriksson EW, Staples CR, Thomer R, Thatcher WW. Differential response of the luteal phase and fertility in cattle following ovulation of the first-wave follicle with human chorionic gonadotropin or an agonist of gonadotropin-releasing hormone. J Anim Sci 1996;74:1074-1083.
  39. Schmitt EJ-P, Diaz T, Drost M, Thatcher WW. Use of a gonadotropin releasing hormone agonist or human chorionic gonadotropin for timed insemination in cattle. J Anim Sci 1996;74: 1084-1091.
  40. Sianangama PC, Rajamahendran. R. Effect of human chorionic gonadotropin administered at specific times following breeding on milk progesterone and pregnancy in cows. Theriogenology 1992;38:85.
  41. Thatcher WW, Lopes FL, Pancarci SM, Drost M, Thatcher M-J, Belschner A. Pregnancy rates in dairy heifers following use of MGA with ECP in a timed insemination system. Theriogenology 2001 ;abstr. This Volume.
  42. Thatcher WW, Staples CR, Danet-Desnoyers G, Oldick B, Schmitt E-P. Embryo health and mortality in sheep and cattle. J Anim Sci 1994;72(Suppl 3): 16-30.
  43. Vasconcelos JL, Silcox MRW, Pursley JR, Wiltbank MC. Synchronization rate, size of the ovulatory follicle, and pregnancy rate after synchronization of ovulation beginning on different days of the estrous cycle in lactating dairy cows. Theriogenology 1999;52:1067-1078.
  44. Wood SL, Lucy MC, Smith MF, Patterson DJ. Estrus and fertility in yearling beef heifers after addition of GnRH to a melengestrol acetate (MGA)-prostaglandin F2~ estrus synchronization protocol. Theriogenology 2000; 54: 207, abstr.

Source: Department of Animal Sciences, IFAS1 and Large Animal Clinical Sciences,  College of Veterinary Medicine at University of Florida, Veterinary Medicine Teaching and Research Center at University of California-Davis
Author: W.W. Thatcher, F. Moreira, J.E.P.Santos, R.C. Mattos, F.L. Lopes, S.M. Pancarcil and C.A. Risco

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