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J. L. Yαniz
*Dept. de Producciσn Animal, Universidad de Lleida, Spain.
Dept. de Producciσn Animal, Universidad de Zaragoza, Spain.
Dept. Anatomνa y Embriologνa, Universidad Autσnoma
de Barcelona, Spain.
This study received financial support from the CTT of the University of Lleida (Grant C-0450 in collaboration with Schering Plough Animal Health, Spain).
KEY WORDS: prostaglandin, estrus synchronization, dairy cattle, update
Estrus synchronization programs used in dairy cattle mainly involve the use of the luteolytic agents prostaglandin or their analogues. Prostaglandin is able to synchronize estrus by inducing the regression of the corpus luteum. Because prostaglandin is only effective in diestrus cows, a double prostaglandin protocol applied 11 to 14 days apart seems to be capable of bringing most cows to estrus. The 14 day rather than 11 day interval gives rise to an improved conception rate, because most cows will be in the late luteal phase of the estrous cycle when they receive the second prostaglandin dose. The time of provoked estrus onset is affected by the estrous cycle stage at the time of prostaglandin treatment. Due to varying estrus-onset times, improved conception rates are obtained after AI at detected estrus rather than fixed-time AI in prostaglandin-treated cows. The administration of estradiol, hCG, or both after prostaglandin treatment improves the synchrony of estrus but does not enhance conception rate. Progesterone-based treatments for estrus synchronization are considered the most appropriate for noncyclic or anestrous postpartum dairy cows; prostaglandin alone is ineffective in these cows because of the absence of a mature corpus luteum. Improved estrus synchrony and fertility rate have been reported using short-term progesterone treatment regimes (79 days) with or without estradiol benzoate combined with the use of a luteolytic agent given 1 day before, or at the time of, progesterone withdrawal. The ovulation synchronization (Ovsynch) protocol, based on the use of GnRH and prostaglandin, was recently developed to coordinate follicular recruitment, CL regression, and the time of ovulation. This protocol allows fixed time insemination and has proven effective in improving reproductive management in postpartum dairy cows. However, it has not proven effective in heifers because of their inconsistent follicular wave pattern. The early luteal phase of the estrus cycle is considered optimal for initiation of the Ovsynch protocol in dairy cows. Presynchronization with prostaglandin to ensure the protocol is started in the early luteal phase has been found to improve pregnancy rates. To date, several prostaglandin-based, fixed-time insemination estrus synchronization protocols are available for use in early postpartum dairy cows with ovarian disorders. This report reviews the most recent developments in prostaglandin-based estrus synchronization programs for postpartum dairy cows and addresses the efficiency of controlled breeding protocols based on such developments for cows with an abnormal ovarian condition.
Increased milk production demands have led to a decline in the reproductive performance of dairy cows due to a prolonged intercalving period.1 Several factors, such as a longer time period from parturition to first estrus, poor estrus expression or detection, improper timing of artificial insemination, and reduced conception rate at first AI, have contributed to a longer intercalving period. In doing so, they have compromised profitability in dairy farming.2-6 The situation is further aggravated by the fact that high yielding early postpartum dairy cows often suffer from one or another ovarian disorder.7-9
Treatments aimed at synchronizing estrus or inducing ovulation allow for the effective management of timed AI in lactating dairy cows, without the need for detecting estrus. Most estrus synchronization protocols are mainly based on the use of the luteolytic agents, prostaglandins or their synthetic analogues.
The use of prostaglandin as a drug for estrus synchronization1017 or as a therapeutic agent1825 has been extensively reviewed in both dairy and beef cows. Furthermore, several recent reports of protocols have been published in which prostaglandins are combined with other hormones for therapeutic estrus synchronization in early postpartum dairy cows with an ovarian disorder.2629 We review these recent developments in prostaglandin-based protocols used to control the estrus cycle in dairy cattle, with special reference to lactating early postpartum dairy cows. Also discussed are prostaglandin-based controlled breeding protocols for cows showing an abnormal ovarian condition.
The luteolytic effect of prostaglandin F2a (PGF2a) in cattle was described by several workers in the early 1970s.3033 Subsequent research efforts thus attempted to improve the reproductive efficiency of dairy cattle by inducing estrus with PGF2a.3442 Several studies demonstrated the capacity of PGF2a and its synthetic analogues, alfaprostol,4347 cloprostenol,48,49 fenprostalene,50,51 and luprostiol5254 to trigger the regression of mature corpus lutea in the ovary, thus provoking and synchronizing estrus.35,5560 When PGF2a was administered to cows with a functionally mature corpus luteum, 85% to 95% reached estrus within 7 days of treatment;6164 70% to 90% showed signs of estrus 3 to 5 days after treatment.65
For PGF2a treatment to achieve its luteolytic effects, the cows must be in the diestrus stage of the estrous cycle (day 7 to 17). Prostaglandin treatment in the early stage of estrous cycle (first 5 days) was found to be ineffective in causing a luteolytic response in cattle.49,66 Consequently, a double protocol in which PGF2a was given at a 7, 11, or 14 day intervals was developed so that cows at a stage in the estrous cycle other than diestrus would have a functional corpus luteum when they received the second PGF2a dose.64,6769 Kristula et al.69 reported that weekly doses of PGF2a allowed AI to be performed earlier, because cows not in the diestrus stage when subjected to the first PGF2a injection were found to have a functional corpus luteum when the second PGF2a injection was given 7 days later. However, several authors report the improved reproductive efficiency of cows detected to be in estrus after the second PGF2a dose using the double regime in which PGF2a doses are given 11 or 14 days apart.63,65,70
Lucy et al.71 showed that when randomly cyclic cows were subjected to the double 14 day prostaglandin protocol, at least 67% of the cows had a corpus luteum at the time of treatment onset on day 7 to day 20 of their estrous cycle, which underwent luteolysis either spontaneously (cows on cycle day 18 to 20) or in response to PGF2a treatment (cows on cycle day 7 to 17). These cows would be on day 9 to 14 of the estrous cycle when the second PGF2a was administered 14 days later. The remaining cows (33%) that had not responded to the first PGF2a dose (cows on cycle day 0 to day 6) would be on day 14 to 20 of the estrous cycle when given the second PGF2a dose. Furthermore, an enhanced estrous response and normal fertility were reported when PGF2a was given at the late, rather than early to middle, stage of the luteal phase in dairy cows72 and in heifers.73,74 Thus, the 14-day interval double prostaglandin regimen seems to show an improved response over the 11 day protocol, because two injections given 14 days apart ensures that most animals are in the late luteal stage (cycle day 11 to 14) when they receive the second PGF2a dose.63,64,75
Recently,16 the successful use of a new estrus synchronization protocol for lactating dairy cows has been described, in which three PGF2a doses are given. In this protocol, known as the Targeted Breeding Program, all the animals that were not detected to be at estrus following the first PGF2a injection were treated with a further two doses of PGF2a at 14-day intervals until artificial insemination at detected estrus or until timed artificial insemination was performed 72 to 80 hours after the third PGF2a dose.
There have been many reports of different intervals to estrus and ovulation following prostaglandin treatment.7678 The time elapsed between PGF2a treatment and the onset of estrus depends on the stage of the estrous cycle at the time of PGF2a treatment61,73,7983. The mean interval to estrus was 48 to 72 h when PGF2a was administered on estrous cycle Day 5 or Day 8 in dairy cows.73,74 Prostaglandin administration in mid-cycle (day 8 to day 11) or later in the luteal phase (day 12 to 15) resulted in a mean time to estrus of 70 and 62 hours, respectively.81,82 There are also reports of higher progesterone concentrations at the time of prostaglandin administration being associated with a delayed onset of estrus.12
The stage of follicular wave development at the time of PGF2a treatment appears to be the factor determining the time of estrus onset.65,84,85 Kastelic and Ginther86 reported that the time from PGF2a administration to ovulation was dependent on the maturity and size of the most emergent dominant follicle, because a small dominant follicle takes longer to grow into an ovulatory follicle. When the dominant follicle had reached the static phase, the time from treatment to ovulation was 3 days, and if a new dominant follicle had emerged at the time of luteolysis, this time period increased to 4.5 days. Smith et al.87 reported that the onset of estrus was significantly and inversely related to the size of the cavity of the smallest follicle over 5 mm in diameter.
Several researchers have noted normal or above normal fertility following synchronization of estrus with PGF2a in cows.56,88,89 Young and Henderson90 found no significant difference in conception rates among cows inseminated at the fixed time of 75 to 80 hours (46%), after a double 11 day interval treatment regimen using a prostaglandin analogue, cows inseminated twice at 72 and at 96 hours (47%) after the same treatment and control untreated cows (50%). However, improved conception rates have been noted after AI at detected estrus compared with timed AI after prostaglandin administration, due to variations in the time of ovulation.9193 Reproductive performance in dairy cattle was also improved following double 14-day PGF2a treatment without assessing ovarian status when compared with a single dose based on detecting a corpus luteum by rectal palpation or by milk progesterone enzyme immunoassay.94 Tenhagen et al.95 noted that timed insemination after double 14-day prostaglandin treatment reduced the number of days open in lactating dairy cows when compared with AI performed at observed estrus. Finally, there have also been many reports of reduced fertility after double PGF2a treatment when artificial insemination was performed at detected estrus rather than normal estrus.72,88,96,97
There is considerable evidence that PGF2a is capable of improving the reproductive performance of dairy cows when given before the end of the voluntary waiting period.98,99 Administering PGF2a during the early postpartum period led to increased first service conception rates related to the associated benefits of enhancing uterine activity,100 thereby decreasing the interval between calving and conception.101103 However, others suggest that the diminished intercalving period may be an effect of luteolysis and an increased number of estrus cycles.102,104,105 In a meta-analysis, Burton and Lean106 explored the effects of prostaglandin given in the early postpartum on the subsequent reproductive performance of dairy cattle. Their pooled data corresponded to 21 independent trails performed on 2,646 cows described in 10 papers. Meta-analysis of the effect of prostaglandin treatment during the early postpartum period revealed no increase in pregnancy rate to first artificial insemination in cows with a normal or abnormal puerperium, while the period from calving to first AI was significantly reduced, thus reducing the number of days open in the dairy farm. These results were, however, not considered conclusive by the authors.
One of the major limitations of the use of prostaglandins to synchronize estrus in dairy cows is the failure of the drug in anestrus or noncyclic cows.60 Progestogens have the advantage that, besides improving estrus synchronization, they also induce estrus and ovulation in an acceptable percentage of anestrus cows.107110 Several works have shown that the estrous cycle in cows can be controlled by prolonging the luteal phase or establishing an artificial luteal phase by the administration of exogenous progesterone or synthetic progestogens10,12, because progesterone suppresses estrus and ovulation by inhibiting the release of luteinizing hormone, impeding the final maturation of follicles.111
Long-term progesterone treatment (14 to 16 days) leads to reduced fertility,112 probably due to development of persistent follicles and reduced oocyte competence.113,114 Lane et al.115 recommended prostaglandin administration when short-duration (79 days) progesterone treatments were started in the early or mid cycle, because the proportion of animals requiring exogenous luteolysis induction increases during this period. Indeed, short-term progesterone treatment using progesterone releasing intravaginal devices or subcutaneous ear implants combined with treatment with a luteolytic agent has proved successful in cattle.39,77,116120
Pregnancy rates equal to or greater than control rates for cows in natural estrus were achieved when progesterone releasing devices were used in conjunction with prostaglandin F2a or one of its analogues.39,116,117,121124 Several reports claim an improved response to estrus synchronization treatment when prostaglandin is administered 48 hours after intravaginal progesterone device removal in Bos taurus125129 and Bos indicus cattle.130131 Using a progesterone-releasing intravaginal device (PRID) prostaglandin procedure, the conception rate was reported to be higher when PRID was inserted in the early (day 1 to 10) rather than late (day 11 to 20) stage of the estrous cycle.132
When comparing the efficiency of prostaglandin treatment alone with that of combined progestin-prostaglandin treatment aimed at controlling estrus cycles in dairy cows, Chupin et al.133 found that combined treatment was more effective than prostaglandin alone in bringing more cows into estrus during the first 96 hours after the end of treatment. Similarly, Gyawu et al.126 observed that the progesterone/prostaglandin combination was more effective in synchronizing ovulation compared with prostaglandin alone. Several authors have also reported increased synchronization rates and fertility after progesterone plus prostaglandin treatment.77,125,134,135 Finally, Mialot et al.136 noted increased reproductive efficiency in cattle when prostaglandin instead of eCG was given 48 hours after PRID removal.
Ryan et al.114 reported that administering GnRH was more effective than giving estradiol benzoate at the start of a progesterone-prostaglandin regime in dairy cows. In contrast, Lane et al.115 reported that 0.75 mg of estradiol benzoate administered at the start of 8 days of progesterone treatment, with prostaglandin given one day before progesterone withdrawal, was more effective than GnRH for synchronizing estrus in heifers. Similarly, synchrony of estrus in dairy heifers sufficient for fixed time insemination was achieved using a protocol that involved the use of a progesterone controlled intravaginal drug releasing device (CIDR) for 10 days, a 10-mg estradiol benzoate capsule delivered at the time of device insertion, and prostaglandin administered 4 days before device removal.120,137,138 In a study undertaken during the AI breeding period in lactating dairy cows, pregnancy rates were higher among cows synchronized with GnRH and a progesterone CIDR followed 7 days later by PGF2a treatment, and device removal 1 day after or at the time of prostaglandin treatment, compared with control unsynchronized cows.139
Use of Estrogen and Human
In the cow, estrogens are known to induce a preovulatory-like LH surge, ovulation,140 and luteolytic activity during the luteal phase.141 These effects could justify the inclusion of estradiol in the different synchronization regimens. Indeed, as noted previously, progestogen-estrogen combinations are widely used. Although synchronizing estrus using prostaglandins, several authors successfully synchronized ovulation by administering estradiol benzoate after prostaglandin treatment in cows142,143 and in heifers.144 A tighter synchrony of estrus with no effect on the conception rate was reported after treating dairy cows with 400 mg of estradiol benzoate 40 to 48 hours after prostaglandin treatment.145 An estrogen-prostaglandin combination protocol for synchronization of estrus was also found to increase the percentage of cows in estrus.146
The hormone hCG induces potent LH activity in ovarian cells, which can even lead to ovulation throughout the estrous cycle.147 The simultaneous administration of hCG and estradiol benzoate 12 hours after treatment with prostaglandins in dairy cows and heifers with mature CL has been reported to shorten the mean time to onset of estrus and increase the precision of synchrony in ovulation. Using this protocol, comparable pregnancy rates were achieved after fixed-time insemination to those recorded when cows were treated with prostaglandin alone148149 or inseminated at natural estrus.150151
The random administration of GnRH during the estrous cycle results in LH release,152 causes ovulation or luteinization of large follicles present in the ovary, synchronizes the recruitment of a new follicular wave,119,153 and equalizes follicle development waves.84,119,153155 Subsequent administration of PGF2a induces the regression of an original or GnRH-induced CL, and allows final maturation of the synchronized dominant follicle.156 Furthermore, there is no apparent detrimental effect of GnRH on the responsiveness of GnRH-induced CL or spontaneous CL to prostaglandin.85
Several reports84,153,157 have described a higher rate of estrus synchronization when GnRH is administered 6 or 7 days before PGF2a (80%) compared with prostaglandin alone (50% to 60%). However, LeBlanc et al.158 reported no advantage of adding GnRH on day 7 of a synchronization program based on double prostaglandin treatment given at a 14-day interval. Similarly, Stevenson et al.159 described a decreased conception rate (48.1%) when GnRH was administered between two PGF2a doses given 14 days apart compared with not including GnRH in the protocol (63.5%).
Pursley et al.160 observed a mean reduction of 27 days to first AI with a voluntary waiting period of 50 days after a GnRH-PGF2a regimen. The same regimen was found to fail to induce estrus in some cows due to incomplete luteolysis after prostaglandin treatment161 or because of differences in pituitary LH release at the time of treatment.162 De Rensis et al.162 compared the effects of intramuscularly administering 2000 IU of hCG 6 or 9 days before prostaglandin treatment and noted that the GnRH- and hCG-prostaglandin combinations led to similar estrus synchronization, treatment efficiency and conception rates in postpartum dairy cows.
To synchronize ovulation within a short time period and enable timed insemination in the GnRH-prostaglandin regime, an additional GnRH dose was included 24,163 48,160 54,85 and 60 hours164 after prostaglandin treatment. The effectiveness of the second dose of GnRH 48 hours after prostaglandin treatment in synchronizing the timing of ovulation has been established for dairy and beef cows.165,166 A second dose of GnRH given 48 hours after PGF2a injection improves the precision of ovulation over an 8-hour period from 24 to 32 hours after this second GnRH dose. The success of this addition to the standard combined GnRH-prostaglandin regime in dairy cattle gave rise to the recently developed Ovsynch or timed artificial insemination (TAI) protocol, which allows successful fixed-time AI without the need for estrus detection.160
In the Ovsynch program, 100 ag of GnRH are given at random during the estrous cycle, followed by 25 mg of PGF2a and a second dose of 100 ag GnRH.160 Ovulation is synchronized because the preovulatory follicles are at a similar stage in development and are responsive to LH at the time of the second GnRH treatment. This program coordinates follicular recruitment, CL regression, and time of ovulation and permits fixed time AI 16 hours after the second GnRH dose is administered. Thus by synchronizing ovulation, reproduction in lactating dairy cows can be effectively managed without the need for estrus detection.167 There have also been reports168 of fertile ovulation in anestrus cows after the Ovsynch program attributable to the incorporation of GnRH.169 Recently, the Ovsynch protocol has been slightly modified so that the second GnRH dose is given 36 instead of 48 hours after prostaglandin treatment.16,170 Pursley et al.171 concluded that AI performed close to 16 hours after the second dose of GnRH in the Ovsynch protocol seems to be optimal, though pregnancy rates per AI and calving rates are comparable to rates achieved after AI performed 0 to 24 hours after the second GnRH dose. Fricke et al.172 and Yamada et al.173 reported that the reproductive performance of dairy cattle is not affected when the GnRH dose is reduced to half (50 ΅g instead of 100 ΅g) in the Ovsynch protocol.
The success of the Ovsynch program has been proven to be influenced by the number of follicular waves or length of the follicular wave,170 as well as the stage of estrous cycle when the first GnRH dose is administered.174176 Moreira et al.176 concluded that the early luteal stage of the estrous cycle (day 5 to 12) was the optimal period for initiating the Ovsynch program. Vasconcelos et al.174 also recorded a higher pregnancy rate when cows were started on the Ovsynch protocol in the early luteal phase compared with the first 3 days or after day 13 of the estrous cycle. These findings are inconsistent with those of Keister et al.,177 who noted similar reproductive performance in dairy cattle whether Ovsynch treatment was initiated at random or on day 7 of the estrous cycle.
Based on the reports that the luteal phase was the optimal time of Ovsynch protocol onset in terms of conception rates, Moreira et al.178 presynchronized cows using two prostaglandin doses given 14 days apart to initiate the Ovsynch protocol at the targeted early luteal phase. Presynchronization was found to increase the pregnancy rate in cyclic lactating dairy cows. Similarly, pregnancy rates in dairy cows were improved when Ovsynch was started on day 12179 or day 14180 after prostaglandin administration, because most cows would be in early diestrus before the beginning of the Ovsynch protocol. However, no beneficial effects were shown by presynchronization before Ovsynch in anestrous cows, given their lack of prostaglandin responsive CL.178
Although many workers168,181184 have reported increased pregnancy rates in cows subjected to Ovsynch treatment, this increase has not been paralleled by conception rates because of the greater number of cows insemination after Ovsynch treatment.157,159. When Burke et al.181 compared the effectiveness of timed-AI versus AI at detected estrus after Ovsynch without administering the second GnRH dose in multiparous animals, they recorded higher conception rates in cows undergoing AI at detected estrus. However, pregnancy rates were similar in both groups. These authors also noticed a mean reduction to first AI in the timed AI program of 9.7 days, compared with AI at detected estrus and a 60-day voluntary waiting period. DeJarnette et al.185 suggested that pregnancy rates in the Ovsynch protocol can be maximized by improving estrus detection, because 20% of the cows display estrus outside the optimal time period for conception by TAI.
Timed AI following the Ovsynch protocol is advocated by authors such as Burke et al.181, Yamada et al.186 and Momcilovic et al.187 as an effective tool for improving reproductive management in dairy cows, since it avoids the need for estrus detection. In heifers, however, timed AI following Ovsynch seems to have no beneficial effects due to an inconsistent follicular wave pattern.170 Lactation stage has also been shown to affect pregnancy rates following Ovsynch in dairy cows, since cows started on the Ovsynch program 76 days after calving show improved pregnancy rates over those initiating Ovsynch between 60 and 75 days postpartum.170
Xu and Burton188 and Xu et al.189 suggested that the reproductive performance of cows undergoing Ovsynch treatment could be improved by administering progesterone during the period between GnRH and prostaglandin treatment. The rationale for this is that progesterone can prevent premature ovulation after spontaneous luteolysis during the treatment period in a small proportion of cows whose dominant follicles fail to respond to GnRH.84,174,190
Prostaglandin Based Estrus Synchronization Protocols for Cows With Ovarian Disorders
It has been recently possible to achieve estrus synchronization and acceptable pregnancy rates in dairy cows with different ovarian disorders detected during the early postpartum period, using various prostaglandin-based protocols in combination with progesterone and GnRH.
Progesterone was included in a GnRH-prostaglandin-GnRH protocol for the treatment of abnormal ovarian conditions in postpartum dairy cows. Following the treatment regime: progesterone for 9 days, GnRH on day 0, and PGF2a on D 7, it was possible to successfully synchronize dairy cows with ovarian cysts during the postpartum period.191 Using the Ovsynch protocol as a therapeutic strategy for ovarian cysts, Bartolome et al.26 recorded similar pregnancy rates in response to timed insemination in cows with and without cysts. Further, Lσpez-Gatius and Lσpez-Bιjar29 successfully synchronized and time-inseminated lactating dairy cows with ovarian cysts using a protocol that combines GnRH and cloprostenol, starting treatment by simultaneously administering GnRH and cloprostenol. Pursley et al.28 observed that anovulatory cows fitted with an intravaginal progesterone device (CIRD) in the period between GnRH and PGF2a administration of the Ovsynch protocol showed higher pregnancy rates (55.2%) than anovulatory cows subjected to Ovsynch without a CIRD (34.7%). In another study, Lσpez-Gatius et al.27 were also able to successfully synchronize and time-inseminate lactating dairy cows with persistent follicles using a progesterone-GnRH-PGF2a treatment regimen.
In conclusion, prostaglandin and its analogues are the main components of different hormone combination protocols used to synchronize estrus or ovulation, giving rise to acceptable pregnancy rates in dairy cows. Prostaglandin alone or in combination with different hormones has proved to be efficient at synchronizing estrus in postpartum dairy cows, improving reproductive efficiency in dairy farms. Future investigations should be directed toward developing cost-effective prostaglandin-based timed-insemination protocols that lead to a high synchrony of ovulation and thus improve pregnancy rates, especially in early postpartum noncyclic or anovulatory dairy cows. Future developments will no doubt serve to improve reproductive performance in dairy farms by reducing the intercalving period to the minimum.
The authors thank Ana Burton for assistance with the English translation.
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