A review of current timed-AI (TAI) programs for beef and dairy cattle

Abstract

This is a review of the physiology and endocrinology of the estrous cycle and how ovarian physiology can be manipulated and controlled for timed artificial insemination (TAI) in beef and dairy cattle. Estrus detection is required for artificial insemination (AI), but it is done poorly in dairy cattle and it is difficult in beef cattle. Protocols that synchronize follicle growth, corpus luteum regression and ovulation, allowing for TAI, result in improved reproductive performance, because all animals are inseminated whether they show estrus or not. As result, TAI programs have become an integral part of reproductive management in many dairy herds and offer beef producers the opportunity to incorporate AI into their herds. Gonadotropin-releasing hormone-based protocols are commonly used in North America for estrus synchronization as part of a TAI program. Protocols that increase pregnancy rates in lactating dairy cows and suckling beef cows have been developed. Protocols that improve pregnancy rates in heifers, acyclic beef cows, and resynchronized lactating dairy cows are also discussed.

Introduction

Almost 80% of dairy producers in North America use artificial insemination (AI) to breed cattle, compared to only 4% of beef producers (1). One of the main reasons for the low adoption rate in beef cattle is that estrus detection required for AI is labour intensive, time consuming, and subject to error. In addition, the efficiency of estrus detection is ≤ 50% in dairy herds across North America, mainly because it is subject to animal, human, and environmental influences. Poor or inadequate estrus detection is the major cause of low insemination risk and reproductive inefficiency in dairy herds. Timed artificial insemination (TAI) protocols that synchronize follicle growth, corpus luteum (CL) regression and ovulation, result in improved reproductive performance, because all animals are inseminated whether they show estrus or not. The TAI programs have become an integral part of reproductive management in many dairy herds and offer beef producers the opportunity to incorporate AI into their herds. This manuscript reviews the physiology and endocrinology of the estrous cycle and provides an overview of how ovarian physiology can be manipulated and controlled for current TAI protocols for beef and dairy cattle.

Artificial control of the estrous cycle

Since reproductive parameters have relatively low heritability, the application of TAI within a reproductive health program deserves exploration to obtain pregnancies over a short interval. Declining efficiency of estrus detection, in particular, may explain the increased interest in the use of controlled breeding programs. In general, these programs are designed to assist in obtaining pregnancies in groups of cows, primarily by restricting the intervals during which estrus detection needs to be done, or by eliminating the need for estrus detection. Although there are numerous controlled breeding protocols in beef and dairy herds, a thorough understanding of estrous cycle physiology and follicular wave dynamics in particular, is necessary before one attempts manipulation of the estrous cycle. It should be noted that not all of the pharmaceuticals described are approved for use in lactating dairy cattle.

Follicle wave dynamics

Each estrous cycle in cattle has either 2 or 3 follicular waves, which consist of a group of growing antral follicles of ≥ 4 mm in diameter from which a dominant follicle is selected, while the remaining follicles become subordinate and undergo atresia (2,3). In both 2- and 3-wave estrous cycles, emergence of the first follicular wave occurs on the day of ovulation (Day 0; 1 day after estrus), whereas the second wave emerges on Day 9 or 10 in 2-wave cycles, and on Day 8 or 9 in 3-wave cycles, with a third wave emerging on Day 15 or 16. Duration of the estrous cycle is approximately 20 days in 2-wave cycles and 23 days in 3-wave cycles. The dominant follicle present at the time of luteolysis will become the ovulatory follicle, and emergence of the next wave is delayed until the ensuing ovulation (4). Follicular wave dynamics and duration of the estrous cycle may be slightly different in high-producing dairy cows (5). The numbers of animals with 2- versus 3-wave cycles are more or less equally distributed in beef cattle, whereas more 2-wave cycles have been reported in lactating dairy cattle (5).There appears to be no clear breed- or age-specific preference for one follicular wave pattern over the other, nor is there any apparent difference in fertility. Two studies suggested that conception rate to first service is reduced in beef and dairy cattle in which the ovulatory follicle came from the second compared with the third follicular wave during estrous cycle (6,7). However, in 2 other studies, pregnancy rate did not differ among lactating dairy cows with different wave patterns of follicle development (8) or in beef cattle in which the length of progesterone exposure of the ovulatory follicle was investigated (9).

Recruitment of follicular waves and selection of a dominant follicle is based on differential responsiveness to follicle stimulating hormone (FSH) and luteinizing hormone [LH, (10)]. Surges in plasma FSH elicit emergence of a follicular wave (11); FSH is subsequently suppressed by products of the growing follicles (i.e., estradiol and inhibin). As a result, FSH is declining when growth profiles of the dominant and subordinate follicles begin to diverge (time of selection), approximately 3 d after wave emergence (3). Within each wave, the follicle that becomes the dominant follicle is also the first to acquire LH receptors while subordinates which still require FSH to maintain growth undergo atresia (12). The dominant follicle grows for approximately 6 d and then enters a static phase. Suppression of LH as a consequence of progesterone secretion by the CL causes the dominant follicle to cease its metabolic functions after 2 or 3 d, and it begins to regress and FSH surges again (13). This FSH surge has no effect on the dying dominant follicle, but is responsible for eliciting emergence of the next wave. The ovarian cycle then repeats itself. Luteal regression allows LH pulse frequency to increase; the dominant follicle increases its growth rate and produces higher concentrations of estradiol which act in a positive feedback on the hypothalamo-pituitary axis, resulting in a surge of LH followed by ovulation. It is clear that follicle wave dynamics can have a profound effect on the efficacy of estrus synchronization programs.

Treatments that synchronize dominant follicle growth and ovulation

Treatments that synchronize follicle growth are generally divided into those that are estradiol-based and those that are gonadotropin-releasing hormone (GnRH)-based.

Estradiol-based protocols

Estradiol treatments are used to synchronize follicle wave emergence and ovulation in beef cattle in South America, Mexico, and Canada (by prescription). The combination of estradiol and progesterone suppresses FSH and LH release and antral follicle growth; estradiol alone (during a time of low circulating progesterone) stimulates LH release, ovulation, and/or luteinization of ovarian follices. Once the estradiol is metabolized FSH surges and a new wave of follicle development emerges. The protocol consists of insertion of a progestin-releasing device (PD) on Day 0 and administration of 2.5 to 5 mg estradiol-17β or 2 mg estradiol benzoate (to synchronize follicular wave emergence), and prostaglandin F2α (PGF) at the time of removal of the progestin device 7 or 8 d later (to ensure luteolysis). A lower dose of estradiol (usually 1 mg) is given 24 h after progestin removal to induce and synchronize an LH surge (~16 to 18 h after treatment) and ovulation [~24 to 32 h later (14)]. Timed AI is typically done 30 to 34 h after the second estradiol treatment. Alternatively, 0.5 mg of estradiol cypionate may be given at progestin removal (15) so animals are handled only 3 times, and TAI is performed 48 to 56 h later. Estradiol is no longer available for use in food producing animals in many countries, including the USA, so most of the remaining discussion will concentrate on alternative protocols.

Gonadotropin-releasing hormone (GnRH)-based protocols

Protocols based on GnRH have been used extensively for TAI in dairy (16) and beef cattle (17) in USA and Canada. In cattle with a growing dominant follicle (> 10 mm in diameter), treatment with GnRH induces LH release and ovulation, with emergence of a new follicular wave approximately 2 d later (18). Treatment with PGF 6 (19) or 7 (16) days after GnRH results in ovulation of the new dominant follicle, especially when a second GnRH injection is given 36 to 48 h after PGF (20). The TAI is recommended at 16 to 20 h after the second GnRH (21). The protocol that utilizes GnRH and PGF for TAI in dairy cattle has been called Ovsynch [(16) Table 1]. Cosynch is a modification of the 7-day Ovsynch protocol in which the second GnRH treatment is given concurrently with TAI [(17) Table 2]. Cosynch protocols are more commonly used in beef cattle because they require animal handling only 3 times. Most TAI protocols, utilized in Canada and USA are variations on the Ovsynch protocol.

Table 1.

Daily injection schedule of the Ovsynch protocol [adapted from (16)]

Monday	Wednesday	Thursday
AM-GnRH
AM-PGF	PM-GnRH	AM-AI

PGF — prostaglandin F2α, GnRH — gonadotropin-releasing hormone, AI — artificial insemination.

Table 2.

Daily injection schedule of the Cosynch protocol [adapted from (17)]

Monday	Wednesday
AM-GnRH
AM-PGF	PM-GnRH & AI

PGF — prostaglandin F2α, GnRH — gonadotropin-releasing hormone, AI — artificial insemination.

Protocols for synchronization of estrus and ovulation with GnRH in beef cattle are listed by the Beef Cattle Reproduction Leadership Team. For additional information, the reader is referred to http://beefrepro.unl.edu/resources.html. Several of these protocols will be discussed in this manuscript.

Ovynch protocols in lactating dairy cattle have resulted in pregnancy rates (defined as the number of pregnant cows over the number treated or eligible) similar to those obtained after AI with estrus detection (22,23). However, conception rate (defined as the number of pregnant cows over the number inseminated) is usually lower in Ovsynch-treated cows because ovulation is not adequately synchronized in approximately one-third of the animals. In cows treated with the Ovsynch protocol 11% ovulated before TAI, 15% did not respond to treatment with PGF, and 9% did not ovulate after the second treatment with GnRH (24), indicating that synchronization rate (defined as cows that had a regressed CL and ovulated within 24 h after TAI) was only 68%. Stage of the estrous cycle when a GnRH-based protocol is initiated affects synchronization and pregnancy rates (25,26). Cattle in which GnRH was administered between Days 1 and 4 or Days 14 and 21 of the cycle had lower pregnancy rates than those treated at other times [32% versus 42%, respectively (25)]. When GnRH is administered during metestrus (Days 1 to 3), the dominant follicle may not ovulate, and begins to undergo atresia at the approximate time that the PGF is injected. The dominant follicle of the second wave (Days 13 to 17) may also not ovulate in response to the first GnRH treatment, and in the absence of ovulation, endogenous PGF may cause luteolysis and ovulation before TAI, resulting in low pregnancy rates. Therefore, estrus detection and AI of cattle that show estrus early, or insertion of a PD between the first GnRH and PGF, is often done to improve pregnancy rates (27–29).

Improving the response to GnRH-based protocols

Presynchronization with PGF

Presynchronization with PGF is commonly used in dairy herds to ensure that cows are at the most appropriate stage of the estrous cycle at the time of the first GnRH treatment. The goal is to have most of the animals between Days 5 and 12 of the estrous cycle. Pre-synchronization with 2 doses of PGF, 14 d apart, and administration of the first GnRH 12 d after the second PGF increase the probability that a LH-responsive follicle will be present at the time of the first GnRH (Table 3). In 2 studies (30,31), pregnancy rate following TAI was higher in cows treated with the “Presynch-Ovsynch” than in those treated with Ovsynch alone (49% versus 37%; 47% versus 38%, P < 0.01).

Table 3.

Daily injection schedule of a presynchronization protocol with 2 PGF injections (14 days apart) and initiation of Ovsynch 12 days after the second PGF [adapted from (28)]

Monday	Wednesday	Thursday
	PM-PGF
	PM-PGF
AM-GnRH
AM-PGF	PM-GnRH	AM-AI

PGF — prostaglandin F2α, GnRH — gonadotropin-releasing hormone, AI — artificial insemination.

The effect of varying the interval between the second PGF of Presynch and initiation of Ovsynch on pregnancy rate in lactating dairy cows has been examined. Although an interval of 12 d between the second PGF and the first GnRH improved pregnancy rate by 10% to 12% (30,31), the dairy industry adopted an interval of 14 d so that all treatments are done on the same days of the week. Recently, Galvão et al (32) reported that a reduction in the interval between Presynch and the first GnRH from 14 to 11 d increased the percentage of animals ovulating to the first GnRH (61% versus 45%, P < 0.01) and consequently pregnancy rate (41% versus 34%, P < 0.05). However, the increase in the proportion of animals ovulating to the first GnRH was observed only in those animals that were cycling. In another study (33), a 12-day interval from last PGF of Presynch to the first GnRH resulted in a numerically higher pregnancy rate (37%) at 32 d after TAI than 14 d (32%) or 10 d (35%), or Control (no presynchronization; 34%). We recently compared ovarian response (i.e., ovulatory response to first GnRH and overall synchronization rate and pregnancy rate) in 241 cycling, lactating dairy cows subjected to the Ovsynch protocol initiated either 9 or 12 d after the second PGF of the Presynch protocol (34). Ovarian responses were determined by plasma progesterone concentrations and transrectal ultrasonography. Cows that had plasma progesterone < 0.5 ng/mL at TAI (regressed CL) and ovulated within 24 h after second GnRH treatment were considered “synchronized.” The percentage of cows that ovulated after the first GnRH (62%) initiated either 9 or 12 d after the second PGF of the Presynch protocol did not differ. However, reducing the interval from 12 to 9 d reduced the overall synchronization rate (73% versus 61%) and pregnancy rate at 32 d (44% versus 34%) and 60 d (43% versus 32%) after TAI.

We have investigated the effects of presynchronization with PGF prior to a Cosynch protocol on estrus synchrony, and pregnancy rate following TAI in beef heifers (35). Presynchronization reduced the proportion of heifers in estrus before TAI (3% versus 24%), suggesting that this may be useful in the successful application of GnRH-based protocols in beef heifers. However, pregnancy rate to TAI was not affected statistically (38% versus 30%), perhaps due to insufficient statistical power. In another study, we investigated whether presynchronization with PGF would increase pregnancy rate in Angus heifers (n = 462) given a PD and 12.5 mg porcine LH (pLH) or 100 μg GnRH to synchronize follicle wave emergence and ovulation (36). Presynchronization with PGF tended to increase ovulation rate to first treatment (64% versus 47%, P < 0.09) but did not affect pregnancy rate (60% versus 54%).

Although presynchronization with PGF appears to be effective in cyclic, lactating dairy cows, benefits in beef heifers are far less obvious. It is assumed that acyclic cattle are also unlikely to benefit from a presynchronization with PGF as they do not have a CL, albeit a recent study has indicated that PGF hastened first ovulation in pre-pubertal beef heifers (37). Other presynchronization protocols that include GnRH or a PD have been proposed in dairy or beef herds with a high incidence of acyclic cows.

Presynchronization with PGF and GnRH

Bello et al (38) developed a novel presynchronization protocol that combines PGF and GnRH. The aim of this protocol was to increase the percentage of animals that respond to the first GnRH injection of the Ovsynch protocol by increasing the probability of an ovulatory-sized follicle at that time. A total of 137 lactating dairy cows were allocated to 4 groups; 1 group received no presynchronization treatment before Ovsynch (Control), whereas the other 3 groups were treated with PGF, followed 2 d later with GnRH administered at 4, 5, or 6 d before the first GnRH in the Ovsynch protocol. The percentages of animals that ovulated following the first GnRH of the Ovsynch protocol were 54%, 56%, 67%, and 85%, respectively, and pregnancy per AI tended to be higher in those animals in which the Ovsynch was initiated at 6 d after presynchronization with PGF and GnRH (G6G; Table 4) than in Controls (50% versus 27%, P < 0.08). More recently, the G6G presynchronization protocol was evaluated in a larger number of animals (39), resulting in pregnancy rates that were similar to those reported by Bello et al (38). However, the pregnancy rate with the G6G presynchronization protocol did not differ from that with the PGF-based Presynch-Ovsynch protocol [50% versus 49%, respectively (39)].

Table 4.

Daily injection schedule of a presynchronization protocol with PGF and GnRH and initiation of Ovsynch 6 days after the GnRH of Presynch (G6G) [adapted from (35)]

Monday	Tuesday	Wednesday	Thursday	Friday
AM-PGF		AM-GnRH
	AM-GnRH
	AM-PGF		PM-GnRH	AM-AI

PGF — prostaglandin F2α, GnRH — gonadotropin-releasing hormone, AI — artificial insemination.

Double Ovsynch protocols

Double Ovsynch, another presynchronization protocol that includes PGF and GnRH [(40), Table 5], involves 2 Ovsynch protocols following one after the other, with the third GnRH treatment administered 7 d after the second. In the first study reported, Double Ovsynch resulted in a higher pregnancy rate than the Presynch-Ovsynch protocol in primiparous (65% versus 45%, P < 0.05) but not multiparous [38% versus 40% (40)] cows. The improved pregnancy rate was probably due to the increased probability of a dominant follicle that would ovulate following the third GnRH, and the elevated circulating progesterone concentrations prior to the administration of PGF (41).

Table 5.

Daily injection schedule of the Double Ovsynch protocol [adapted from (37)]

Monday	Wednesday	Thursday	Friday
			AM-GnRH
			AM-PGF
AM-GnRH
AM-GnRH
AM-PGF	PM-GnRH	AM-AI

PGF — prostaglandin F2α, GnRH — gonadotropin-releasing hormone, AI — artificial insemination.

Progesterone and progestins

Progesterone alters ovarian function in cattle by suppressing estrus and preventing ovulation by blocking LH release (42). Progesterone also suppresses LH pulse frequency (43) which causes reduced growth of the dominant follicle in a dose-dependent fashion. However, progesterone does not suppress FSH secretion (12,13), and follicular waves continue to emerge in the presence of a functional CL. Progestins (synthetic and natural progesterone) given for longer than the CL life-span (i.e., for > 14 d), result in synchronous estrus upon withdrawal, but fertility is low. Progestins used to control the estrous cycle in cattle have less suppressive effects on LH secretion than the CL-secreted progesterone and are associated with high LH pulse frequency which causes continued growth of the dominant follicle [persistent follicles (44)] and aged oocytes which result in low fertilization rates and early embryonic death (45).

Progesterone-impregnated intravaginal devices (PD) are now available in most countries (including Canada). Label directions for AI usually state that the device should be in the vagina for 6 or 7 d; PGF is given 24 h before or at the time of device removal, and estrus detection begins 48 h later. Because of the short treatment period, the incidence of persistent follicles is reduced. Progestin devices can be used as a presynchronization strategy or during the TAI protocol. Progestin treatment also induces cyclicity by increasing LH pulsatility in postpartum cows and pubertal heifers (46), and there are several other reports regarding the use of progestin device treatment protocols for acyclic cows (47–51).

Presynchronization with progesterone

We examined the effects of presynchronization with a PD on follicle size, ovulation rate to the first injection of GnRH, and pregnancy rate in lactating beef cows subjected to a Cosynch protocol (52). Presynchronization with a PD for 7 (plus PGF) or 15 d (without PGF) increased the proportion of cows ovulating to the first GnRH treatment (77% versus 55% or 75% versus 49%, respectively). However, the effect of presynchronization on pregnancy rate was influenced by parity. Presynchronization with a PD for 7 d plus PGF increased pregnancy rate from 40% to 64% (P < 0.01) in primiparous cows, but did not affect pregnancy rate in multiparous cows.

Supplementing progesterone before AI in cattle

The effects of progesterone secretion on embryonic development and fertility in cattle have been reviewed (53). One of the causes of reduced fertility, particularly in high producing dairy cows, is inadequate circulating progesterone concentration. A simple and practical approach to supplement progesterone is via an intravaginal device.

Two early studies showed that the use of a PD prior to the second injection of PGF in dairy cows synchronized with 2 injections of PGF 14 d apart increased fertility. Folman et al (54) reported that more (P < 0.05) cows became pregnant to AI when they received a PD for 8 d before the second injection of PGF (66%) than when they were inseminated after a single treatment with PGF (39%). In another study in pasture-based dairy cows (55), 593 cows were treated with 2 doses of PGF separated by 14 d, followed by estrus detection and AI, or 608 cows were treated with a PD 5 d prior to the second PGF treatment. Progesterone supplementation before the second injection of PGF increased the expression of estrus (90% versus 83%) and pregnancy rate (65% versus 60%).

Supplementing progesterone in GnRH-based protocols

The emergence of a new follicular wave is synchronized only when GnRH treatment causes ovulation (18). If the first GnRH does not synchronize follicular wave emergence, ovulation following the second GnRH may be poorly synchronized (28), resulting in disappointing pregnancy rates following TAI (29). Also, ~20% of heifers show estrus before the injection of PGF, potentially reducing fertility to TAI (56,57). Prevention of the early ovulations by addition of a PD to a 7-day GnRH-based protocol improved pregnancy rates after TAI in beef heifers (28,29) and cows (27). A GnRH-based protocol with the addition of a PD is illustrated in Figure 1.

Figure 1.

A 7-day Ovsynch protocol that includes a progestin device between first GnRH and PGF treatments. PGF — prostaglandin F2α, GnRH — gonadotropin-releasing hormone, AI — artificial insemination.

Several studies have reported that supplementation with exogenous progesterone during the Ovsynch protocol may increase pregnancy rates in lactating dairy cows. El-Zarkouny et al (31) compared pregnancy rates in 182 lactating cows treated with the Ovsynch protocol, with or without a PD. Based on serum progesterone concentrations, only 44% of the cows were cycling at the time of initiation of treatment. Pregnancy rates per AI at 29 d (59% versus 36%) and 57 d (45% versus 20%) after TAI were greater in cows treated with Ovsynch plus a PD than in those treated with Ovsynch alone. Fertility was particularly enhanced in acyclic animals (pregnancy rate of 64% versus 27%). In a second experiment by the same authors, in a dairy herd with 80% of cows cycling at the beginning of treatment, adding a PD did not improve pregnancy rate (43% versus 32% in Ovsynch and Ovsynch plus PD groups, respectively).

The benefit of supplementing progesterone in lactating dairy cows subjected to an Ovsynch protocol was also demonstrated in a large study in Mexico (58). Cows received 2 treatments of PGF 14 d apart and were inseminated after the second PGF following estrus detection. Those not detected in estrus (n = 1175) by 12 d after PGF were allocated into 2 groups: 1 group was subjected to the Ovsynch plus PD protocol and the other group to Ovsynch only. Although the proportion of acyclic cows was not determined, it could be assumed that a vast majority of the cows in this study were non-cycling because they were not detected in estrus after the second PGF treatment. A comparable percentage of animals (89% and 95%) with plasma progesterone > 1 ng/mL 14 d after TAI suggests that both TAI protocols resulted in a similar response and induction of cyclicity. However, pregnancy rate was greater in cows subjected to Ovsynch plus PD protocol (31% versus 23%).

Most recently, Colazo et al (59) determined the efficacy of a PD in a 7-day Ovsynch protocol with or without presynchronization with 2 injections of PGF on ovarian response, plasma progesterone concentrations, pregnancy rate and pregnancy loss in 608 lactating dairy cows. Cows given a PD during the Ovsynch protocol had a higher synchronization rate (78% versus 71%) because fewer cows had premature ovulations (6% versus 11%). Administration of a PD also tended to result in increased plasma progesterone concentrations at PGF treatment (4.4 ± 0.2 ng/mL versus 3.9 ± 0.2 ng/mL, P < 0.08) and increased pregnancy rate in cows subjected to Ovsynch without presynchronization (41% versus 25%, P < 0.01). However, adding a PD during the Ovsynch protocol did not increase pregnancy rate in presynchronized cows (42% versus 40%). Although administration of a PD in cows subjected to Ovsynch without presynchronization improved the overall synchronization rate and subsequent pregnancy rate, the increase in pregnancy rate was also observed when only synchronized cows were considered (52% versus 38%).

Regarding acyclic cows, inclusion of a PD increased pregnancy rate from 22% to 35%. However, as discussed, pregnancy success was highly dependent on ovulatory response to the first GnRH. In other words, pregnancy rate in cows that did not ovulate to first GnRH was extremely low and did not differ significantly whether (9%) or not (4%) they received a PD during the Ovsynch protocol. Thus, the common recommendation to provide acyclic cows with progesterone supplementation via a single intravaginal device before breeding was not fully supported in our study.

Note that most cows in the Ovsynch group had been previously inseminated and diagnosed as non-pregnant 32 d after AI (resynchronization). Hence, our finding has a practical implication in that supplementation with exogenous progesterone during resynchronization starting on Day 32 after the previous AI will enhance subsequent pregnancy rate. Similarly, the incorporation of a PD in dairy cows subjected to a 5-day TAI protocol during resynchronization starting on Day 34 after the previous AI improved pregnancy rate [51% versus 43%, P < 0.05 (60)].

On the contrary, cyclic cows that were presynchronized with PGF would have adequate plasma progesterone concentrations at initiation of the Ovsynch protocol. Presynchronization also increases the probability of ovulation after the first injection of GnRH (46% versus 28% in our study), which would result in an additional CL and even more circulating progesterone. Hence, we infer that progesterone supplementation before TAI would not benefit cows subjected to a Presynch-Ovsynch protocol because their ovarian synchrony is already optimized and progesterone concentrations would be elevated prior to administration of PGF, leaving little room for further improvement.

Others have also reported no benefit in supplementing progesterone pre-TAI in presynchronized lactating dairy cows. El-Zarkouny et al (31) showed that adding a PD did not improve pregnancy rate at 29 days after TAI (45% versus 48% in Ovsynch and Ovsynch plus PD, respectively) in a dairy herd that utilized presynchronization with PGF and had ~80% cyclic cows at the beginning of the Ovsynch protocol. Similar results were reported by Galvão et al (61) in lactating dairy cows of which > 80% were cycling.

In yet another study (51), the use of a PD in cows treated with Ovsynch led to a 10% increase in pregnancy rate (50% versus 40%, P < 0.05). Neither cyclicity status at initiation of Ovsynch nor its interaction with administration of a PD affected pregnancy rate. However, the PD increased pregnancy rate in cows with low circulating progesterone concentrations at the time of administration of PGF in the Ovsynch protocol (17% to 33% and 19% to 38% in acyclic and cyclic cows, respectively). Results from this study suggest that the addition of a PD will increase the fertility of cows that may not respond to the first GnRH of the Ovsynch protocol (acyclic cows or cyclic cows in late diestrus).

Reducing the duration of follicular dominance and lengthened proestrus

The 5-day Cosynch protocol

Bridges et al (62) compared a 7-day Cosynch plus PD protocol with TAI at 60 h and a 5-day Cosynch plus PD protocol with TAI at 72 h in postpartum beef cows. In that study, pregnancy rates were 11% higher with the 5-day protocol. Santos et al (63) reported 7% higher pregnancy rates in dairy cows subjected to a 5-day Cosynch protocol with TAI at 72 h. The hypothesis proposed was that the 5-day protocol provided for a longer proestrus with increasing estradiol concentrations due to continuous gonadotropin support for the dominant follicle. The ovulatory follicle of cows in the 5-day program benefited from the additional gonadotropin support and the extra time to grow and develop. However, due to a shorter interval between the first GnRH and induction of luteolysis in the 5-day protocol, 2 injections of PGF 6 to 8 h apart were necessary to induce complete regression of the GnRH-induced CL.

However, Rabaglino and coworkers (64) tested the 5-day Cosynch + PD protocol with 1 or 2 PGF treatments in 2 experiments in dairy heifers and attained 53% and 59% pregnancy rates, respectively, after TAI. They concluded that a single PGF treatment is sufficient to induce luteolysis in dairy heifers subjected to a 5-day Cosynch + PD protocol.

We compared a 5-day versus a 7-day Cosynch + PD protocol in dairy heifers given a single dose of PGF at device removal (65). Pregnancy per AI did not differ between 5-day (59%) and 7-day (58%) protocols. Hence, our study does not suggest any benefit of one protocol over the other in dairy heifers. Interestingly, ovulation response to first GnRH treatment was only 25% in heifers subjected to the 5-day Cosynch + PD protocol (Figure 2), and a greater proportion of heifers that did not ovulate became pregnant compared to those that did ovulate (65% versus 45%, respectively).

Figure 2.

The 5-day Cosynch plus progesterone device protocol for beef and dairy cattle. PGF — prostaglandin F2α, GnRH — gonadotropin-releasing hormone, AI — artificial insemination.

Given that GnRH induces ovulation in a small percentage of heifers, we questioned whether GnRH administration is required at the beginning of the 5-day Cosynch + PD protocol. An experiment was designed to determine whether the initial GnRH was necessary to achieve acceptable pregnancy rates in dairy heifers subjected to a 5-day Cosynch + PD protocol (65). All heifers received a single injection of PGF. Pregnancy per AI did not differ whether or not heifers received GnRH at PD insertion (68% versus 71%). We concluded that initial injection of GnRH in a 5-day Cosynch + PD protocol was not essential to achieve acceptable pregnancy rates in dairy heifers and thus a second administration of PGF was not necessary.

In a follow-up study, we investigated whether the modified 5-day Cosynch + PD protocol (without the initial GnRH; Figure 3) is suitable for the use of sexed semen in dairy heifers (66). In a two-by-two experimental design, cycling heifers were divided to receive either 2 PGF treatments 14 d apart with insemination approximately 12 h after estrus detection, or the modified 5-day Cosynch + PD protocol (without the first GnRH and a single administration of PGF at the time of device removal) with TAI 72 h later. Heifers were inseminated with either conventional or sexed semen from 1 of 4 sires. The overall pregnancy rate was higher in heifers inseminated after estrus detection than in those subjected to TAI after the modified 5-day Cosynch + PD protocol (70% versus 63%, respectively). More importantly, heifers inseminated by TAI with sexed semen had a pregnancy rate of 61% which did not differ from those inseminated by TAI with conventional semen (64%).

Figure 3.

The modified 5-day Cosynch protocol for dairy heifers. PGF — prostaglandin F2α, GnRH — gonadotropin-releasing hormone, AI — artificial insemination.

The timing of insemination has been the focus of research in heifers subjected to the 5-day Cosynch + PD protocol, as approximately 20% of heifers showed estrus and ovulated within 72 h after device removal. Some differences may exist between dairy and beef heifers or whether initial GnRH is given or not. Kasimanickam et al (67) reported that beef heifers inseminated at 56 h had, on average, a 10% higher AI pregnancy rate than heifers inseminated at 72 h. Both groups of heifers received the initial GnRH and the second GnRH was given concurrently with TAI. Conversely, Lima et al (68) reported an increased pregnancy rate in dairy heifers receiving the final GnRH concurrent with AI at 72 h after PGF compared with 16 h before AI; both groups of heifers did not receive an initial GnRH. More research is needed to optimize the timing of induction of ovulation with GnRH relative to TAI in heifers subjected to the 5-day Cosynch + PD protocol.

Conclusions

There are numerous controlled breeding protocols for beef and dairy cattle, with GnRH-based protocols the most commonly used in North America. The addition of a PD between first GnRH and PGF improves pregnancy rate in heifers, acyclic beef cows, and resynchronized lactating dairy cows. However, the addition of a PD may not improve pregnancy rate in lactating dairy cows previously presynchronized with PGF. Presynchronization with 2 injections of PGF increases pregnancy rate in lactating dairy cows but not in beef heifers. Presynchronization with a PD increases pregnancy rate in suckling beef cows. The 5-day plus a PD TAI protocol enhances fertility in lactating dairy cows and heifers. The high pregnancy rate achieved with this protocol, in particular during resynchronization, is very encouraging. However, the initial injection of GnRH in a 5-day Cosynch + PD protocol is not essential to achieve acceptable pregnancy rates in dairy heifers inseminated with either conventional or sexed semen.