The authors have declared that no competing interests exist.
This review focused on the various methods for controlling estrous cycles in well-managed dairy cows. Because up to 70% of dairy cows may stay non-pregnant after an AI procedure, an effective approach for identifying and reinseminating open cows is essential for dairy herds to achieve optimal reproductive performance. Overall, well-managed dairy farms with effective estrus detection programs inseminate 50% or more of non-pregnant cows after behavioral estrus is detected. Cows not detected in estrus are admitted in a resynchronization of ovulation procedure to receive a timed AI (TAI) service to avoid a long interbreeding interval. In Egypt, a widely used program involves starting the Ovsynch protocol (GnRH-7 d-PGF2-56 h-GnRH-16 to 20 h-TAI) 32 days after an initial AI, regardless of pregnancy status. Previous studies have proven that there was no difference in pregnancy/artificial insemination (P/AI) between Ovsynch+P4 and Presynch-Ovsynch, both protocols were equally effective in improving the fertility of cows with a CL 15 mm. The review also addressed different methods for synchronization of ovulation and different factors affecting the selection of the management program.
The profitability of dairy cows is depending mainly on reproductive performance
Although there are numerous reproductive management strategies available for dairy farms, determining the best management program to implement remains a major challenge for dairy producers owing to the complicated interactions of multiple biological and management factors affecting dairy herd dynamics and economics
So, the present study aimed to evaluate how the application of different synchronization protocols such as Ovsynch, modified Ovsynch, presynch, and modified presynch protocols would be used to maximize the reproductive performance of Holstein dairy cows in Egypt.
In dairy cows, the estrous cycle is the time between two standing heats. The estrous cycle is a cyclical pattern of ovarian activity that allows female animals to go from a state of reproductive receptivity to one of reproductive non-receptivity, allowing for the formation of pregnancy after mating. An estrous cycle in cattle lasts 18-24 days on average
GnRH starts a chain reaction of hormones that regulate the estrous cycle
Following ovarian stimulation with FSH and LH, estrogen and progesterone are generated. Because it produces prostaglandin F2, the uterus aids in reproductive control
During the follicular, estrus, and luteal phases of the cycle, the combination of hormone secretion and metabolism maintains the correct hormonal balance
During the ruminant estrous cycle, changes in a pre-ovulatory follicle and corpus luteum, patterns of LH, estrogen, and progesterone release, and changes in ovarian blood flow all occur
Maximizing reproductive performance in high-producing cows is one of the top aims of dairy herd management
To attain an appropriate heat detection rate, cows must exhibit estrous symptoms for a significant amount of time, and estrous behaviors must be carefully observed by herdsmen. Due to delayed resumption of ovarian cyclicity and failure of oestrus expression, increased milk production and changes in management techniques can result in a longer duration of post-partum anoestrus
Of note, the success of the artificial insemination program in most cases depends mainly on the detection of estrous. However, because peak estrus activity generally occurs at night, accurate control of the period of estrus is difficult, and determining the true commencement of standing estrus without 24 hours of observation may be challenging
Regular observation of the cows is required for the successful detection of heat. These should be observed at least twice a day, once early in the morning and once later in the evening. At least 20 minutes should be spent on each observation. The more females on heat at the same time, the more heat activity there will be
The optimal time to inseminate a heifer or cow is a few hours before ovulation, which happens about 24-38 hours after standing heat begins. This means cows or heifers should be inseminated in the latter two-thirds of a heat cycle or within a few hours after the cycle has ended
The environment (temperature, season, and light), age and body weight, hormonal imbalance, diet, and amount of production are all aspects that influence the expression of estrus behavior in bovine species. The temperature has a significant impact on the expression of estrous in bovines since sexual activity is reduced in extreme cold and hot settings, resulting in poor conception. It also has an impact on the duration of the estrous cycle
The ability to correctly identify cows in estrus and inseminate them at the right time is crucial in reproductive control. In recent decades, technology for recognizing cows in estrus has progressed from tail paint in the 1970s to automated activity monitors and mount detectors
Understanding the endocrine and neurological processes that drive estrus in dairy cows makes it easier to devise tactics for increasing estrous behavior identification. A concept map on estrus in cattle has been created (
Estrus is caused in the absence of progesterone by the action of estradiol on the hypothalamus (
The size of the follicle at the time of GnRH-induced ovulation influenced the formation and maintenance of pregnancy in beef cattle
The lower percentage of cows pregnant after GnRH-induced ovulation from a physiologically immature dominant follicle is likely related to oocyte competence difficulties and insufficient maternal tissue preparation for pregnancy
Estrus synchronization is a significant reproductive control method in the dairy cattle sector, where the majority of animals are bred through artificial insemination. Estrus synchronization lowers the cost of estrus detection and eliminates mistakes. Manipulation of the length of the luteal phase of the estrus cycle is the core premise of estrus synchronization
Although numerous synchronization programs involving one or more hormones have been created and deployed on commercial farms, the fundamental concept of estrus synchronization is to modify the length of the estrus cycle
Despite the fact that estrus synchronization is associated with a reduced conception rate during the synchronized estrus, improvements in estrus detection efficiency and accuracy can improve practically all reproductive performance.
In fact, despite the advantages of estrus synchronization, commercial dairy producers have yet to implement it. When dairy farmers consider whether or not to use estrus synchronization, they consider the expense of drugs as well as the time it takes to implement the program. Detecting estrus in a large herd of synchronized cattle can be more difficult because so many animals are in estrus at the same time, making it impossible to tell which ones are in real estrus
Since the introduction of the Ovsynch protocol two decades ago, significant progress has been made in the understanding and application of synchronization programs for the management of reproduction in dairy herds
Furthermore, timed AI protocols have also demonstrated benefits in pasture-based milk production systems because of the ability to increase the insemination rate. In general, successful use of the Ovsynch protocol requires some fundamental physiological principles to be respected, including induction of ovulation to synchronize follicle growth in the first 2 days of the program such that a young antral follicle is recruited; maintenance of high concentrations of progesterone during the development of the ovulatory follicle. Having a good quality pre-ovulatory follicle of moderate diameter that is highly oestrogenic and responsive to gonadotropins to synchronously ovulate 12 to 18 hours after insemination, and having an active corpus luteum to result in very low progesterone concentration at AI, and having a healthy corpus luteum to result in very low progesterone concentration at AI. The present estrous and ovulation synchronization methods are inefficient, and future progress would almost probably demand new hormone composition and delivery technology, lowering the need for intervention and assuring producer acceptance
The dairy industry's consolidation, combined with the steady increase in herd size, has necessitated the implementation of systematic reproduction management programs. The development of synchronized ovulation procedures, which provide artificial insemination at a predetermined period with acceptable fertility, has become an important part of breeding management in a variety of production systems
For the selection and successful implementation of the estrus synchronization planned, knowledge of the hormonal profile and functional structures present in the ovaries at various stages of the estrous cycle is highly crucial
The hormones that exist spontaneously are prostaglandin (PG). PGF will be released from the womb within 16 to 18 days of the heat of the animal during the regular estrous cycle of a non-pregnant animal. This PGF release works to damage the corpus luteum (CL). The CL is an ovarian structure producing the hormone progesterone and avoiding the return of the animal to the estrus. The release of PGF from the uterus causes the animal to return to the estrus every 21 days
Despite these drawbacks, using prostaglandins to synchronize estrus in cattle is the easiest way. These products can synchronize estrus and fertility in cyclic females, such as virgin heifers, but they can't produce estrous cycles in non-cycling cows
In dairy cows with a functioning corpus luteum, prostaglandin F2 is often utilized to synchronize estrus. When dairy cows are given PGF2, around 50-60% of the treated cows go into estrus within 2-6 days
Once the stage of the estrous cycle in the cows is identified, two injections of prostaglandins are administered at 10-to-14-day intervals. Before or between injections, estrus detection is not required. Regardless of where they were in their estrous cycle when the first injection was given, all cycling cows should respond to the second. All females experiencing estrus after the initial PGF2 injection can be bred, which can change the program
Recent studies used intravaginal (IVG) prostaglandins instead of intramuscular or subcutaneous. of notes, Wijma et al. (2016) found that IVG delivery of two doses of 25 mg dinoprost (a natural form of PGF2α) 12 hours apart successfully produced complete luteal regression and resulted in circulating progesterone (P4) profile similar to that of milking dairy cows given a single 25-mg IM dosage of PGF2α. Circulating P4 concentrations in cows treated with IVG or IM PGF2α did not differ 48, 60, or 72 hours after therapy, and a similar proportion of cows in both groups exhibited complete CL reduction by 60 and 72 hours following treatment
Wijma et al. (2016) also found that giving two 25-mg dinoprost doses 12 hours apart was the most effective way to get complete CL regression compared to smaller or larger doses (i.e., 25 and 125 mg) or a similar dose (i.e., 50 mg) given as a single therapy. P4 concentration dynamics in cows given two 25-mg PGF2α doses 12 hours apart were similar to those in cows given a single 25-mg IM dosage of dinoprost, suggesting that this dose and treatment frequency for intravaginal infusion is a feasible choice for future investigations examining CL regression and ovulation synchronization
Even after the corpus luteum has regressed, synchronization of estrus with progestogens maintains high amounts of progesterone in the female's system. Estrus synchrony occurs 2 to 5 days after progestin withdrawal. Melengesterol acetate (oral feeding), Syncro-Mate-B (Ear Implant), and CIDR are commercial products that fit within this category (Intra-vaginal device). Only 48% of cows treated on day 3 had their estrus synchronized, but when treatment began on day 9 of the estrous cycle, the synchronization was 100%. The longer cattle were given progestin, the higher the rate of estrous synchronization, but the lower the fertility of the synchronized animals
This therapeutic regimen was used to develop the Syncro-mate B commercial product, as well as the PRID (Progesterone releasing intravaginal device) and CIDR (Controlled intra vaginal drug release device). Although the corpus luteum has shrunk, increased pulsatile secretion of gonadotropin during the period when exogenous progestin is preventing estrus leads the persistent follicle to grow
The decreased fertility of cows bred at synchronized estrus after long-term progestin administration is attributed to early ovarian meiosis or aberrant embryo development produced from persisting follicle ova. The use of progestogens in cattle for less than 14 days did not lower the percentage of calves conceived. Furthermore, short-term progestogen exposure induces some anestrus (postpartum or prepubertal) heifers to cycle
Ovsynch, a GnRH and PGF2α regimen, was created to synchronize ovulation in dairy calves, with the goal of synchronizing ovulation within an 8-hour interval (from 24 to 32 hours after the second GnRH treatment), allowing TAI without detection of estrus and pregnancy rates of 30 to 40%
Fertility to AI in dairy heifers is frequently regarded as the gold standard to obtain in lactating dairy cows, and that such fertile responses in heifers should be achieved in TAI programs for both heifers and lactating cows. Lactating dairy cows have a number of problems, including the biological responses and demands of lactation, metabolism, and production disorders include metabolic, mammary, and uterine diseases, all of which reduce fertility. Due to basic reproductive differences, however, developing a fertile TAI program in heifers has proven difficult. Application of a simple OvSynch-program
Presynch techniques that include gonadotropin-releasing hormone (GnRH) and prostaglandin (PG) F2 induce ovulation in anovulatory cattle and increase the proportion of cows that synchronize during an Ovsynch procedure maximize pregnancies per A.I. In a seven-day Ovsynch protocol, adding a second PGF2α injection 24 hours after the first boost luteal regression, which leads to more pregnancies per A.I. especially in multiparous cows
The intervals between PGF2α therapy and estrus and ovulation relied on the stage of development of the dominant follicle at the time of treatment, according to preliminary research involving estrus monitoring and following comprehensive ultrasound analyses of ovarian follicular wave patterns
There was a rise in the number of synchronized animals and a reduction in the variability in the time to estrus in beef cows and heifers when a GnRH analog was administered 6 days before injection of PGF2. This reduction in time variability could be explained by the commencement of a new follicular wave in response to GnRH, resulting in the presence of a new dominant follicle at the time of PGF2α injection
GnRH-induced follicle turnover or the initiation of a new follicular wave would be most useful if ovulation was prompted in response to the first administration of GnRH, resetting follicular development, and producing a new dominant follicle containing at least 25.8% of the dairy cattle in the Ovsynch group had insufficient luteal regression
Late embryonic and early fetal mortality, which average 13 percent and 11 percent, respectively, impair breastfeeding dairy cows' reproductive effectiveness
In Bos indicus cows, intervention with intravaginal P4 devices coupled with eCG at device removal enhanced ovulation rates, plasma P4 concentrations, and pregnancy rates in suckled beef cattle with a higher incidence of anestrous or a poor body condition score
The efficiency of TAI procedures is limited in postpartum anestrous cows because their pulsatile release of LH is insufficient to support the final phases of ovarian follicular growth and ovulation
Because of its cost-effectiveness, pregnant mare serum gonadotropin (PMSG) is commonly used for estrus synchronization programs in small and large ruminants
Treatment with PMSG is thought to improve ovarian follicular development and fertility in dairy cows at the end of the estrus synchronization program
The rate of submission to insemination and P/AI define the rate of pregnancy in dairy herds once the voluntary waiting period has ended. The reduced rate of oestrus detection and, as a result, reduced submission to insemination found in nursing dairy cows is one of the constraints to optimal reproduction in many dairy farms
Replacement to avoid issues during synchronization, heifers are grown to a pre-breeding target weight of at least 65 percent of their predicted mature weight, and reproductive tract scores of 2 or higher on a scale of 1 to 5 are assigned to heifers two weeks before synchronization treatment begins
Several factors have been proven to alter the length of the estrous cycle, including age, species, and body weight, amount of nourishment, time of the year, hormonal changes, lactation, suckling, and degree of milk yield
Ovulation has been observed to occur 10-12 hours after the heat cycle has ended
On a scale of one to five, a score of one suggests that the cow is malnourished, while a score of five means that the cow is overweight. A BCS of 3 indicates that the animal has adequate energy stores to support a pregnancy around the time of breeding; a BCS of less than 3 suggests that the animal does not have enough energy reserves to maintain a pregnancy. Because the cow has mobilized body fat stores to fulfill the energy requirements for milk yield in early lactation, negative feeling balance (NEB) is indicated by a drop in BCS
The state of a cow's body has a direct impact on one's ability to reproduce. In dairy cows, there is a lot of evidence that body condition affects reproductive performance. Low body condition can have a detrimental impact on female fertility for a variety of reasons, including a prolonged interval between parturition and the commencement of ovarian activity
Undernutrition can affect pituitary activity, which includes gonadotropin production and release, as well as the responses of target organs to gonadotropin-releasing hormone or gonadal hormones. The capacity of an animal to maintain a high-frequency mode of pulsatile LH release has been related to its metabolic status. One important way that energy limitation impairs reproductive activity appears to be suppression of an increase in luteinizing hormone pulse the frequency that has been necessary for the growth of ovarian follicles inhibits pulsatile release by the hypothalamus
The time between calving and first ovulation in primiparous cows has been shown to be longer than in multiparous cows. Because of the requirements for growth other than lactation, this link is related to more nutritional shortages being imposed on young cows. According to recent research, primiparous cows' first ovulation postpartum was postponed under optimal management compared to multiparous cows.
Multiparous cows have superior reproductive performance than primiparous cows
The breed plays an important role in the response of cows to the synchronization program. There are considerable differences in the effects of estrus synchronization between zebu and taurine breeds due to their different estrus cycles. There has been documented diversity in the length, duration, and severity of the estrus cycle within and across breeds
Recently, slight changes in ovarian follicular dynamics that could affect the use of assisted reproduction like synchronization or superovulation have been addressed.
Because prolactin function peaks during intense lactation, and sensory cues from sucking decrease the prolactin inhibiting factor (PIF), which comprises dopamine and GnRH-related peptide, lactation has been demonstrated to diminish ovarian activity. Because dopamine and GnRH have essential interactions with gonadotropins, the GnRH component is important. Because the LH releasing factor is blocked, no LH is released, and hence no final follicle maturation, oestrus, or ovulation occurs
Even in the presence of estrus-inducing doses of estradiol, stress has been shown to delay, shorten, or entirely block the manifestation of estrus in cows
Larson and Ball (1992) found that pregnancy rates from fixed TAI after synchronization with PGF2 were varied, especially in lactating dairy cows compared to heifers, owing to changes in the time of ovulation in response to AI, resulting in varying estrous cycle duration in cows
Pursley et al. (1995 and 1997) developed Ovsynch to build a TAI program that put less emphasis on detecting estrus because all cows were inseminated at a specific time relative to hormone injection
TAI methods minimized the annoyances of ovulation synchronization and AI on detected estrus, and they suggested that they could provide an efficient and effective means of capturing selecting genetic features with economic implications in a fixed time insemination program. They also noted that changes in pasture and diet, breed composition, physical condition, postpartum interval, climate, and geographic location will impact TAI protocol performance
The initial service pregnancy rate was a useful metric for assessing fertility, with 60-70 % being considered ideal in well-managed herds
The progesterone hormone is in charge of stimulating cyclicity, follicular growth, and pregnancy maintenance. The plasma protein, cholesterol, and mineral profiles of animals indicate their nutritional status and are associated with their fertility
PGF2 therapy resulted in a decrease in progesterone levels on day 9 (48 hours later) in all controlled breeding programs (Ovsynch and CIDR synch) due to PGF2's luteolytic action on the corpus luteum
Significant increase in plasma P4 profile observed on day 7 of treatments with CIDR and Ovsynch protocols (4.97 ±1.68 and 3.75 ±0.47 ng/ml) over initial (0 days) values, with dramatic fall to almost basal values on induced estrus within 48-60 h after PG injection has previously been identified in anestrus cows using CIDR and Ovsynch protocols
Some researchers reported lower first service conception rates (CR) in targeted breeding programs compared to spontaneous oestrus breeding
Additionally, serum P4 levels are higher 14 days later than they were at the time of the first treatment. At any given time, a certain percentage of cycling cows have physiologically low progesterone levels. During lactation, the percentage of cows with consistently low P4 levels in serum (1 ng/ml) decreased. Inactive ovaries or cystic ovarian disease are associated with low progesterone levels at PG shots of the ovsynch protocol
Regarding the level of estrogen (E2) during the ovsynch programs, it was reported that GnRH-induced ovulation of small follicles (11.3 mm) in beef cows was associated with lower P/AI and serum E2 concentrations at AI, as well as higher pregnancy loss