Oestradiol directly induces preovulatory LH peak and induces ovulation if a preovulatory (mature) follicle is present on the ovary at the time of treatment. The follicles that are too small at the time of treatment to have LH receptors will not ovulate even if an LH peak is induced.
Simultaneously however administration of oestradiol will induce oestrus behaviour in the treated cow regardless of the status of her follicular development. This way we run a risk that some cows that are not yet in heat or have follicles that are too small to respond will be inseminated.
In the European Community the use of oestradiol is forbidden in food producing animals.
Those approaches do not exclude each other as the purpose of the pharmacological oestrus management lies not only in the improvement of heat detection rates. If oestrus synchronisation for a group breeding or induction of heat in anoestrus is required then a selected pharmacological system should be applied in selected animals. Moreover with some systems such as those based on prostaglandins oestrus detection still is the condition for a success.
There is no set rule that would determine which animals should be the first choice. All depends on the purpose with which the pharmacological oestrus management is applied in general: organisational, therapeutic etc.
There are however some universal rules that should be observed:/p>
Question 1: When is the best time to AI after single or double prostaglandin injection?
The best results are achieved with insemination at observed oestrus. With the double prostaglandin injection it is theoretically possible to perform a fixed time insemination at 72 and 96 hours after the second injection of prostaglandin. The results can however be lower that when the cows are inseminated according to the oestrus detection.
In general these animals can be inseminated or left to be served with the rest after the whole treatment is completed. Serving them after they show signs of oestrus after the first PG injection saves however the cost of the second treatment. Care should be taken that they are properly marked and AI recorded to avoid that they are not erroneously included in the second PG administration.
This can sometimes be observed. The reason could lie in the fact that with such an intensive prostaglandin treatment the cows have had limited chance to experience one full luteal phase after post partum anoestrus. This in turn has been associated in the literature with shortened luteal phase post AI and embryonic loss.
In general this administration route is in practice far more difficult than it seems. It requires good restraint of the animal and additional measures to ensure proper hygienic conditions.
Moreover data from literature show that variable results can be obtained with a lower dose of prostaglandin administered intravulvosubmucosally. Chohan (1998) observed comparable results both in the occurrence of oestrus and fertility in buffalo treated ivsm or i.m. with cloprostenol racemate. Chauhan et al., (1986), however, found a transient decline in progesterone levels and partial luteolysis in cows treated with one quarter of the regular cloprostenol dose given ivsm.
Canizal et al., (1992) found luteolysis in 92% of heifers treated with 25mg of PGF2α i.m., compared to 15% of the animals treated with 6.25mg i.m. and 38% in those treated with 6.25mg administered ivsm. They also observed delayed luteolysis in 23% of heifers of the group treated intravulvosubmucosally that required longer than 48 hours for progesterone concentration to fall to the baseline levels.
Question 5: Is there a difference in time-to-oestrus between various prostaglandin analogues?
No, there should be no difference. With comparable efficacy of the prostaglandin analogues used it is the degree of follicular development at the moment of the treatment that determines the time-to-oestrus post PG administration and not the type of analogue.For more information seeOestrus synchronisation-PG.
With alternative administration routes and reduced prostaglandin dose, differences can be observed but again not between the analogues but rather the dose chosen.
Experience using this program in field conditions indicate quite some flexibility during this “time window” but the best results are obtained with an earlier AI in heifers (16-17h) and later in cows (20-24h).
In general the use of a reduced dose of GnRH in systems like Ovsynch where success depends on a sequence of synchronised ovarian events should be discouraged.
The Ovsynch was proven to be the most successful if the rate of the ovulation after the first GnRH was maximal. Therefore the first GnRH injection should not be at the reduced dose.
Question 3: Will I see a significant decrease in the results when using Co-Synch instead of Ovsynch?
The data from the field experiments and controlled laboratory trails indicate that in general the results from the use of Ovsynch and Co Synch protocols are comparable with a slight advantage of the Ovsynch protocol observed in some herds.
In some herds and individuals lower intensity of oestrus behaviour can be observed after the Ovsynch protocol. This however should have no major consequences as the animals are inseminated at fixed time and there is no need for oestrus observation.
Question 5: Is it worth applying Ovsynch in anoestrus cows?
In general the Ovsynch system gives best results in cycling dairy cows. In the herds with high anoestrus incidence still important benefits can be achieved from introduction of the Ovsynch protocol such as induction of cyclicity and faster introduction of all cows into an intensive breeding schedule. Therefore even if the results measured as conception rate to first AI post Ovsynch are lower than in cycling animals the overall pregnancy rate and reduction in days open are improved in comparison to no treatment.
Question 6: Can I use Ovsynch in beef cows and how?
In general Ovsynch can be used in suckled beef cows but as the incidence of anoestrus in this group of animals is usually high, the results are on average lower than achieved with progestagens.
If used in beef cows the Ovsynch is usually combined with partial temporary weaning of the calf. To eliminate the blocking effect of suckling on the hypothalamo-pituitary axis it is necessary to remove the calf at least 48h before the second GnRH injection. See use of Ovsynch in beef cows.
Question 1: How long will Crestar block oestrus if not removed?
The activity of Crestar implant lasts approx. 21 days however the duration of its blocking effect beyond 10 days depends largely on the release and metabolism of the active in individual animal and can vary. Therefore in the population we can observe cows coming back to heat as early as 15 days and as late as 21 days post implant insertion. Such cows should be rather inseminated in the next oestrus.
Question 2: Should I use the PMSG injection in cycling cows?
The function of the PMSG injection given at implant removal is to enhance the synchronous growth of the follicles (FSH-like activity) and to facilitate their maturation and ovulation (LH-like activity). Although the Crestar system functions without the PMSG injection in cycling animals results from numerous field trials indicated improvement of the results when the injection was used.
Question 3: Should I use prostaglandin injection in all cattle types or only in dairy?
Data from the field trials indicate that it is the dairy cows that most often require additional prostaglandin injection when treated with progestagens.
In beef herds that rarely show more than 25% of cows cycling at the beginning of the breeding season an additional prostaglandin injection will probably not bring much benefit. Some practitioners however always use prostaglandins with progestagens, including Crestar when treating cows. Such approach should be always evaluated for its cost-benefit.
Question 4: What dose of PMSG should be used with Crestar in zebu cows and crosses?
Bos indicus (zebu) breeds show higher sensitivity to PMSG/FSH than bos Taurus (standard breeds). Therefore to avoid multiple ovulations and twinning it is recommended to reduce the dose of PMSG (Folligon) used with Crestar to 300-350iu per cow.
Question 5: Cow showing heat approx. 8 days after implant insertion.
In a small percentage of animals (<1%) this has been observed and most probably is associated with their individual absorbtion of norgestomet from the implant and its metabolism. Such cows can have the implant removed and can be inseminated according to heat observation after having the presence of the preovulatory follicle confirmed by rectal examination. In such animals it is advisable to replace the PMSG injection with GnRH (e.g. 2.5ml of Receptal) at AI to ensure timely ovulation and high pregnancy rates.
Question 6: What is the quality of oestrus behaviour observed with the use of Crestar?
Usually as it is typical for progestagen-based synchronisation systems the quality of oestrus behaviour observed during oestrus induced by Crestar is very good.
Question 7: Does it make sense to add a GnRH at AI after the complete Crestar program?
If the complete Crestar program has been applied, the PMSG injection additionally stimulates growth and ovulation of the dominant follicle. The injection of GnRH at AI is therefore not necessary. It can be added in the situation of temperature stress in highly producing dairy cows but should be clearly evaluated in terms of cost-benefit.
Question 8: How is Crestar used in suckled cows?
The presence of suckling calf has a strong blocking effect on the GnRH release and can negatively affect the efficacy of Crestar program. Therefore normally a partial or temporary weaning of the calf is practised with Crestar. The most practical method proved to be a removal of the calf at 48h before the implant removal and return after the insemination has been completed.
In cattle we know that after repeated use of PMSG indeed antibodies are produced however there is no indication that they interfere with the efficacy of PMSG as it is at present used in practice.