The objective of most studies are to push cycle forward to have a follicle with at least 6 mm diameter at time of first GnRH injection that as usual we can see it at d 6 of estrous cycle.

Ovulation of a growing dominant follicle occurred 100 % of the time following GnRH administration, however ovulation of a dominant follicle in the static or regression phase occurred 33 % and 0 % at time of GnRH injection respectively.

We reported starting ovsynch protocol on day 6 of estous cycle increased fertility compare to starting at random stage of estrous cycle in dairy cows (Dirandeh 2014. Animal Reproduction Science). Bello et al. (2006) have indicated that pregnancy outcome to Ovsynch is highly associated with the ovulatory response to initial GnRH injection. Vasconcelos et al. (1999) has shown that dairy cows at mid cycle (d 5 to 9 of the estrous cycle) have a greater probability of ovulating toGnRH injection than cows early (d 1 to 4) or late (d 10 to 16) in the cycle. Similarly, Dirandeh et al. (2014) reported.

Dear Dirandeh, 

Thank you for your answer to my question. I also want to know GnRH on day 0 can induce a new follicular wave if there is no any dominant follicule at time of the first GnRH administration. And the dominant follicule from this wave can be ovulated and a new CL can be occured until day 7. At the end,  PGF on day 7 can regress or lyse this CL... Do you have any finding about this?

As I said gnrh injection only affect follicle that have at least 6 mm so if we have f less than 6 gnrh cause growth if follicle and response to pg will decrease 

Then we can say there is a correlation between diameter of follicule at the time of first GnRH and success of PGF on day 7 (Because PGF can work only on active CL...). Have you check ovarium of cattle random time and what is incidence of absence of follicules with 6mm and higher diameter? I want to know the possiblity of absence of 6 mm or higher follicle. It is well known that there are three follicular waves (mean) in a estrus cycle. And I think it is important to find or catch the follicular growing wave at the time of first GnRH injection. However, it is theoretical information. So we can say the success of Ovsynch is depend on presynch and to create a suitable follicular wave or follicule (>6mm) for first GnRH injection, isn't it? 

Yes.

if you can do a best presynchronization to start ovsynch on day 5-7 of cycle you will have higher Preganancy rate,

almost 80 % of cows have F > 6 mm at d 6 of cycle

Dear Umut , I would like to ask you a question. I am a doctoral student working with LH receptors (LHR) and ovulation in  Bos taurus and Bos indicus cows in Brazil.  What kind of cows you work? We found differences in the LHR expression in these subspecies and  i think this may be important in your question .

Dear Eduardo,

We are working on Bos taurus, I mean simmental and brown swiss breeds. Also Holsthein is a breed in our region. However, at the momment our breeds are simmental and brown swiss and native breed in Kars region (Turkey). Native breeds also are members of Bos taurus in fenotypicly. 

Hi, Umut,

GnRH application induces:

- ovulation of a large follicle

- or luteinzation of this follicle or other large one

- intiate a new follicular wave, from this a large follicle will be developed to reach a meximum size in 5-7 days

So, I think even when no large follicle present, a follicular wave will be intiated.  

Hi Ahmet Ali,

You pointed out GnRH application induces a new follicular wave, and large follicle will be developed to a maximum size in 5-7 days. My ask is whether or not this large follicule developing from first GnRH injection can be ovulated with second GnRH on day 9 in Ovsynch. Do you have any experience about this issue?

Thank you. 

Hi Umut, 

Actually this is the aim of the ovsynch program, is to ovulate a follicle initiated from the first GnRH treatment, PG is used on D 7 to regress a CL and GnRH on D 9 to ovulate this follicle.

NB. This program is not a 100% one, I mean that some animals not ovulate on the second GnRH for may purposes.

I have experience for this program, but in buffaloes, the difference is not large. See attached

Hi Ahmet Ali,

Thank you for your answers and sci. paper... 

I want to share my experience. I planed the ovsynch protocol for 8 cows in a little family herd. The region is far away from my office and there is no way to apply ovsynch. I gave GnRH and PGF to herd owner and I left an application paper. However, he accidentally gave two times GnRH a part from 7 days (instead of PGF). He mentioned the situation and  I said him to continue with PGF 7 days after last GnRH and 2 days later GnRH injection again. I applied artificial insemination and 7 cows from 8 cows would have calf. I think accidentally "double GnRH 7" days intervals increased success of ovsynch protocol. I know the number of animals was low, nevertheless it was interesting experiment for me. In my opinion, first GnRH induced ovulations and provided CL formation in 3-4 days in cows with follicules sensitive to GnRH and second GnRH 7 days after has similar effect rest of the cows. And second GnRH warrantied the formation of CL rest of the cows. And I think, approximately all cows had CL to sensitive to PGF... Maybe double GnRH can be investigated in different time apart. For example 5, 7 or 9 days intervals may be investigated to determine double GnRH considered CL formation, isn't it? 

Hi. As you may know, newest information about the Ovsynch protocol show that the first GnRH administration on day 0, induce a new follicular wave in majoriyy of treated cows instead of ovulation a dominant follicle. So, after luteolysing any Cl on day 7, the dominant F. can continue its grow and reply to second GnRH on day 9. Good Luck

We have also some experiences.  The ovsynch protocol  spend some cost and time. It is more useful in dairy management. In order to get good  estrus and conception rates, I think, it is needed to make combinations with progestagens.Good succeses...