Cows had the greatest chance to ovulate a follicle and create a new accessory CL when they were on d 6 or 7 of the estrous cycle when the first GnRH of Ovsynch was administered. More than 90% ovulated a first wave dominant follicle and formed an accessory CL. When the PGF of Ovsynch was administered 7 d later, cows that ovulated had both a primary 13 or 14 d old CL and a 7 d old accessory CL. Cows with both CL (d 13 CL and d 7 CL) had correspondingly greater circulating concentrations of progesterone at time of the PGF of Ovsynch compared with cows that did not receive GnRH and only had a d 13 CL (5.22 vs.3.53 ng/mL, respectively). Thus, presence of a young (d 7) accessory CL during Ovsynch positively impacted progesterone concentrations before artificial insemination (AI).
What is the Best Way to Ensure that Cows are on d 6 of Estrus Cycle at Time of 1st GnRH of Ovsynch?
To induce the accessory CL following the first GnRH of Ovsynch cows must be pre-synchronized (prior to Ovsynch) to ensure that cows are in a stage of the estrous cycle that has both a high probability of ovulation of a dominant follicle to the first GnRH and control of subsequent CL regression with PGF.
In two previous studies , (published and unpublished data), we observed rate of induction of accessory CL at 4, 5, 6, 7, or 8 d of the estrous cycle at the time of the first GnRH of Ovsynch.
The “d 6” interval (referred to as G6G; Figure 3) resulted in a greater percentage of cows ovulating to the first GnRH and inducing accessory CL compared with the other intervals.
If cows responded to both PGF and GnRH pre-synchrony treatments and were on d 6 of the cycle at time of first GnRH of Ovsynch, 97% of cows contained accessory CL, had a greater progesterone concentrations, and a greater probability of a pregnancy.
What Current Programs Seem to be Working the Best to Achieve Greater Conception Rates?
There are several programs that enhance fertility of dairy cows that utilize the technologies described above to enhance progesterone. We recommend three programs for first AI: Double Ovsynch , G6G , and Presynch-11 .
The recommended resynchronization programs (setting cows up for AI following a pregnancy diagnosis) are: G6G, GGPG (for cows without a CL at diagnosis), and an abbreviated Presynch-11 (see Calendars left for details).
This article provided a basis for the concept that low progesterone in lactating dairy cows due to enhanced steroid metabolism may be the underlying cause of the low fertility that has plagued dairy herds for the past 2 decades.