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Abstract
Synchronization of ovulation (Ovsynch) is an effective method for controlling time
of first and subsequent AI in lactating dairy cows. However, validation of the original
Ovsynch program did not include testing the optimal time to deliver the final treatment
of GnRH. In Experiment 1, the effect of administering the final dose of GnRH on the
same day as prostaglandin F2alpha (PGF2alpha) administration was tested. Lactating
dairy cows (n = 218) were randomly assigned to receive either Ovsynch (OV; cows were
given 100 microg GnRH, then 7 days later cows were administered 25mg PGF2alpha followed
by a subsequent treatment of 100 microg GnRH 2 days after the PGF2alpha or the modified
version of Ovsynch (MOV; cows were given 100 microg GnRH, then 7 days later cows were
administered 25mg PGF2alpha followed immediately with 100 microg GnRH). In both treatment
groups, AI took place 16 h after the final administration of GnRH. In Experiment 2,
cows (n = 457) were randomly divided into four treatment groups that were administered
GnRH 0, 12, 24 and 36 h following PGF(2alpha). The 36 h treatment group served as
control. Pregnancy diagnoses were performed by palpation per rectum 36 days post-AI
in Experiment 1 and by ultrasonography on Day 28 in Experiment 2. In Experiment 1,
pregnancy rate/AI (PR/AI) was greater (P<0.025) in OV versus MOV. In a subset (n =
85), percentage of cows with both synchronized ovulations and regressed CL following
administration of PGF2alpha were similar (P>0.1) between OV and MOV, respectively.
All cows that became pregnant in the MOV subset group showed regression of the CL
in response to the PGF2alpha. Diameter of the ovulatory follicle at the time of final
GnRH administration was greater (P<0.05) in OV versus MOV. In Experiment 2, the synchronization
rate was once again similar among treatments (P>0.28). There was a linear effect of
treatment on follicle size (P<0.05) and PR/AI (P<0.0001) as time increased between
administration of PGF2alpha and GnRH, with the greatest PR/AI at 36 h. There was a
trend for a greater percentage of cows with short luteal phases in the 0 h group (P<0.10).
In summary, delivering the final treatment of GnRH of the Ovsynch program at the same
time as PGF2alpha, or in the 24h following PGF2alpha, resulted in lower fertility
compared to controls.