Abstract:
Our objective in this study was to consider the eff
ect of service numbers on the
percentage of lactating cows that responded to the
resynchronization and fertility during
warm season. Lactating dairy cows (n= 750) in five
groups of 150 with different service
numbers according to the last service (AI1= First s
ervice, AI2= 2nd, AI3= 3rd, AI4= 4th,
AI
≥
5) were assigned to receive Ovsynch56 starting on
day 28 after fixed time AI (TAI).
Ultrasonography was done during Ovsynch56 injection
s and also for confirmation of
pregnancy in all cows at 32 and 60±4 days after fix
ed timed AI. Results showed
proportion of cows that ovulated in response to the
first GnRH injection of Ovsynch was
greatest (P= 0.01) in the first service cows compar
ed with other services. In addition, cows
that ovulated in response to the first GnRH of Ovsy
nch had greater response to PGF2
α
of
Ovsynch (91.8 vs. 74.2%, respectively) and finally
greater conception rate (CR) at 32 d
after AI (32.1 vs. 24.6%, respectively) than those
that did not ovulate. Conception rate at
32 and 60 days after AI was similar up to the third
service (29.1 and 21.8% at days 32 and
60, respectively), but decreased after that (19.75
and 10.4% at days 32 and 60,
respectively, P= 0.02). In conclusion, results of t
his study showed service numbersaffected
the proportion of cows that responded to resynchron
ization protocol. Conception rate was
reasonable up to the third service, but, after the
third breeding, proportion of cows that
responded to resynchronization decreased and
CR
dropped below 20%.
Keywords: C
onception rate, Ovsynch, Pregnancy loss, Ultrasonog
raphy
