Resynchronized Timed Inseminations

Only 30 to 45% of inseminated cows are pregnant at 40 d after insemination, and nonpregnant cows need to be reinseminated as quickly as possible. Strategies to accomplish this can be rather aggressive with resynchronization of

Fig. 2 Presynch/Ovsynch® protocol for timed AI at the first postpartum service.
Fig. 3 Presynch/Heatsynch protocol for timed AI.

follicle development prior to an early ultrasonographic pregnancy diagnosis, as part of a TAI protocol for nonpregnant cows.

Ovsynch® Initiated 7 Days Prior to Pregnancy Diagnosis

A study was conducted to determine the effects of resynchronization with GnRH beginning on day 21 after insemination on PR and losses of pregnancy to the first service in lactating dairy cows.[5] On day 21 after a prior insemination, cows in the resynchronization group received an injection of GnRH, whereas the control group received no treatment. Pregnancy was diagnosed by ultrasound on day 28. Nonpregnant cows on day 28 received a PGF2a injection followed by GnRH on day 30 and TAI on day 31. In contrast, nonpregnant cows of the control group initiated the Ovsynch® protocol at day 28

and were TAI 10 days later on day 38 after the previous service. For resynchronized and control cows, PR at days 28 (33.1 vs. 33.6%) and 42 (27.0 vs. 26.8%) after the initial insemination did not differ. Administration of GnRH on day 21 after insemination had no effect on the losses of pregnancy between resynchronized and control groups from 28 to 42 d (17.9%) after the first insemination. Pregnancy rate after the resynchronization period was similar for both groups and averaged 29.4%. The resynchronization and control groups were reinseminated at 31 and 38 days after the previous service.

Initiation of Ovsynch® and Heatsynch at 23 Days After AI

Based on the distribution of intervals to estrus in nonpregnant cows that returned to estrus following a previous insemination (Fig. 4), it is feasible to inject

First TAI Presynch/Ovsynch

20-

0

15-

to

(I)

ID

<->

(I)

CL

S'

2 5 8 10 12 14 16 18 20 22 24 26 28 30 Days from AI to Next Estrus

ECP+TAI

GnRH+TAI

Fig. 4 Strategy for resynchronization.

GnRH at day 23 (i.e., 22 24 days) after insemination to synchronize the follicular wave and ensure that a PGF2a-responsive CL is present at day 30. Cows diagnosed nonpregnant at ultrasound on day 30 receive PGF2a, and ovulation is synchronized with either ECP or GnRH (Fig. 4). The timing of the Ovsynch® protocol is standard with the ovulatory dose of GnRH given 48 h after injection of PGF2a and a TAI at approximately 16 h after GnRH. Our experience with ECP for resynchronization is such that ECP (1 mg) is given 24 h after injection of PGF2a and all cows are TAI at approximately 36 h after injection of ECP. Results evaluating 593 nonpregnant cows indicate the following distribution of cows according to stages of the estrous cycle at the time of pregnancy diagnosis: diestrus 75%, metestrus 5.8%, proestrus 9.6%, ovarian cysts 7.9%, and anestrus 1.6%.[6] For the 445 diestrus cows, PR for resynchronization was 28.6% (63/ 220) for cows subjected to PGF-ECP-TAI and 25.8% (58/ 225) for cows subjected to PGF-GnRH-TAI. Pregnancy losses between days 30 and 55 averaged 11.8% and did not differ between groups. Choosing the proper stage to initiate the protocol with GnRH (e.g., day 23) takes advantage of the reoccurring follicular wave and CL to reduce the time for reinsemination (Fig. 4). Reinsemination of nonpregnant cows occurred at approximately 32 days after the first service. Future cow-side pregnancy tests may allow detection of nonpregnant cows at an early stage (e.g., day 23) so that resynchronization protocols can be initiated only in cows known to be nonpregnant.

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