Optimizing Ovulation Synchronization with Pre-synchronization and BST
by Geoffrey E. Dahl
TAKE HOME MESSAGES
- Presynchronizing cows before the voluntary waiting period improves responses to ovsynch.
- Initiating bST simultaneously with the first GnRH injection in the ovsynch protocol enhances pregnancy rates.
Whereas heat detection was previously a limitation to getting cows pregnant, recent advances in timed breeding regimens (i.e. Ovsynch) have allowed producers to overcome heat detection problems and still get cows bred. But, reproductive performance remains an area of dairy management that must be optimized to ensure continued productivity and a sustainable operation. New approaches to enhance pregnancy rates in response to ovsynch include pre-synchronizing cows and initiating bST coincident with the second injection of GnRH in the Ovsynch protocol. Both of these manipulations improve pregnancy rates to first service AI in response to Ovsynch.
Before considering the effects of pre-synchronization and bST, it is best to briefly review the basic Ovsynch protocol. The first step is an injection of GnRH, between day 5 and 10 of the cycle. This causes ovulation of a dominant follicle and subsequent development of a corpus luteum (CL). Seven days after the initial GnRH, an injection of prostaglandin F2α ?(PGF-2α), which will cause regression of the CL formed in response to the previous GnRH injection and any residual CL remaining from a spontaneous ovulation. Two days later a second injection of GnRH is administered to induce ovulation, and after 12-18 hrs the cow is bred by AI with pregnancy resulting about 30% of the time. Because ovulation is synchronized with a timed breeding, Ovsynch eliminates heat detection, a continuing problem on many farms. Pregnancy rates have repeatedly been shown to be as high in response to Ovsynch as to breeding to visually observed estrus (Thatcher et al., 2002).
Despite the successful implementation of Ovsynch on many farms, there are limitations to its use. For example, cows that are not between day 5 and 10 of the cycle at the first GnRH injection have lower pregnancy rates in response to Ovsynch. Thus, ensuring that a greater percentage of cows begin Ovsynch between day 5 and 10 increases the success rate of the program. This can be accomplished through strategic use of PGF-2α injections prior to initiating Ovsynch, that is, presynchronizing cows (Thatcher et al., 2002). The first step in presynchronization is establishing a voluntary waiting period (VWP), that is, the first day when a cow should be bred after calving to achieve a target calving interval for the herd. Then, calculate the date 12 and 26 days prior to the VWP; these are the target dates for injections of PGF2α. The 2 injections of PGF-2α at a 14 day interval will synchronize approximately 90% of the cows (of those cycling) to be around day 5 of the cycle at the VWP, which is 12 days after the second PGF2α injection. Thus a much greater number of cows are expected to be in the ideal stage of the cycle following presynchronization relative to the 30-35% that would be expected in the ideal stage under non-synchronized conditions.
In contrast to the effects of presynchronization, which influence the number of cows at the ideal stage of the cycle to begin Ovsynch, the effects of bST on pregnancy rate appear to be related to improved embryo development. In an initial study, Moreira et al. (2000), found that cows that began Ovsynch and bST at a VWP of 63 days had greater first service pregnancy rates relative to cows that did not receive bST. In a subsequent study, Moreira et al. (2001) confirmed the previous finding that bST initiation coincident with Ovsynch improved subsequent pregnancy rates by 15%, and the effects were additive to improvements observed with presynchronization.
The observed effects of bST on Ovsynch success are consistent with observations on the influence of bST on embryo survival. In tissue culture, addition of bST or IGF-I enhanced embryonic development (Moreira et al., 2002b), and a similar positive effect of bST has been observed in pregnancy rates of superovulated embryos following transfer from cows treated with bST (Moreira et al., 2002a). More recently, Badinga et al. (2002) found that GH may limit PGF2α production by the uterus, thus extending the life of the CL. Collectively then, these results are consistent with the positive effect of bST on pregnancy rates following Ovsynch.
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