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Modifications to Ovsynch improve fertility during resynchronization: Evaluation of presynchronization with GnRH 6 days before Ovsynch and addition of a second PGF treatment
Recent modifications to Ovsynch, such as presynchronization with GnRH and addition of a second PGF have, at times, appeared to improve synchronization and fertility to the timed AI protocol; however, combining these two methods may further optimize the hormonal environment. Lactating Holstein cows were randomly assigned to a 2x2 factorial design to compare main effects of presynchronization with GnRH (±GnRH) 6 d before beginning an Ovsynch protocol, and a second PGF injection (±PGF) administered 24 h after the first on P/AI. For first TAI, cows were presynchronized with two injections of PGF 14 d apart, and cows detected in estrus after the second PGF injection were inseminated and removed from the experiment. Nonpregnant cows were resynchronized using an Ovsynch protocol initiated 32 ± 3 d after AI. Blood samples collected from all cows at the first GnRH (G1), at the PGF, and at the last GnRH (G2) injections of the Ovsynch protocol were assayed for progesterone (P4) concentrations. At 32 d after TAI, pregnancies per AI (P/AI) was greatest for cows presynchronized with GnRH and receiving a second PGF injection, intermediate for cows receiving the second PGF injection only and for cows presynchronized with GnRH only, and least for controls [(39.6% (91/230), 35.6% (73/205), 31.8% (77/242), 30.3% (67/221), respectively; P = 0.18]. Interestingly, treatments affected P/AI only for resynchronized cows [41.0% (57/139), 32.8% (39/119), 27.9% (41/147), 24.6% (35/142), respectively; P = 0.02], but not for cows receiving first TAI [(37.4% (34/91), 39.5% (34/86), 37.9% (36/95), 40.5% (32/79), respectively, P = 0.96]. Fewer (P<0.01) cows presynchronized with GnRH had low (< 0.5 ng/mL) P4 at G1 compared to cows not presynchronized (12.8% vs. 24.8%), and P4 was greater (P = 0.05) at the PGF injection for cows presynchronized with GnRH (4.4 vs. 4.0 ng/mL). Surprisingly, differences in P4 at PGF were only detected for resynchronized cows (P = 0.09) and not for cows receiving first TAI (P = 0.23). Cows receiving the second PGF injection had less (P < 0.01) P4 at G2 compared to cows not receiving the second PGF injection (0.2 vs. 0.4 ng/mL). We conclude that presynchronization with GnRH 6 d before beginning an Ovsynch protocol increased P4 at the PGF injection of an Ovsynch protocol, and a second PGF injection 24 h after the first decreased P4 at TAI resulting in more P/AI in resynchronized cows. Supported by Hatch project WIS01171
Keywords: Ovsynch; Resynchronization; Fertility