Stage of cycle, incidence, and timing of ovulation, and pregnancy rates in dairy cattle after three timed breeding protocols.
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In experiment 1, 705 cows were assigned to three treatments: 1) the Ovsynch protocol (a GnRH injection given 7 d before and another 48 h after one PGF2alpha injection); 2) PGF2alpha + Ovsynch (one PGF2alpha injection given 12 d (d -22) before initiating Ovsynch (d -10); and 3) 2xPG12 (two PGF2alpha injections 12 d apart; d -15 and -3, followed 48 h later by a GnRH injection. All cows were inseminated (d 0) 16 to 20 h after the GnRH injection on d -1. Cyclic status was estimated by serum progesterone. More cows were in early diestrus at d -10 for PGF2alpha + Ovsynch (36%) and 2 x PG12 (29%) versus Ovsynch (19%). Multiparous cows receiving PGF2alpha + Ovsynch had greater pregnancy rates via ultrasonography at d 28 after AI (42%) than contemporaries after Ovsynch (28%) or 2xPG12 (27%) but did not differ significantly at palpation 10 to 30 d later (28, 19, and 17%, respectively). Pregnancy of first-parity cows was similar across treatments at 28 d (41%) or at palpation (33%). Pregnancy rates for 128 anestrous cows were lower, regardless of treatment. Overall embryo survival from d 28 until palpation was 72% but was only 44% in 2xPG12 cows that were anestrus through d -10. Experiment 2 included the three treatments above plus controls (one GnRH injection 7 d before PGF2alpha and AI after estrus). Preovulatory follicles were 6 to 11% larger near estrus in controls than on d -1 in cows receiving GnRH. More controls ovulated by 32 h after onset of estrus than were treated cows by 32 h after GnRH, but percentages (79 to 94%) were similar by 40 h. In multiparous cows, PGF2alpha before Ovsynch increased pregnancy rates, whereas the 2xPG12 protocol produced similar pregnancy rates as Ovsynch across parities. Ovulation was effectively induced by 40 h after GnRH.
