Stage of Cycle, Incidence, and Timing of Ovulation, and Pregnancy Rates in Dairy Cattle after Three Timed Breeding Protocols

Abstract

In experiment 1, 705 cows were assigned to three treatments: 1) the Ovsynch protocol (a GnRH injection given 7 d before and another 48h after one PGF2α injection); 2) PGF2α + Ovsynch (one PGF2α injection given 12 d (d –22) before initiating Ovsynch (d –10); and 3) 2xPG12 (two PGF2α injections 12 d apart; d –15 and –3, followed 48h later by a GnRH injection. All cows were inseminated (d 0) 16 to 20h after the GnRH injection on d –1. Cyclic status was estimated by serum progesterone. More cows were in early diestrus at d –10 for PGF2α + Ovsynch (36%) and 2 x PG12 (29%) versus Ovsynch (19%). Multiparous cows receiving PGF2α + Ovsynch had greater pregnancy rates via ultrasonography at d 28 after AI (42%) than contemporaries after Ovsynch (28%) or 2×PG12 (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 2×PG12 cows that were anestrus through d –10. Experiment 2 included the three treatments above plus controls (one GnRH injection 7 d before PGF2α 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 32h after onset of estrus than were treated cows by 32h after GnRH, but percentages (79 to 94%) were similar by 40h. In multiparous cows, PGF2α before Ovsynch increased pregnancy rates, whereas the 2×PG12 protocol produced similar pregnancy rates as Ovsynch across parities. Ovulation was effectively induced by 40h after GnRH.