Pregnancy Rates Per Artificial Insemination for Cows and Heifers Inseminated at a Synchronized Ovulation or Synchronized Estrus

Abstract

Two synchronization protocols were tested for lactating dairy cows and heifers. Nulliparous dairy heifers (13 to 23 mo; n =155) and primiparous and multiparous dairy cows (60 to 289 d postpartum; n=310) were assigned randomly to two treatments. Controls received 25mg of PGF2α and were artificially inseminated according to the a.m.–p.m. rule following detected estrus. All controls that were not detected in estrus were injected with 25mg of PGF2α at 14-d intervals until artificial insemination (AI) at a detected estrus or until timed AI at 72 to 80h after a third sequential injection of PGF2α. Treated cows and heifers received a protocol that used GnRH and PGF2α to synchronize ovulation (Ovsynch).Cows and heifers that were treated with Ovsynch were injected i.m. with 100μg of GnRH at a random stage of the estrous cycle. Seven days later, cows and heifers in this group received 25mg of PGF2α, followed by a second injection of 100μg of GnRH 30 to 36h later. Subsequently, the treated cows and heifers received AI 16 to 20h after the second injection of GnRH. Pregnancy rates per AI were similar (38.9% vs. 37.8%) for control cows and cows treated with the Ovsynch protocol, respectively. However, pregnancy rate per AI was greater for control heifers (74.4%) than for heifers treated with Ovsynch (35.1%). Evaluation of serum progesterone concentrations at each hormonal injection indicated that the first injection of GnRH synchronized luteal function of lactating dairy cows but not of heifers. In summary, one fixed-time AI at a synchronized ovulation provided similar pregnancy rates per AI as did AI following the a.m.–p.m. rule after estrus had been induced by PGF2α in lactating cows, but the fixed-time AI was not effective for heifers because of the lack of synchronization.