Abstract
This paper reports a new method for synchronizing the time of ovulation in cattle using GnRH and PGF2α. In Experiments 1 and 2, lactating dairy cows (n=20) ranging from 36 to 280 d postpartum and dairy heifers (n=24) 14 to 16 mo old were treated with an intramuscular injection of 100 μg GnRH at a random stage of the estrous cycle. Seven d later the cattle received PGF2α to regress corpora lutea (CL). Lactating cows and heifers received a second injection of 100 μg GnRH 48 and 24 h later, respectively. Lactating cows were artificially inseminated 24 h after the second GnRH injection. Ovarian morphology was monitored daily by trans-rectal ultrasonography from 5 d prior to treatment until ovulation. In Experiment 3, the flexibility in the timing of hormonal injections with this synchronization protocol was evaluated by randomly assigning 66 lactating dairy cows to 3 different treatment groups. Lactating cows received the injection of PGF2α 48 (Group 1), 24 (Group 2), and 0 h (Group 3) prior to the second injection of GnRH, which was administered at the same time in each group to ensure the second injection of GnRH was given when follicles were at a similar stage of growth. In Experiments 1 and 2, the first injection of GnRH caused ovulation and formation of a new or accessory CL in Math Eq cows and Math Eq heifers. In addition, this injection of GnRH initiated or was coincident with initiation of a new follicular wave in Math Eq lactating cows and Math Eq heifers. Corpora lutea regressed after PGF2α in Math Eq cows and in Math Eq heifers. All cows and Math Eq heifers ovulated a newly formed dominant follicle between 24 and 32 h after the second injection of GnRH. Ten of 20 cows conceived to the timed artificial insemination. In Experiment 3, the conception rate in Groups 1 and 2 were greater than in Group 3, (55 and 46 % vs 11%, respectively). In summary, this protocol could have a major impact on managing reproduction in lactating dairy cows, because it allows for AI to occur at a known time of ovulation and eliminates the need for detection of estrus.