Evaluation of Timed Insemination Using a Gonadotropin-Releasing Hormone Agonist in Lactating Dairy Cows

¹ University of Florida, Gainesville 32611

An experiment compared the effectiveness of a timed AI and an AI at observed estrus. Lactating dairy cows were injected with PGF₂, at 30 ± a range of 3 d postpartum, with a GnRH agonist at 65 ± a range of 3 d, and with PGF₂, 7 d later; control cows (n = 128) were inseminated at detected estrus, but cows in the timed AI group (n = 171) received a second injection of GnRH agonist 48 h after the injection of PGF₂, and were inseminated 16 h later. Control cows that were not inseminated within 7 d were resynchronized with GnRH agonist, followed 7 d later with an injection of PGF₂. Pregnancy rate was 30.5% for control cows and 29.0% for cows in the timed AI group, and conception rate was 41.5% for control cows and 26.5% for cows in the timed AI group. Days open for cows that conceived by 120 d postpartum was 83.6 d for control cows and 79.2 d for cows in the timed AI group. Treatment by month interactions indicated that pregnancy and conception rates and days open for cows that conceived were more consistent across months for cows in the timed AI group. Differences (control vs. timed AI) were not detected for overall pregnancy rate by 120 d postpartum (58.8 ± 4.7% vs. 56.2 ± 4.4%). Conception and pregnancy rates at first synchronization were influenced positively by body condition and plasma concentrations of progesterone at 65 d postpartum. Timed AI was an effective alternative for reproductive management.

Key Words: cows • gonadotropin-releasing hormone agonist • insemination • pregnancy rates

Submitted on December 18, 1995
Accepted on April 29, 1996

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